chapter 6 reproduction and environmental biology · the intent of this chapter is to summarize...

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Reproduction and Environmental Biology

Bruce A. BArton And terence P. BArry

Chapter 6

6.1 INTRODUCTION

The intent of this chapter is to summarize general physiological, organismal, health, and related biological information about walleye and sauger as a reference for field biologists, managers, and fish culturists. The chapter is not intended to be an exhaustive review of the physiology and reproductive biology of walleye and sauger, of which relatively little is known. Additional detail is provided on fish reproductive endocrinology at the beginning of this chap-ter as a foundation for understanding the endocrine events that occur during reproduction of these species. Further details of basic anatomy and physiology of walleye and other percid fishes can be found in published texts, notably Craig (1987, 2000). A short but excellent popu-lar article describing the structures and mechanics underlying sensory processes in walleye, particularly vision and sound detection, is provided by Carlson (2010).

6.2 THE REPRODUCTIVE ENDOCRINE SYSTEM

Reproductive development and spawning in fish is regulated by environmental cues such as photoperiod and temperature that are transduced into hormonal signals that, in turn, medi-ate the processes of gamete development, maturation, and spawning. A brief review of the reproductive endocrine system of teleosts (i.e., the brain–pituitary–gonad axis) is presented to provide context for understanding the reproductive physiology of the walleye and sauger.

6.2.1 Brain

Gonadotropin-releasing hormone (GnRH) is a brain neurohormone that stimulates the re-lease of pituitary gonadotropins in fish (Peter 1983; Millar 2005; Kah et al. 2007). It is the pri-mary hypothalamic factor controlling reproduction, and multiple forms of GnRH have been identified in fish. Evidence that luteinizing hormone-releasing hormone analog (LHRHa, a synthetic GnRH agonist) stimulates spawning in walleyes (see Section 6.4.3) indicates that stimulatory hypothalamic control is important in this species, although no investigations to date have identified the specific GnRHs of walleye or sauger. The stimulatory actions of GnRH may be opposed by the potent inhibitory actions of dopamine, particularly during the

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final stages of gametogenesis (Dufour et al. 2010). The dual neuroendocrine control of repro-duction by GnRH and dopamine has been demonstrated in many, but not all, adult teleosts. Evidence that the dopamine antagonist, pimozide, accelerates oocyte development in wall-eyes is consistent with the hypothesis that dopamine inhibits gonadotropin (gonadotrophic hormone, GtH) secretion in walleyes (Pankhurst et al. 1986).

Recent evidence suggests the brain factor, Kisspeptin, may be an important regulator of GnRH and consequently gondotropin secretion in fish (Zohar et al. 2010) and is thought to integrate environmental and metabolic signals and pass this information onto the reproductive axis. Kisspeptin neurons may emerge as the mediators of puberty and reproduction in fish, as in mammals (Oakley et al. 2009); however, this system has yet to be studied in walleye or sauger.

6.2.2 Pituitary Gland

Axons of GnRH neurons from the hypothalamus directly innervate gonadotropin cells in the adenohypophysis region of the teleost pituitary gland. The gonadotropin cells are re-sponsible for the synthesis, storage, and release of the two key gonadotropins in most fish, follicle-stimulating hormone (FSH) that mediates early gonadal growth and development, and luteinizing hormone (LH) that mediates gamete maturation and spawning (Levavi-Sivan et al. 2010). The pituitary also produces other hormones that may be involved in reproduction, including growth hormone (GH), prolactin (PRL), adrenocorticotrophic hormone (ACTH, or adrenocorticotropin), melanocyte-stimulating hormone (MSH), and thyroid-stimulating hormone (TSH) (Ball and Baker 1969) (common abbreviations included for reader). To our knowledge, no walleye pituitary hormone has yet been purified and characterized. Exogenous hormones and artificial analogs are used to induce gamete maturation and spawning in wall-eyes (see Section 6.4.3 and Chapter 13) and support the hypothesis that the reproductive pi-tuitary hormones of walleye are similar in form and function to those of other teleostean fish species (e.g., salmonids, ictalurids).

6.2.3 Gonads

The gonads (ovaries and testes) of bony fishes act as endocrine glands and consist of differ-ent cell types that have specific roles in reproduction, including the synthesis of sex steroids.

6.2.3.1 Testes

The testes of teleosts contain three primary cell types: (1) the Leydig cells, which are the primary steroidogenic cells; (2) the Sertoli cells, which support and sustain developing germ cells and may also play a role in steroid synthesis; and (3) the germ cells themselves (spermatozoa and their precursor cells). The testes of fish produce three primary steroids that regulate spermatogenesis. These are testosterone (T), 11-ketotestosterone (11-KT), and 17α,20β-dihydroxy-4-pregnen-3-one (17,20-P). All three of these steroids have been charac-terized in walleyes, and their pattern of change during reproductive maturation and spawn-ing suggests they regulate the processes of early sperm development, differentiation, and spermiation (final maturation), respectively (Pankhurst et al. 1986; Malison et al. 1994a; Barry et al. 1995). The steroid 11-KT is a male-specific hormone, and measuring its levels in the blood can be used as a method to differentiate between the sexes. Teletchea et al. (2009)

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reported a 95% success rate using 11-KT levels to sex zanders (European pikeperch, a close relative of the walleye).

In many fish species, steroid hormones are conjugated into water-soluble metabolites (e.g., glucuronides and sulfates) for removal from circulation (Fostier et al. 1983; Kime 1993) and these steroid conjugates may also have a role as pheromones. Pheromones probably ex-plain the observation that male walleyes held upstream from females can significantly affect female reproductive physiology (Barry et al. 1995).

6.2.3.2 Ovary

Three major cell types of the teleost ovary are (1) granulosa cells, (2) thecal cells, and (3) germ cells (oocytes). Granulosa and thecal cells combine to make up the ovarian follicle and interact to produce the two major female sex steroids, estradiol-17β (E2) and 17,20-P (Naga-hama 1983), which regulate oocyte growth and development (vitellogenesis), and final oocyte maturation, respectively (Pankhurst et al. 1986; Malison et al. 1994a; Barry et al. 1995). Testosterone, also found in female fish including walleyes, is a precursor to E2 production, but may also have other functions. Plasma T content in female walleyes can be higher than E2 near final maturation (Pankhurst et al. 1986; Malison et al. 1994a; Barry et al. 1995). Evi-dence for the role of 17,20-P in regulation of final oocyte maturation in walleyes is presented in Section 6.4.2.

6.2.4 Other Endocrine Systems

6.2.4.1 Interrenal Tissue (Adrenal Homolog)

The corticosteroid hormone, cortisol, is produced by the interrenal tissue, or adrenal homolog, of fish and mediates the organism’s adaptation to stress (see Section 6.7 for de-tails). In some fish species, corticosteroids can have positive effects on gametes and act as a maturation-inducing steroid (MIS). In general, however, corticosteroids have negative effects on reproduction in most fishes (Milla et al. 2009). Barry et al. (1995) provided evidence to suggest that final gamete maturation in walleyes can be inhibited by stress and cortisol. Cap-tive walleyes held under highly stressful conditions (blood plasma or serum cortisol levels of 50–100 ng/mL, see also Section 6.7) did not spawn but could be manipulated to spawn with hormone injections. The stress imposed by these conditions may operate at the level of the brain or pituitary, as in vitro evidence revealed that cortisol had no direct effect on final oocyte maturation in walleyes (Barry et al. 1995). DiStefano et al. (1997) evaluated the effects on reproductive function of walleyes subjected to stressors associated with changing water flow rates and temperatures in the tailwaters of a major Missouri River dam and found physiologi-cal evidence that the walleyes in the tailwaters were in a state of chronic physiological stress, which was suggested as a possible explanation for observed reproductive failures of walleye populations downstream from the dam.

6.2.4.2 Pineal Gland

The pineal gland is a major source of melatonin, a hormone that plays a key role in trans-ducing the photoperiod signals that regulate seasonal reproductive cycles in fish (reviewed by

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Kulczykowska 2001). Presumably, melatonin has that function in Sander but, to our knowl-edge, there is currently no published research on the role of the pineal gland in reproduction in walleye or sauger.

6.3 REPRODUCTIVE DEVELOPMENT

The current state of knowledge on the environmental and hormonal regulation of repro-ductive development and spawning in walleyes is limited to data available on the endocrine regulation of sexual development, oocyte maturation, and spawning in this species reviewed by Malison and Held (1996). No attempt has been made to summarize the data on repro-ductive physiology of the closely related zander although some references to this species are made here to illustrate general principals. Further details on the reproductive biology of zander can be found in the review by Lappalainen et al. (2003).

6.3.1 Fish Age and Size

Sexual maturity in walleye and sauger is dependent on both the size and age of the fish (Scott and Crossman 1973). Males mature earlier (e.g., age 2–4) and at a smaller size (e.g., 280 mm total length [TL]) than do females (e.g., age 3–6, 360 mm TL), and males also attain a smaller overall size than females (see also Chapter 7). Rennie et al. (2008) demonstrated lower food consumption and food conversion efficiencies in male walleyes than in females, and sex steroids may cause these differences. The female-specific hormone, E2, stimulated food consumption in percids (i.e., yellow perch and walleye), whereas male hormones (i.e., androgens such as T and 11-KT) can inhibit food consumption and increase metabolism in these species (Malison et al. 1985).

6.3.2 Environmental Effects

The reproductive cycles of walleye and sauger are controlled by seasonal changes in photoperiod and water temperature (see also Chapters 5 and 7). In the walleye’s most south-ern range (~35°N latitude), walleyes spawn as early as February, and in the most northern range (~55°N latitude) the spawning season can go until mid-July (Hokanson 1977; Malison and Held 1996). Water temperatures less than 10°C generally are required for successful spawning in most stocks (e.g., 6–9°C, Koenst and Smith 1976), although there is consider-able variation among populations (see details in Chapter 5). For example, in the north-central United States (e.g., Wisconsin, Minnesota), walleyes begin to spawn when water tempera-tures are from 3.3°C to 6.7°C, and spawning activity peaks when temperatures are between 5.6°C and 10°C (Becker 1983). In Canada, walleyes spawn when water temperatures range from 6°C to 11°C, and spawning activity peaks when temperatures are 7–9°C (McPhail and Lindsey 1970; Scott and Crossman 1973; Nelson and Paetz 1992). Spawning usually lasts for 1–2 weeks (see Chapter 7 for further ecological details). Saugers begin spawning in Canada at water temperatures of about 4–6°C (Scott and Crossman 1973). By comparison, natural spawning of zander in Europe is reported to occur at temperatures of 8–22°C (Mann 1996) or 8–16°C (Szkudlarek et al. 2007). Similarly, Volga pikeperch reportedly spawn at 10–22°C (Freyhof and Kottelat 2008).


6.3.3 Gonad Development

6.3.3.1 Males

Just after spawning in early spring, male walleyes have a low gonadosomatic index (GSI, defined as gonad weight/total body weight × 100), with only a few scattered lobules of sper-matogonia present in the testes (Malison et al. 1994a). Testosterone and 11-KT levels are low (~0.25 and 5 ng/mL, respectively). In summer, more spermatogonia begin to appear. In early autumn, there is a marked rise in GSI coincident with an increase in spermatogenic activity (Figure 6.1). By mid-autumn, approximately 50% of the germ cells are mature spermatozoa. Testosterone and 11-KT levels are approximately 1 and 5 ng/mL, respectively. By midwinter, almost all of the germ cells in the testes are spermatozoa, and semen can readily be expressed from the fish by applying pressure to the abdomen (Malison et al. 1994a). By early spring, T and 11-KT levels increase to about 2 and 30 ng/mL, respectively, although the physical appearance of the testes is little changed compared with that in midwinter (Malison et al. 1994a). After the release of spermatozoa following spawning, GSI, T, and 11-KT all sharply decline, and, anatomically, the gonads consist primarily of nongerminal tissue. Photographs and figures showing these changes are presented in Malison and Held (1996).

6.3.3.2 Females

Female walleyes have group-synchronous ovarian development. Two populations of oo-cytes are distinguishable at any time during the annual reproductive cycle, a heterogeneous group of nonvitellogenic oocytes that are less than 300 μm in diameter, and a group of larger, synchronously developing, vitellogenic oocytes that are greater than 1 mm in diameter and that are destined to be spawned in the upcoming spring (Malison et al. 1994a).

Figure 6.1. Seasonal changes in the gonadosomatic index of male (○) and female (■) walleyes captured from Minnesota lakes and rivers. Each point represents the mean ± SE of 2–17 indi-viduals for each sampling time (from Malison et al. 1994a).

0

5

10

15

GO

NA

DO

SO

MA

TIC

IND

EX

(%

)

J A S O N D J F M A M J

MONTH

FEMALE

MALE

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Malison et al. (1994a) documented gonadal development and sex steroid levels during the annual reproductive cycle of female walleyes. Just after spawning in early spring, female walleyes have low GSIs, with ovaries composed primarily of oogonia and nonvitellogenic oocytes. Serum levels of E2, the ovarian steroid that regulates yolk (vitellogenin) production, are less than 1 ng/mL. By late spring, the ovaries are filled with larger previtellogenic oocytes. By midsummer, yolk-filled vesicles appear in the oocytes. By autumn, large (>1 mm in diam-eter), yolk-filled oocytes are present in the ovary, and E2 levels are at their highest annual con-centration (>3 ng/mL). Oocyte diameter and GSI (Figure 6.1) continue to increase until late January, at which time the oocytes appear almost fully mature. Estradiol-17β levels steadily decline throughout the winter and early spring to less than 1 ng/mL just before spawning. Photographs and figures showing these changes are presented in Malison and Held (1996).

6.4 SPAWNING AND FERTILIZATION

6.4.1 Fecundity

The number of spawned eggs is proportional to body size, and in walleyes the average is about 60,000 eggs/kg body weight (Nickum 1986; see also Chapters 7 and 13). However, egg number can range from 28,000 to 120,000 eggs/kg body weight (Smith 1941; Wolfert 1969; see also Chapter 7). Individual females normally ovulate and release all of their eggs in one night (Ellis and Giles 1965). O’Donnell (1938) observed that 200–300 eggs were released during multiple spawning acts that were repeated at 5-min intervals.

6.4.2 Final Oocyte Maturation

Shortly before spawning, walleye oocytes undergo final maturation and ovulation. During the final maturation of fish oocytes, the oil droplets coalesce and the egg nucleus, or germinal vesicle (GV), migrates from the center of the oocyte to the periphery and then “breaks down” (i.e., the nuclear membrane undergoes dissolution preparatory to ovulation). The process of germinal vesicle breakdown (GVBD) is easily observed in walleye oocytes after they are cleared using a suitable “clearing” solution (e.g., ethanol, formalin, and glacial acetic acid; 6:3:1, v/v). Ovulation (expulsion of the egg from the surrounding follicle layer) occurs within several hours or days after GVBD. See Barry et al. (1995) for photographs of walleye oocytes undergoing GVBD.

The steroid hormone, 17,20-P, is probably the oocyte maturation-inducing steroid (MIS) in walleye and sauger, as it is in many other teleosts (Scott et al. 2010). Pankhurst et al. (1986) and Barry et al. (1995) measured significantly elevated serum levels of 17,20-P coincident with final oocyte maturation in walleyes. The peak levels of 17,20-P (1.5–2.5 ng/mL), how-ever, were 10–100-fold lower than those measured in some salmonid and cyprinid species (Scott and Canario 1987; Scott et al. 2010), suggesting that percids may require only low levels of 17,20-P to induce GVBD. Alternatively, 17,20-P may be rapidly conjugated and removed from the blood, or another steroid besides 17,20-P could be the MIS in walleyes. Regarding the latter, the steroid, 17α,20β,21-trihydroxy-4-pregnen-3-one (20β-S) is the MIS in some percid species (Thomas and Trant 1989; Sorensen et al. 2004), and was as effective as 17,20-P at inducing GVBD in walleyes.


6.4.3 Artificial Induction of Spawning

Injections of human chorionic gonadotropin (hCG), luteinizing hormone-releasing hor-mone (LHRH), carp pituitary extract, and some maturational steroids have all been used to successfully induce final oocyte maturation in walleyes captured during the spawning sea-son (Nelson et al. 1965; Lessman 1978; Hearn 1980; Pankhurst et al. 1986; Heidinger et al. 1990; Barry et al. 1995). Timing of injections appears to be important for effective results. Single intramuscular injections of hCG (500 IU/kg) and synthetic LHRHa (i.e., des-Gly10 [D-Ala6] LHRH-ethylamide, 100 mg/kg) both stimulated final oocyte maturation and ovulation in walleyes treated 2 weeks before the normal spawning period, but the synthetic LHRHa induced oocyte maturation faster than did hCG (Barry et al. 1995). In another experiment conducted 3 weeks before normal spawning, hCG (500 IU/kg), synthetic LHRHa (100 mg/kg), and 17,20-P (100 mg/kg) all induced final oocyte maturation, but in this case hCG was more effective than LHRHa (Barry et al. 1995).

6.4.4 Out-of-Season Spawning

Malison et al. (1994b) manipulated environmental cues (water temperature increase from 2°C to 10°C over 1 week) and hCG injections (150 IU/kg on day 0 and 500 IU/kg on day 2) to successfully advance spawning in walleyes by several months. More than 80% of the fe-males treated with hCG spawned 6–8 d after injection. Hormone injections were not needed to induce spermiation in males. Eggs collected from the hCG-injected fish were successfully fertilized, incubated, and hatched, and the survival rates of these fry were comparable with those produced during the normal spawning season (Dabrowski et al. 2000). Similar methods have been used to induce spawning out of season in zanders (Schlumberger and Proteau 1996; Demska-Zakęś and Zakęś 2002; Zakęś and Szczepkowski 2004; Rónyai 2007; Zakęś 2007; Mueller-Belecki and Zienert 2008) and were reviewed by Zakęś and Demska-Zakęś (2009).

6.5 TEMPERATURE AND OXYGEN REQUIREMENTS

6.5.1 Temperature

Temperature is the most important factor controlling the rate of metabolism in fish. For fish generally, a 10°C increase in temperature results in a two- to threefold increase in meta-bolic rate (i.e., Q

10 = 2–3). Many factors affect how fish respond to changes in temperature;

important among these are life stage, acclimation history, and digestive state (Elliot 1981), as well as duration of exposure. In a classic paper, Fry (1947) outlined the concept of scope for activity, defined as the difference between the maximum energy output possible (active me-tabolism) and the minimum required to survive (standard metabolism) at a given temperature. In simple terms, scope for activity represents the energy available to the fish for all other func-tions (e.g., routine activity, disease resistance, predator avoidance, growth and reproduction) after basic metabolic needs are satisfied. For any given species, a temperature range exists within which the scope for activity is greatest, or wherein the fish is most efficient physi-ologically, and which is often reflected in their behaviorally selected preferred temperature, or preferenda.

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Walleye and sauger are classified as temperate mesotherms (Hokanson 1977). General temperature requirements for walleye, sauger, and, for comparison, zander are summarized in Table 6.1, much of which is based on the thorough review of percid temperature requirements by Hokanson (1977; but see additional details on temperature requirements in Chapters 5, 7, and 13). Walleyes prefer temperatures around 20–23°C; although they can be found in nature from 0°C to 30°C (Ferguson 1958; Christie and Regier 1988), they may avoid temperatures higher than 24°C (Fitz and Holbrook 1978). More precisely, their final preferenda range from 20.6°C to 23.2°C (data cited in Coutant 1977 and Hokanson 1977), which is consistent with the physiological optimum temperature of 22.6°C reported in the review of Hokanson (1977; see also Casselman 2002). The physiological optimum determined for saugers was slightly less at 22.0°C (Hokanson 1977).

Optimum temperatures for walleye growth range from 20°C to 26°C, depending on source (various authors in McMahon et al. 1984; Hokanson and Koenst 1986). In a review of walleye attributes for aquaculture, Summerfelt (2005; see also Chapter 13) concluded that there was no clear agreement on optimum temperature for growth, citing examples that ranged from 22°C to 26°C. Past reports indicate the optimum growth of juveniles occurs between 19°C and 25°C, notably at 22°C (various authors in Colby et al. 1979). Nickum (1986) reported that 20°C is generally satisfactory for fingerling rearing in intensive culture. However, Sum-merfelt and Summerfelt (1996) reviewed growth rates and temperatures for juvenile walleyes raised in intensive culture from a number of previous studies and identified a narrower range for optimum growth of 23–24°C. Cai and Summerfelt (1992) determined the physiological optimum for juvenile metabolism as 25.3°C (under light intensity of 45 lx). Metabolic re-quirements for maintenance increase rapidly from 20°C down to 12°C and at the low end of the temperature range, adult growth ceases at temperatures less than 12°C (Kelso 1972).

The upper lethal temperature for juvenile walleye is 31.6°C (Hokanson 1977), although Wrenn and Forsythe (1978) reported higher lethal temperatures for adults. Wrenn and Forsythe (1978) found that maximum temperatures in which adult walleyes continued to grow was about 32–33°C when held consistently at temperatures 4°C above a natural ambient that increased seasonally (March–September) from 10°C to about 28–29°C. Upper lethal temperatures for walleye fry are 31–33°C (Smith and Koenst 1975; Wrenn and Forsythe 1978). Hokanson (1977) provided an upper incipient lethal temperature for juvenile sauger of 30.4°C. By comparison, the upper incipient lethal temperature for juvenile zander is 34.3–35.0°C (two separate studies cited in Hokanson 1977) and their physiological optimum is 27.3°C (Hokanson 1977). The up-per tolerance limit for hybrid walleye, or saugeye, has not been determined, but in the develop-ment of a bioenergetics model for saugeye, Zweifel et al. (2010) determined experimentally the optimum temperature for respiration of 28°C, and, based on this value, established an “artificial” upper tolerance temperature at 7°C above the optimum, i.e., 35°C, for use in their model.

Walleye spawn once each year in early spring, when water temperatures warm to 5°C or higher, depending on geographic location (Scott and Crossman 1973; Becker 1983; see Section 6.3.2 and Chapters 5, 7, and 13 for further details). Similarly, saugers are reported to spawn when temperatures reach 3.9–6.1°C (Scott and Crossman 1973). Steady warming rates (e.g., >0.2°C/d) during egg incubation are required for good survival (Busch et al. 1975). Egg mortality, however, can be high (>90%) when temperatures are above 21°C (data cited in Colby et al. 1979). Hokanson (1977) found that lower and upper tolerance limits of em-bryonic walleyes were less than 6°C and 19.2°C, respectively. Similarly, Smith and Koenst (1975) indicated that upper lethal temperatures for walleye embryos are near 19°C.


Table 6.1. Summary of temperature preferences and limits of walleye, sauger, and zander de-scribed in text. See details on geographic variation of temperature preferences in Chapter 5.

Preferred range Physiological Growth (°C) Upper lethal Reference(°C) optimum (°C) limit (°C) Walleye 15–2a Piper et al. (1982)20–23 Ferguson (1958); Christie and Regier (1988)20.6– 23.2 In Coutant (1977) and Hokanson (1977) 22.6 In Hokanson (1977); Casselman (2002) 25.3b Cai and Summerfelt (1992) 20–26 McMahon et al. (1984); Hokanson and Koenst (1986) 19–25b Various authors in Colby et al. (1979) 20b Nickum (1986) 23–24c Summerfelt and Summerfelt (1996) >12d Kelso (1972) 31.6e In Hokanson (1977) 32–33 Wrenn and Forsythe (1978) 31–33 Smith and Koenst (1975); Wrenn and Forsythe (1978) Sauger 18.6–19.2 (lake) 22.0 22.0 30.4e In Hokanson (1977)22–28 (stream) Zander 24–29 27.3 28–30 34.3–35.0e In Hokanson (1977)

a Fryb Juveniles (including fingerlings)c Advanced juvenilesd Lower limit below which growth ceasese Incipient lethal

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At temperatures of 8°C up to 15°C, walleye eggs hatch in 21–14 d (Ney 1978), or 168 and 210 temperature units (TU), respectively. Koenst and Smith (1976) found that it took 3 weeks at 10°C and 1 week at 20°C for walleye eggs to hatch, or 210 and 140 TU, respectively. Similarly, incubation at constant hatchery temperatures has resulted in hatching times of 4 d at 23.9°C (96 TU) to 33 d at 4.5°C (149 TU) (data presented in Colby et al. 1979). However, in rapidly fluctuating temperature regimes, the TU requirement will be considerably higher (Piper et al. 1982). Hatching in sauger takes 25–29 d at temperatures ranging from 4.5°C to 12°C (Scott and Crossman 1973). See Chapter 13 for more complete details of time–temper-ature relationships for hatching.

Climate change will probably have a profound effect on the distribution and sustainability of coolwater fishes such as walleye and sauger (Casselman 2002; Shuter et al. 2002; see Chap-ter 9). For example, using Hokanson’s (1977) estimate of 22°C as the physiological optimum, Shuter et al. (2002) predicted that walleye yield in Ontario could increase as lakes become warmer. This potential yield, however, would be offset by other factors associated with cli-mate change. An increasing temperature trend would result in an overall drop in lake levels through evaporation and a decline in dissolved organic carbon input, resulting in a predicted decrease in sustainable harvest of walleyes (Shuter et al. 2002). In the Great Lakes basin, Casselman (2002) predicted that continued warming will have a negative curvilinear effect on coolwater fishes in general, speculating that with a 1°C increase, recruitment of coolwater fishes would decrease by a factor of about 2.7 times. Recent empirical evidence indicates that walleye spawning in Minnesota is already occurring earlier in the spring as a result of earlier ice-out related to warmer temperatures at that time, possibly as a result of climate change (Schneider et al. 2010).

6.5.2 Dissolved Oxygen

Early field studies indicated that the greatest abundance of adult walleyes occurs where minimum dissolved oxygen (DO) levels do not fall below 3–5 mg/L (Dendy 1948). In a re-view of DO requirements for walleye and other fishes, Barton and Taylor (1996) indicated that, in general, embryos and larvae thrived at DO concentrations at 5 mg/L and higher, but adults were capable of tolerating DO levels as low as 2 mg/L, at least in the short term (Table 6.2). Hoff and Chittenden (1969) reported an acute lethal concentration for adult walleyes of 1.2 mg/L (range, 1.1–1.6 mg/L) at 24°C. Colby et al.’s (1979) general synopsis indicated that 2 mg/L may be tolerated by walleye adults in the laboratory, but no experimental work was cited. Scherer (1971) found adult walleyes could tolerate 2 mg/L DO for a short period, but behavioral disruption and eventual loss of equilibrium occurred at DO levels less than 1.5 mg/L. By comparison, Müller et al. (2006) determined the acute lethal DO concentration for fingerling Volga pikeperch was 1–1.3 mg/L, whereas that of zander and of the hybrid cross was 0.8–0.9 mg/L, although 4.5 mg/L has been cited as the lower limit of the optimal DO range for adult zanders (Kuznetzova 1955, cited in Marshall 1977).

However, behavioral responses have been evident when DO was less than 6 mg/L (Scher-er 1971). At less than 6 mg/L, walleyes displayed increasing mobility and spent less time re-maining under shelter as DO levels progressively declined. At very low DO levels (1–2 mg/L), the fish remained outside of the shelter, which corresponds to a loss of avoidance response to light (Scherer 1971). By observing the decline of opercular movement, Gee et al. (1987) estimated the incipient lethal level, expressed as partial oxygen pressure (PO

2), for walleyes


as 32.8 torr (approximately 2.3 mg/L at 10°C). Unlike many temperate freshwater fishes, however, walleyes do not appear to employ aquatic surface respiration when DO reaches criti-cally low concentrations (Gee et al. 1987). Perturbation of normal behavior patterns at low DO levels could seriously impair feeding performance of walleyes as these fish feed at night or early morning when DO levels would be at their daily minimum.

Survival of the early life stages is seen as a key in self-sustaining walleye populations (Noble 1972; Scott and Crossman 1973). These early stages include the period when the newly hatched larvae leave their spawning area and disperse throughout the lake or river (Scott and Crossman 1973). This period is relatively short as the yolk is absorbed quickly (Nickum 1986; see Chapter 13). Respiration at the larval stage is typically through the skin, which constitutes more than 99.9% of the total respiratory surface (skin plus gills) of walleyes at the time of hatch, as gills are poorly developed; this proportion declines nonlinearly as gills develop and reaches less than 50% when larvae are approximately 0.7 g in size (Rombough and Moroz 1997). Generally, in nonsalmonid fishes newly hatched larvae are considerably more sensitive to low DO levels than are older fish (Doudoroff and Shumway 1970). Walleye larvae, in particular, are too small to fight currents (Walburg 1972) and, thus, cannot actively avoid episodes of low DO.

More data are available on DO requirements of young walleyes (Table 6.2). From experi-mental evidence, it appears that concentrations much below 3.5 mg/L are likely to lead to re-duced survival of embryos and young larvae. For example, larval survival to 20 d at 17°C was 42, 39, 15, and 0%, at DO saturations of 100, 50, 35, and less than 25%, respectively (Siefert

Table 6.2. Summary of low dissolved oxygen (DO) effects on walleye adults, embryos, and larvae (from Barton and Taylor 1996).

Life stage DO level Effect Reference (mg/L) Embryo 5–6 Optimum for incubation, 12–13°C Oseid and Smith <5 Reduced hatching success (1971) 3 Severely reduced hatching success; 10–13% shorter larvae than at 7 mg/L Embryo–larva 5.0 95% survival after 20 d Siefert and Spoor 5.3 No mortality after 1 d (1974) 3.4 40% survival <3 1-h exposure, >25% mortality 2.5 No survivors Adult (age 1+) 1.5–2 Loss of light-avoidance behavior Scherer (1971) <1.5 Loss of equilibrium (probably fatal) Adult 2 Tolerance limit in laboratory Colby et al. (1979) Adult 1.1–1.6 Mortality Hoff and Chittenden (1969)

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and Spoor 1974); these saturation values (at 0 m altitude) are equivalent to about 9.7, 4.9, 3.4, and less than 2.4 mg/L. Similarly, DO concentrations below 3.4 mg/L can delay hatching as well as reduce embryo size at hatching (Colby and Smith 1967; Siefert and Spoor 1974; see Table 6.2). In experiments incubating walleye eggs at various DO levels at temperatures of either 12–13°C or 4–5°C, Oseid and Smith (1971) found that walleye eggs incubated at DO levels at the low end of a range from 2 to 7 mg/L required 1–4 d longer to hatch than did those at the high end and recommended optimal DO levels for walleye egg incubation of 5–6 mg/L. In comparison, a DO concentration of 4.5 mg/L was reported to approach the lethal limit for embryos and fry of zander (Kuznetzova 1955, cited in Marshall 1977).

Other environmental requirements of walleyes and saugers, including water quality needs such as appropriate light levels, turbidity, and pH, are not discussed here as they are covered in detail in Chapters 5, 7, and 13 in the contexts of habitat requirements, natural populations, and aquaculture, respectively.

6.6 METABOLISM AND SWIMMING

6.6.1 Metabolic Rate

Despite their popularity in North American fisheries, a dearth of published information exists about walleye or sauger on their metabolism and metabolic rates. Cai and Summerfelt (1992) examined oxygen consumption of walleyes (75–251 mm) in tanks to determine their mass-specific metabolic rate at two different temperatures. At 20°C, mean weight-adjusted oxygen consumption of walleyes was 231 mg O

2·kg–1·h–1, whereas at 25°C oxygen consump-

tion increased by 20% to 277 mg O2·kg–1·h–1. Cai and Summerfelt (1992) demonstrated that

metabolic rates, expressed as oxygen consumption per unit weight, decreased with increasing fish size at both temperatures. Similarly, ammonia (NH

3) excretion was also inversely propor-

tional to fish size in a linear fashion. The mean metabolic rate of 76 mg O2·kg–1·h–1 in walleyes

fasted for 153 h was reduced by 56% compared with that of 172 mg O2·kg–1·h–1 in fed fish

(Cai and Summerfelt 1992). Similar to other temperate fishes, small walleye larvae appear to have a high rate of metabolism compared with their larger juvenile and adult counterparts; rate of oxygen uptake in newly hatched larvae has been estimated at approximately 1,200 μg O

2·g–1·h–1 (I. Manns, unpublished, cited in Rombough and Moroz 1997). However, this rate is

relative as larval walleyes have a much greater respiratory surface relative to body mass than do juveniles and adults (Rombough and Moroz 1997).

6.6.2 Swimming Capacity

Stamina and performance capacity has been assessed in walleye associated with swim-ming. Critical swimming speed (Ucrit

x) reflects the highest swimming speed the fish can

sustain for x minutes and has often been applied in the context of fish passage through fish-ways or culverts, particularly for larval or juvenile fishes. Houde (1969) measured swimming performance in larval walleyes of various sizes and observed that most larvae less than 12 mm TL drifted with the current when water velocity exceeded 5 cm/s despite the positive rheotaxis exhibited by the larvae. Some larvae greater than 8 mm, however, were able to swim against the current. As larvae increase in size, their swimming ability increases; for larvae between


7.0 and 9.5 mm, this ability increases exponentially (Houde 1969). From 1-h sustained swim-ming tests, Houde (1969) concluded that larval walleyes of 20 mm TL can sustain current velocities of 7.5 cm/s at 13°C. Jones et al. (1974) observed walleyes (4–500 g) to be capable of sustained swimming speeds of 2.2–4.7 body lengths (BL)/s.

Tarby (1981) compared metabolic rates of juvenile and adult walleyes (72–674 g) at rest (standard) and while swimming at 1 BL/s. Unlike Jones et al. (1974), Tarby (1981) found that walleyes would not swim beyond 1 BL/s, and individuals greater than 350 mm would not swim at that speed. Tarby (1981) suggested a possible behavioral inhibition in the larger walleyes to swim in the smaller diameter swim tunnel used compared with the larger swim-ming tube used by Jones et al. (1974). Using multiple regression analysis, Tarby (1981) de-termined that oxygen consumption (Q) was highly correlated with both fish weight (W) and water temperature (T) for both standard metabolism (log Q = –1.663 + 0.908 log W + 0.037T, R2 = 0.983) and that for swimming (log Q = –1.269 + 0.974 log W + 0.025T, R2 = 0.979). Regardless of swimming speed (0–45 cm/s), Beamish (1990a) showed oxygen consumption of juvenile walleyes (5–7 g) followed a similar pattern, increasing from 5°C to 15°C, but decreasing at temperatures above 15°C. At the highest temperature of 23.5°C, walleyes were unable to swim for the entire 1-h test period faster than 35 cm/s, but were able to swim at all test velocities for the full hour at the other temperatures. Beamish (1990a) calculated the metabolic cost of swimming for juvenile walleyes to range from 111 to 663 mg O

2·kg–1·h–1 for

swimming speeds from 20 to 45 cm/s, respectively. Extending these findings further, Beamish (1990a) calculated that the energy required for a walleye to swim 1 km would range from 2.14 to 5.68 J/g at speeds ranging from 20 to 45 cm/s, respectively. When walleyes were challenged under different photoperiod regimes to test for seasonality effects, no differences in oxygen consumption while swimming were evident relative to natural photoperiod compared with those tested under a constant photoperiod (Beamish 1990b).

Peake et al. (2000) assessed swimming performance in wild walleyes (180–670 mm fork length [FL]) at 6, 12, and 20°C and found that Ucrit

60 (sustained swimming, 60 min) ranged

from 0.30 to 0.73 m/s, whereas Ucrit10

(prolonged swimming, 10 min) ranged from 0.43 to 1.14 m/s. In both cases, Ucrit increased significantly with both fish length and water tempera-ture. In comparison with other temperate species, Peake et al. (2000) concluded that walleyes have a relatively narrow scope for swimming activity, which may explain why walleyes (and possibly saugers) perform poorly when passing through fishways (Schwalme et al. 1985). A model that incorporates walleye length and water temperature was developed to estimate the maximum water velocity needed to pass walleyes through culverts (Peake et al. 2000). Peake et al. (2000) also found that walleyes (160–570 mm FL) were capable of higher swimming speeds (1.6–2.6 m/s) during short bursts of activity than the Ucrit values would indicate, but the fish may not be inclined behaviorally to do so in the wild, such as in a fishway. Factors other than water velocity are important, such as their behavioral motivation and willingness, in dictating whether walleyes will enter a culvert or fishway; however, Peake (2008) noted that once walleyes entered, most were capable of complete passage at least when velocities were below 120 cm/s. The results of Peake et al. (2000) support the earlier finding of Tarby (1981) who indicated that walleye swimming, particularly that of larger individuals, was in-hibited when water velocity exceeded 1 BL/s.

Castro-Santos (2005) developed a new model based on swim speed relative to fatigue time in an attempt to predict an optimum “ground” swimming speed (difference between swim speed and flow velocity) for maximizing distance traveled in either sprint (<20 s) or prolonged

212 Chapter 6

(20 s–200 min) swimming modes. As one would expect, time to fatigue (as loges) decreased as

swim speed (as BL/s) increased, but there was a clear separation between slope values (b) for sprint and prolonged swimming modes for walleyes (315–326 mm FL) and other nonclupeids tested; regressions for the modes for walleye were: (1) sprint (S): y

S = –0.13x

S + 4.02 (SEb

S ±

0.02); (2) prolonged (P): yP = –0.41x

P + 6.65 (SEb

P ± 0.05). Based on this model the predicted

optimal ground speed for walleye was 7.69 BL/s in sprint mode, and in prolonged swimming mode was 2.44 BL/s. Castro-Santos (2005) clearly pointed out, however, that, while one can predict the distance-maximizing behavior from the swim speed–fatigue time relationship, the nonclupeid fishes tested, including walleye, failed to switch to sprint mode to optimize their ground speed for distance traversed even though they had the physiological capacity to do so. During the flume trials Castro-Santos (2005) also noticed that most fish, including walleyes, generally tended to swim within 20 cm of the bottom of the flume and avoided the walls, stay-ing more than 20 cm away from either wall.

6.7 RESPONSES TO STRESS

6.7.1 Handling

Stress can be considered as a state of threatened homeostasis that is re-established by a complex suite of adaptive responses (Chrousos 1998), which have been grouped as primary (e.g., endocrine), secondary (e.g., metabolic), and tertiary (e.g., whole-animal performance) re-sponses (Wedemeyer et al. 1990; Barton and Iwama 1991; Barton 2002). One useful indicator of stress in fish is the elevation of circulating (plasma or serum) cortisol, a primary stress hormone, in response to a stimulus. Measurement of cortisol has become popular, in part, because of its sensitivity to both the severity and duration of the stressor (Barton 2002). Cortisol is synthesized and released into circulation by the interrenal tissue (adrenal homolog) in the head kidney of teleost fishes after stimulation by adrenocorticotropin (ACTH) from the pituitary. The pituitary is first stimulated by corticotropin-releasing hormone (CRH), or factor (CRF), which is released chiefly from the hypothalamus of the brain (Wendelaar Bonga 1997). This cascade of endocrine events from brain to interrenal tissue is referred to as the hypothalamic–pituitary–interrenal (HPI) axis and is controlled by negative feedback of circulating cortisol on the axis.

Compared with other fish groups, notably the salmonids, little work has examined the physiological responses of walleyes or saugers to stressors in the environment. Like most teleosts, walleyes exhibit typical physiological responses to stress, which are reflected by changes in characteristic indicators (e.g., circulating cortisol, glucose, lactate, and ion concen-trations; Wedemeyer et al. 1990). Judged by changes in plasma cortisol, walleyes appear to be particularly responsive to stress. In a comparison with a variety of salmonids and other fresh-water fishes subjected to an identical handling stressor under similar conditions, walleyes had the highest 1-h poststress levels of plasma cortisol (Barton 2002). When juvenile walleyes acclimated to 21°C were subjected to a 30-s handling stressor in a net with aerial emersion, plasma cortisol increased more than 20-fold from 11 to 286 ng/mL within 15 min and blood lymphocyte numbers declined by 67% in 3 h (Barton and Zitzow 1995). Poststress elevations in plasma cortisol were accompanied by a decline in plasma osmolality, which was amelio-rated by adding salt (0.5% sodium chloride [NaCl]) to the recovery water (Barton and Zitzow 1995). Similar to the results of Barton and Zitzow (1995), Barry et al. (2004) also found that


plasma cortisol increased rapidly to higher than 150 ng/mL in walleyes acclimated to 15°C, but increased more rapidly and to higher levels (ca. 200 ng/mL) when fish were acclimated to 21°C. Plasma glucose also increased approximately threefold from a resting value of 60 mg/dL in both groups. Walleyes from the 21°C-acclimated group demonstrated a greater glyce-mic response (190 mg/dL) than those acclimated to 15°C (165 mg/dL) (Barry et al. 2004).

Walleyes also exhibited a diurnal rhythm in plasma cortisol, ranging from less than 5 ng/mL during the day to greater than 36 ng/mL at midnight (Barry et al. 2004). Barry et al. (2004) suggested that changes in plasma cortisol reflected the diurnal activity pattern of the fish, in-dicating that a nighttime increase in cortisol is concurrent with its nocturnal feeding behavior. This finding contrasts with an apparent resting plasma cortisol concentration of about 2 ng/mL measured in juvenile walleyes captured in their natural habitat (i.e., vegetated shallows in a prairie pond) late at night by means of electrofishing and sampled immediately (Barton et al. 2003). The difference between these two findings for nighttime cortisol concentrations is not known, but may relate to different environments from which the walleyes were sampled—one being artificial tanks with flow-through water and the other being their natural environment.

No studies have been conducted on the stress response of saugers. However, Barry et al. (2004) compared plasma cortisol, glucose, and chloride (as a blood ion indicator) responses of female walleye × male sauger hybrids to an acute handling stressor with purebred wall-eyes and found no differences between the hybrid and the parent strain. Comparatively little work has been done examining cortisol stress responses in zanders, but Fatemeh et al. (2008) reported that a 30-s aerial emersion increased plasma cortisol levels in zanders to 59 ng/mL, which is much less than that reported by Barton and Zitzow (1995) in walleyes subjected to a similar stressor. Fatemeh et al. (2008) observed that prestress cortisol titers in zanders were relatively high (41 ng/mL), which suggests that feedback of these levels on the HPI axis may have suppressed additional cortisol output when the fish were handled. In another study, however, Brown et al. (2001) observed cortisol levels above 150 ng/mL in zanders exposed to 33‰ seawater, a presumably milder stressor than aerial emersion, even though preexposure concentrations were around 50 ng/mL.

Responses of other physiological variables (e.g., blood glucose, lactate, chloride) of wall-eye, sauger, and zander are within ranges of other temperate freshwater teleosts (e.g., glucose, from 50–150 mg/dL prestress to 100–250 mg/dL poststress; lactate, from 20–30 to 40–80 mg/dL; chloride, approximately 10% decline from 100–130 meq/L; osmolality, approximately 10% decline from 290–320 mOsmol/kg; see others in Barton et al. 2002), although plasma glucose levels approached 300 mg/dL in zanders exposed to seawater (Brown et al. 2001), which is comparatively high.

6.7.2 Transport

Recent studies examining stress responses in walleyes were done in the context of stress associated with transport to stocking sites. Barton et al. (2003) assessed juvenile walleye re-sponses to capture, loading, transport, and stocking procedures using the physiological stress indicators, plasma cortisol and chloride. Typically plasma cortisol rose above 150 ng/mL by 1–3 h after transport in one trial and above 200 ng/mL in a second trial; plasma chloride dropped 28–30% from prestress levels and did not recover after 24 h. Results from both sets of trials suggested a cumulative stress effect from various individual components of the cap-ture–loading–transport operation, with the most stressful component of the operation being

214 Chapter 6

the handling associated with loading the fish into the truck tanks (Barton et al. 2003). Freeze-branding walleye fingerlings did not appear to impose additional stress on the fish beyond that already caused by processes of capture, loading, and transport (Barton and Haukenes 1999). Although cortisol levels recovered to prestress values within 24 h, plasma chloride levels did not, suggesting that the fish had not fully recovered from the capture and branding process by the next day (Barton and Haukenes 1999).

An attempt to reduce the stress associated with transporting juvenile walleyes was made by Forsberg et al. (1999) by equipping transport tanks with ram-air ventilation (RAV) by means of air scoops. After 6 h of transport, plasma cortisol and glucose levels in walleyes from RAV-equipped tanks (47 ng/mL and 67 mg/dL, respectively) had recovered to preload values (41–48 ng/mL and 54–62 mg/dL), whereas those in walleyes from control tanks at 6 h (72 ng/mL and 91 mg/dL) were still significantly elevated (Forsberg et al. 1999). Just as importantly, blood pCO

2 and bicarbonate (HCO

3–) concentrations in the walleyes from RAV-

equipped tanks were significantly lower and closer to preload values than those from control tanks after 6 h of transport. Water quality in RAV-equipped tanks was also superior to the regular transport tanks, and the reduction in physiological stress in the transported walleyes was attributed to the ability of RAV to vent CO

2 from the airspace above the water, thus keep-

ing in-tank CO2 levels low (Forsberg et al. 1999).

Adding salt (e.g., 0.5–1% NaCl and other mineral salt formulations) to transport tanks is becoming a common practice in fish transportation (Wedemeyer 1996). Forsberg et al. (2001) compared the benefits of using buffered salt (0.45% NaCl + 0.05% sodium bicarbon-ate [NaHCO

3]) with salt (0.5% NaCl) in the transport media for hauling juvenile walleyes

as a means of reducing stress. The highest levels of cortisol measured in the walleyes after transport was 175–204 ng/mL, which is similar to findings of Barton and Zitzow (1995) and Barry et al. (2004). Buffered salt did not alter the physiological stress responses (circulating cortisol, sodium, and chloride) or blood acid–base status (pCO

2, HCO

3–) that occurred during

loading, transport, and stocking relative to those in the salt-only treatment (Forsberg et al. 2001). Similarly, other attempts to mitigate the stress associated with loading and transport in juvenile walleyes by reducing loading density and adding 0.5–0.7% AquaHaul®, a com-mercial salt-based formulation, to the transport medium did not appreciably alter their physi-ological responses (cortisol and chloride) to loading and transport procedures (Barton and Haukenes 1999).

Forsberg et al. (2001) noticed that juvenile walleyes were lethargic after being stocked as a result of the stress experienced from the transport and stocking procedure, and their recovery was slow (possibly up to a day). The stress-induced altered behavior is consis-tent with what Forsberg et al. (2001) observed in laboratory trials and also the reduced avoidance-response ability in juvenile walleyes subjected to acute handling documented by Barton and Haukenes (1999). In the latter study, the reduced capacity to respond to a nox-ious stimulus was observed in walleyes along with elevated levels of plasma cortisol and decreased plasma chloride concentrations, particularly in those with only 1 h of recovery time.

High-energy finishing diets have been suggested as a means of helping the fish to over-come adverse conditions after stocking, such as lack of food availability and presence of predators, during their posttransport recovery from stress. Czesny et al. (2003) subjected ju-venile walleyes (104 mm), fed a diet with either high lipid or low lipid, to a 1.5-BL/s current with and without a predator species (muskellunge) present. Although Czesny et al. (2003)


did not find any apparent adverse effect of the predator presence on circulating cortisol con-centrations, they did observe that walleyes fed the high-lipid diet and fasted for 6 weeks preferentially used body lipids for energy rather than proteins compared with walleyes fed the low-lipid diet. Their results suggest that using such diets might improve stocking success of hatchery-reared juvenile walleyes.

6.7.3 Angling

Killen et al. (2003) documented physiological effects in walleyes captured and pro-cessed during live-release angling tournaments. Mean plasma cortisol concentrations in walleyes from the time they were captured until weighed and released were significantly elevated (100 ng/mL) compared with those walleyes subjected to angling only. In addi-tion, both plasma and white muscle lactate concentrations were elevated and white muscle energy stores (phosphocreatine, adenosine triphosphate, glycogen) were significantly de-pleted in tournament walleyes compared with control fish. Killen et al. (2003) concluded that walleyes undergo significant physiological disturbance that may include moderate cell damage and ion loss during live-release tournaments. Later studies by Killen et al. (2006) confirmed these findings and showed that the major physiological disturbances occurred during periods of anaerobic metabolism associated with the stressors of angling and the weigh-in procedure, a time when walleyes may be subjected to extended periods of air exposure. Killen et al. (2006) suggested that minimizing both the time on the hook and the time that walleyes are exposed to the air during weigh-in, particularly when the water is warm, would probably reduce the stress experienced by the fish and improve their survival during tournament events. Indeed, Reeves and Breusewitz (2007) found that high water temperature (25°C) greatly exacerbated hooking mortality in both bleeding and nonbleed-ing walleyes caught by angling, but mortality was comparatively low when the temperature was below 20°C. Similarly Schramm et al. (2010) found that walleyes handled extensively during live-release fishing tournaments experienced high mortality when water tempera-tures exceeded 18°C, whereas below 14°C mortality was low.

6.8 FISH CONDITION AND HEALTH

6.8.1 Body Condition

Carlander (1997) provided a thorough compendium of length and weight relationship data for walleye and sauger. Weight–length regression slopes (b) for 92 walleye populations ranged from a low of 2.23 to a high of 4.03, with a median of 3.05 (Carlander 1997). Similarly the slopes of weight–length regressions ranged from 2.47 to 3.50 for 20 sauger populations.

The relative weight (Wr) compares the weight (W) of an individual fish with an estab-

lished standard for a conspecific of the same length developed from empirical data throughout North America, and is calculated from the formula: W

r = 100 × W/W

s, where W

s (standard

weight) is determined from a published standard weight equation for the equivalent length (Anderson and Neumann 1996; Neumann et al., in press). Thus, a W

r of 100 indicates a

fish at the “midpoint” of the range, or what would be considered as a “normal” fish in good condition. From 114 walleye populations in which more than 42,000 fish were examined,

216 Chapter 6

Murphy et al. (1990) reported slopes ranging from 2.3 to 3.9, with the median b around 3.15. Using the regression-line-percentile (RLP) technique, which uses the 75th percentile of mean weights for 1-cm length intervals from each population as a standard upon which to base the regression, Murphy et al. (1990) established a standard weight (W

s) equation for walleye as

log10

(Ws) = –5.453 + 3.180 × log

10(L), where W

s is in grams and L is TL in millimeters. This W

s

value can then be used to calculate Wr for walleye of a given length (Anderson and Neumann

1996). This equation is recommended for walleyes of 150 mm TL and longer (Neumann et al. in press). For smaller walleyes of 30–149 mm, Flamming et al. (1999) established the W

s

equation as log10

(Ws) = –4.804 + 2.869 × log

10(L).

From 49 data sets representing 33 sauger populations across their range, Guy et al. (1990) calculated a W

s equation for sauger as log

10(W

s) = –5.446 + 3.157 × log

10(L), using

the midpoint of the range of predicted weights. In this analysis, weight–length regression slopes ranged from 2.7 to 3.8. However, based on the RLP approach, a revised W

s equation

for the sauger data is log10

(Ws) = –5.492 + 3.187 × log

10(L), which is now the recommended

equation to use for sauger (Neumann et al., in press). Using the RLP method from 32 popu-lations of the walleye × sauger hybrid (saugeye) in which 3,371 fish were examined, Flam-ming et al. (1993) proposed a standard weight equation for saugeye as log

10(W

s) = –5.692

+ 3.266 × log10

(L).Condition factor measurements, based on length and weight data, can be a useful tool

for a “first-cut” assessment of the condition and general well being of fish populations and is used extensively in hatcheries. The metric condition factor, K = W (g) × 105/L3 (mm) (Anderson and Neumann 1996; Neumann et al., in press), however, can vary considerably within a species due to sexual maturation and gonad development, food intake and nutri-tional status, and seasonal changes (Barton et al. 2002). Condition factor will also be affect-ed by the length measurement used—standard length (SL), FL, or TL—which emphasizes the importance of including the length-measurement method when calculating K. For wild walleye populations Carlander (1997) tabulated K-values that ranged as follows: K(SL), 0.71–2.03 (n = 15); K(FL), 0.78–1.19 (n = 4); and K(TL), 0.49–1.71 (n = 30). While ranges overlap, a clear difference is apparent between K(TL) and K(SL) ranges. The TL:SL ratio for walleye populations ranges from 1.136 to 1.227 (n = 26) and for TL:FL ranges from 1.050 to 1.065 (n = 6) (Carlander 1997); no length-based trend in the data were evident. Ratios of FL:SL range from 1.088 to 1.133 (n = 14) for walleye and from 1.102 to 1.24 (n = 7) for sauger (Carlander 1997).

Other condition indices and physiological indicators are also applied routinely to evalu-ate fish health and condition (Iwama et al. 1995; Barton et al. 2002). Ratios of the mass of particular organs or tissues relative to total body mass can be used as indices of change in nutritional or energy status, or chronic exposure to toxicants. An example is the hepatoso-matic index (HSI), also called the liver-somatic index (LSI), which is determined as (liver weight/body weight) × 100. The HSI of walleye appears to be around unity; for example, a healthy population of adult walleyes in a large northern Alberta lake had an average HSI of 1.27 (SE = 0.27, n = 30) (C. Mushens and B. Barton, Applied Aquatic Research, unpub-lished data). The ratio of gonadal tissue mass to body mass, or GSI (see Section 6.3.3), can be used as a useful indicator of gonadal status or reproductive state, particularly in female walleyes where GSI can range from less than 1 in summer after spawning to approximately 15 in early spring when fully mature (e.g., see Figure 6.1).


6.8.2 Health and Disease

6.8.2.1 Bacterial Pathogens

Bacterial diseases of walleye and sauger, listed in Table 6.3 with their causative agents, are those that affect most temperate freshwater fishes. The diseases of concern in walleye culture are discussed by Muzzall (1996) and are covered more thoroughly in Chapter 13. Major bac-terial diseases known to infect walleyes and saugers include columnaris, which is of particular concern in hatcheries (see Chapter 13), bacterial gill disease (BGD), and furunculosis.

There appear to be no bacterial diseases specific to walleye with the possible exception of “bleach” disease, a condition documented in walleyes in northern Canada (Stewart and Bernier 1999). This condition causes bleaching of the skin pigment and renders the flesh inedible. Bleach disease is a bacterial disease associated with relatively warm water, but its causative agent is unknown (Stewart and Bernier 1999).

6.8.2.2 Viral Pathogens

Few of the viruses that are common to many North American freshwater fishes appear to affect walleyes or saugers, or at least have not been detected. Viral diseases that have been reported (Table 6.3) include viral hemorrhagic septicemia (VHS), which is a major health

Table 6.3. Bacterial and viral diseases and their causative agents known to occur in walleye (from McAllister 1996; Muzzall 1996; Chapter 13, this volume; and other sources cited in this chapter).

Disease Causative agent

Bacterial Columnaris Flavobacterium columnare, formerly Flexibacter columnaris Bacterial gill disease (BGD) Flavobacterium branchiophilum Furunculosis Aeromonas salmonicida Fin rot Mycobacteria Bleach disease Unknown Viral Viral hemorrhagic septicemia (VHS) Viral hemorrhagic septicemia virus (VHSV) Infectious pancreatic necrosis (IPN)a Infectious pancreatic necrosis virus (IPNV) Lymphocystis Lymphocyctis disease viruses (LCDV-1 and LCDV-2) Walleye dermal sarcoma (WDS) Walleye dermal sarcoma virus (WDSV) Walleye epidermal hyperplasia (WEH)b Walleye epidermal hyperplasia viruses (WEHV1 and WEHV2) Walleye diffuse epidermal hyperplasia Herpesvirus vitreum

a No pathology reported (Muzzall 1996). b Also referred to as walleye discrete epidermal hyperplasia (McAllister 1996).

218 Chapter 6

concern for wild stocks of walleye and other fishes, particularly in the Great Lakes region, and is discussed extensively in Chapter 13. There are a few other viral conditions that are of concern and some appear to be specific to Sander spp. These diseases are lymphocystis, wall-eye dermal sarcoma (WDS), and walleye epidermal hyperplasia (WEH) (Table 6.3), and are discussed in detail in the following text and by McAllister (1996). Mortality from infection by any of these viral agents is relatively low.

Lymphocystis is a skin disease caused by at least two strains of lymphocystivirus (an iridovirus), lymphocystic disease viruses 1 and 2 (LCDV-1 and LCDV-2), and is known to affect more than 125 marine and freshwater fishes comprising 34 families in nine orders (Kurkjian et al., undated). Lymphocystis occurs in walleyes throughout their entire range (e.g., Yamamoto et al. 1976; Amin 1979; Bowser et al. 1988; Stewart and Bernier 1999; Sny-der and Crites 2003) and appears as clusters of whitish or pinkish tumor-like fleshy growths erupting from the skin or fins (e.g., images in Stewart and Bernier 1999; Snyder and Crites 2003), although the so-called “tumors” are not neoplastic (Walker 1969). Microscopically the individual infected cells become excessively large, up to 0.7 mm in diameter, as does the nucleus proportionately, and cells become encapsulated (Walker 1969). The incidence of lymphocystis has been observed as high as 18% in spawning walleyes (Margenau et al. 1988), and Ryder (1961) similarly found that spawning walleyes in the Nipigon River, a tributary of Lake Superior, had infection rates of 25% to 29%. By comparison, the rate of lymphocys-tis incidence in far-northern Canadian walleyes is less than 1% (Stewart and Bernier 1999). Resultant mortality, however, is probably negligible (Ryder 1961), although few estimates of mortality in the field are available.

Walleye dermal sarcoma, also called simply “dermal sarcoma” (DS), is primarily a cuta-neous neoplasm, or skin tumor, characterized by fine-textured, pink to white nodules, about 1–10 mm in diameter, and often found in patches (Walker 1969; Martineau et al. 1990; McAl-lister 1996). Walleye dermal sarcoma has been observed for years in wild walleyes from Oneida Lake, New York, as well as in Lakes Champlain, Erie, and Huron (Walker 1969; Bowser et al. 1988) and Canadian prairie provinces (Yamamoto et al. 1976), and is apparently limited to the genus, possibly the species. Walleye dermal sarcoma is prevalent in both early spring at spawning and in the fall (Bowser et al. 1988). For example, WDS can affect up to 10% of the walleye population in the Northwest Territories, northern Canada (Stewart and Bernier 1999), and up to 27% in Oneida Lake (Martineau et al. 1991, 1992) around the time of spawning. In Oneida Lake, the prevalence of WDS in walleyes increased from age 3 to age 6, but decreased thereafter (Getchell et al. 2000a). Getchell et al. (2000a) found that spawning walleyes in Oneida Lake between 1995 and 1999 infected with WDS in one year did not show signs of infection in the next.

Walleye dermal sarcoma is caused by a type C retrovirus, walleye dermal sarcoma virus (WDSV) (Martineau et al. 1991, 1992; Zhang et al. 1996). The virus can be transmitted to con-specifics and cause infection experimentally by inoculation (Martineau et al. 1990; Bowser et al. 1997), oral gavage (Bowser et al. 1997), topical application (Bowser et al. 1997), and water-borne exposure (Bowser et al. 1999). Holzschu et al. (1995) determined the complete nucleotide sequence of a DNA clone of WDSV and suggested from their molecular analysis that transcrip-tion, RNA processing, and induction of the tumor involves accessory genes of WDSV. Lairmore et al. (2000) found that the retroviral cyclin (rv-cyclin) encoded by WDSV was a potent stimu-lator of cell proliferation in transgenic mice and postulated that a rv-cyclin has a major role in the development of WDS. The encoded rv-cyclin homolog, specifically the open reading frame


(Orf) A protein, has intracellular targets in both the nucleus (full form) and cytoplasm (truncated form) (Rovnak et al. 2001) and its functional role may be in interactions with large transcription processes involved in viral expression (Rovnak and Quackenbush 2002).

Infection with WDS appears to be temperature dependent. In experimental transmission experiments, Bowser et al. (1990) found that the highest level of tumor development was at 15°C, followed by 20°C and then 10°C. After experimental infection and subsequent tumor development at 15°C, tumor regression was also highest at 15°C and 20°C compared with 10°C (Getchell et al. 2000b). The WDSV replicates naturally in walleyes at 4°C, but reverse transcriptase activity from virus particles was similar at 15°C and 4°C, suggesting that the enzyme does not have any specific adaptation to function more efficiently at the colder tem-perature (Fodor and Vogt 2002a). Although WDS is present in walleyes in both early spring and fall (Bowser et al. 1988), high levels of WDSV RNA were detected in spring tumors but not in fall tumors or in inocula from fall tumors (Bowser et al. 1996).

Transcripts of WDSV were found to be abundant in the neoplastic cells of dermal sar-comas and, based on their findings, Poulet et al. (1995) suggested sarcomas were associated with elevated virus transcription activity in these cells. Both tumor-free walleyes and those with tumors may be infected with WDSV; however, those shown to have characteristic WDS tumors contain the transcriptionally active virus, but the WDSV found in walleyes without tumors was silent (Poulet et al. 1996). Walleyes challenged experimentally with WDSV a sec-ond time showed a significantly lower incidence of tumors than did naïve fish challenged for the first time (Getchell et al. 2001), suggesting that walleyes can develop natural resistance to the virus after an initial infection, which may partly explain the decline of WDS incidence in Oneida Lake walleyes after age 6.

While the tumors are most often associated with the skin in adult walleyes, WSD has been experimentally transmitted to juveniles and found to be invasive of other tissues, mainly muscle (Earnest-Koons et al. 1996). Invasive infection of WDS can also occur naturally in adults, but the incidence appears to be low (Bowser et al. 2002). Walleye dermal sarcoma has also been transmitted experimentally to saugers and—similar to walleye—the tumors appear mostly on the skin, but invasive tumors have also been identified (Holzschu et al. 1998).

Walleye epidermal hyperplasia, a condition similar to but nevertheless distinct from WDS, was first described in walleyes from Oneida Lake, New York, (see Walker 1969) and later from Manitoba and Saskatchewan (Yamamoto et al. 1985). It occurs in walleyes also during the spring spawning period and in the same populations that experience WDS and lymphocystis (Walker 1969; Yamamoto et al. 1985; Bowser et al. 1988). Getchell et al. (2004) reported an increased incidence of WEH-related lesions in older walleyes, with less than 20% of age-8 and older fish being affected, but concluded it is unlikely that all walleyes of a given year-class would become infected and experience high mortality.

The causative agents of WEH are two retroviruses, walleye epidermal hyperplasia virus type 1 (WEHV1) and type 2 (WEDV2), which are closely related to each other and with WDSV, and constitute a newly recognized genus, the epsilonretroviruses (LaPierre et al. 1998a; Fodor and Vogt 2002b). Recent studies indicate that WDSV, WEHV1, and WEHV2 are not only structurally similar, but also functionally similar, and induce cell proliferation using similar mechanisms employing rv-cyclins (LaPierre et al. 1998b, 1999). In McAllister (1996) WEH is called walleye discrete epidermal hyperplasia to differentiate it from walleye diffuse epidermal hyperplasia, another similar but less-defined skin lesion condition attributed to a herpesvirus, Herpesvirus vitreum (Kelly et al. 1980, 1983).

220 Chapter 6

The underlying cause of the prevalence or outbreaks of skin lesions, including lympho-cystis and WDS, is still unclear (McAllister 1996), but it may be population-density or en-vironmentally related. An attempt was made to associate these conditions in walleyes with water-borne contaminants, but a relationship was not established (Smith et al. 1992).

6.8.2.3 Other Pathologies

Myofibrogranuloma (MFG), or “sandy flesh,” is a myopathy specific to adult walleyes (Mayes 1976; Economon 1978; Holloway and Smith 1982). Sandy flesh condition is a degra-dation of skeletal muscle that appears as a rough, coarsely fibrous, yellowish-brown granular lesion within the tissue (Mitchum 1995). This condition is common in wild walleye popula-tions in the U.S. Midwest, but generally does not appear to cause mortality or produce disease or abnormal behavior (Mitchum 1995). However, MFG, which is also present but not com-mon in walleyes in northern Canada, has been known to kill the fish in some instances (Stew-art and Bernier 1999). Physiologically, MFG can cause an elevation in serum calcium levels in both male and female spawning walleyes, possibly as a result of acute lysis of muscle tissue cells leading to increased extracellular calcium (Shoemaker and Holloway 1997). An increase in the calcium content of muscle, up to 55 times that in healthy fish, has been associated with walleyes affected with MFG (Kelly et al. 1987). Affected walleyes show no outward signs of the pathology, and filleting is usually required to detect MFG, although X-ray analysis can also be used to determine its presence (Holloway and Shoemaker 1993).

6.8.2.4 Parasites

Summerfelt (2005) provided a list of parasites in walleye, sauger, and blue pike compiled from Hoffman (1999), which is modified and presented as Table 6.4 for walleye and sauger (blue pike removed); additions were made from Colby et al. (1979), Mitchum (1995), and Stewart and Bernier (1999). From these compilations, at least 87 species of parasites, exclud-ing mollusk glochidia, are known to occur in walleye and 91 in sauger. This listing does not include the fungal condition common in hatcheries caused by the fungus Saprolegnia spp., which is discussed in Chapters 7 and 13.

6.9 SUMMARY

This chapter updates and summarizes some of the physiological and related biological in-formation about walleye and sauger and provides a background for understanding the mecha-nisms underlying how these species function in their environment, notably during maturation and reproduction. The chapter also provides a summary of current knowledge about health and condition in walleye and sauger that can provide a context for subsequent chapters that discuss life history, population dynamics, exploitation and management, and their culture. Appreciating how these fishes interact with their environment physiologically and behavior-ally as individuals is important for understanding how populations may respond and adapt to altered environmental conditions and management practices, particularly in light of increas-ing evidence of the effects of climate change on fisheries resources (see Chapter 9).


Table 6.4. Parasites known to occur in walleye and sauger, modified from Summerfelt (2005) (•) with additions from Mitchum (1995) (○) and Stewart and Bernier (1999) (♦), excluding Saprolegnia spp. Also included are those mentioned in Colby et al. (1979) (◊) beyond those reported by Summerfelt (2005).

Parasite Walleye Sauger Parasite Walleye Sauger Fungi

Nemathelminthes, Nematoda

Branchiomyces sanguinis ● Camallanus ancylodirus ● Protozoa, Ciliophora C. lacustris ● Apiosoma sp. ○ ○ C. oxycephalus ●○ ● Ichthyophthirius multifilis ● C. oxycephalusb ○ Trichodina sp. ○ ○ Capillaria catenata ◊ ● Protozoa, Myxozoa Contracaecum spiculigerum ● Henneguya aysmmetrica ● C. brachyurum ● Myxobilatus aysmmetricus ● Contracaecum sp. ◊ ● Myxobolus sp. ● Contracaecum sp.a ● Platyhelminthes, Trematoda, Monogenea Cucullanellus cotylophora ● Dactylogyrus extensus ● Cystidicola lepisostei ● Gyrodactlus mizellei ● ● Dactinitoides cotylophora ◊ ● G. schmidti ● Eustrongylides sp.a ● ● Urocleidus aculeatus ● ● Oxyuroidea (accidental?) ◊ ● Platyhelminthes, Trematoda, Digenea Philometra cylindracea ◊ ● Allocreadium lobatum ◊ ● Rhabdochona canadensis ● Apophallus americanusa ◊ ● Rhaphidascaris acus ● A. venustusa ◊ ● Rhaphidascaris sp. ● Azygia acuminata ◊ ● Spinitectus carolini ◊ ● A. angusticauda ◊ ● S. gracilis ◊ ● A. bulbosa ◊ ● Spinitectus sp. ◊ ● A. longa ● Thynnascaris brachyura ● ● Bucephaloides ozakii ● T. brachyuraa ● Bucephalus pusillus ● ● Acanthocephala Bunoderina sacculata ◊ ● Echinorhynchus salmonis ● Centrovarium lobotes ● ● Echinorhynchus sp. ● Cleidodiscus aculeatus ◊ Leptorhynchoides thecatus ◊ ● Clinostomum complanatuma ● ● Metechinorhyncus salmonis ● Clinostomum marginatuma ◊ Neoechinorhyncus crassus ● Cotylurus communisa ● ● N. cylindratus ● ● Crassiphiala bulboglossaa ◊ ● N. tenellus ● ● Crepidostomum cooperi ● Pomphorhynchus bulbocolli ◊ ● Crepidostomum sp. ◊ ● Annelida, Hirudinea Diplostomulum scheuringia ◊ ● Batracobdella phalera ● D. spathaceuma ● Cystobranchus verrilli ◊ ● Diplostomulum sp.a ● ● Iillinobdella moorei ◊ Neascus sp.a ● ● Macrobdella decora ● Phyllodistomum superbum ● ● Myzobdella lugubris ●○ ●○ Posthodiplostomum minimuma ◊ ● Percymoorensis marmorata ● Prosorhynchoides pusilla ● Piscicola geometra ◊

222 Chapter 6

Table 6.4. Continued.

Parasite Walleye Sauger Parasite Walleye Sauger Prosorhynchoides pusilla ● Piscicola geometra ◊ Ptycogonimus fontanus ● P. punctata ● ● Sanguinicola occidentalis ◊ ● Placobdella pediculata ● Tetracotyle communisa ● ● Mollusca, Bivalvia Tetracotyle sp.a ● Glochidia (various)a ● T. diminutaa ● Arthropoda, Crustacea Uvulifer ambloplitisa ◊ ● Argulus appendiculosus ● ● Platyhelminthes, Cestoda A. biramosus ● Abothrium crassum ◊ A. canadensis ◊ ● Biacetabulum macrocephalum ● A. stizostethi ● ● Bothriocephalus claviceps ● ● A. versicolor ◊ ● Bothriocephalus sp. ♦ Ergasilus caeruleus ● ● B. cuspidatus ● ● E. centrarchidarum ● ● Dibothriocephalus latus ◊ E. confusus ◊ ● Diphyllobothrium latuma ● ● E. luciopercarum ◊ ● Diphyllobothrium sp.a ♦ ● E. versicolor ● Eubothrium sp. ● Lernaea cruciata ● Proteocephalus ambloplitisa ● ● L. cyprinacea ● P. fluviatilisa ● L. variabilis ● P. luciopercae ● ● Lernaeocera sp. ● P. macrocephalus ◊ ● P. pearsei b ◊ ● P. pinguis ● P. stizostethi ● ● Proteocephalus sp. ◊ ● Triaenophorus crassus ● T. nodulosus ● T. nodulosusa ● ● T. stizostedionis ◊ ● Triaenophorus sp. ●♦

a Larval forms b Immature adults


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