behavioral and ecological determinants of home range size in juvenile pumpkinseed sunfish (lepomis...

Ethology 102, 900-914 (1996) 1996 Blackwell Wissenschafts-Verlag, Berlin ISSN 0179-1613

Department of Biological Sciences, Binghamton Universip, Binghamton

Behavioral and Ecological Determinants of Home Range Size in Juvenile Pumpkinseed Sunfish (Lepomis gibbosus)

KRISTINE COLEMAN & DAVID SLOAN WILSON

COI.I,MAN. k: & WIIXIN. D S. 1996: Behavioral and ecological determinants o f home range size in juvenile pumpkinseed sunfish (Iepomis ~ihhosus). Ethology 102, 900-914.

Abstract

Numerous species of freshwater fish are known to be site specific, yet the ecological and behavioral factors influencing home range size have rarely been examined. In this study we examine aggression, predator avnidance and feeding rate in juvenile pumpkinseed sunfish as a function of location within their home range. We also compare the behavior of individuals within their home range with individuals that were displaced from their home range by being transplanted from one pond to another. Juvenile pumpkinseeds had a higher feeding rate in the center of their home range than on the periphery. Transplanted juveniles did not adopt a restricted home range over a 3-wk period and had a lower feeding rate than residents in the same pond. Aggression and predator avoidance do not appear to be important determinants of home range size in our study.

Correspnnding authot: Kristine COI.I~.MAN, Department o f Biological Sciences, Binghamton University, Binghamton, N Y 13902-6000, USA.

Introduction

The concept of home range, or the area in which an animal spends most of its time, is widely studled in terrestrial vertebrates such as mammals, reptiles, and birds (eg. HARVEY & CLUTTON-BROCK 1981; ROSE 1982; CALL & GUTIERREZ 1992; PRESTRUD 1992), and even some invertebrate species (KEASAR & SAFRIEL 1994). Home range size can be influenced by a variety of factors including diet DILL^^ al. 1981; GITTLEMAN & HARVEY 1982; NORMAN &JONES 1984; HEWS 1993), predators (CLARKE et al. 1993), and intraspecific interactions (BROWN 1969; SCHOENER & SCHOENBR 1982; GRANT et al. 1992; SALSBURY & ARMITAGE 1994).

Home ranges have been documented in freshwater fish but ecological and behavioral determinants of home range size are poorly understood. GERKING (1950, 1953,1959) reviewed a diffuse Literature on freshwater fish, concluding that individuals of many species have Limited home ranges that they return to when dsplaced, even

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Home Range Size in Pumpkinseed Sunfish 901

during the nonbreeding season. Since GERKING’S studies, the subject has received remarkably little attention and almost all that is known is based on mark and recapture studies without behavioral observations (see GUNNING & SHOOD 1963; KUDRNA 1967; L,EWIS & FLICKINGER 1967; BERRA 1973; HILL & GROSSMAN 1987; MUNDAHL & INGERSOLL 1989). The main exception to this statement is the Salmonids, whose feeding territories and social behavior have been relatively well studied, at least over the short term (KEENLEYSIDE & YAMAMOTO 1962; NOAKES & GRANT 1986; GRANT & NOAKES 1987; GRANT & KRAMER 1990).

Behavioral observations are more extensive on marine fish (e.g. CLARKF. 1970; THRESHER 1976; HIXON 1981; FRICKE & HISSMANN 1994) which commonly maintain defended home ranges (i.e. territories). BARLOW (1 992) has recently reviewed this literature and called attention to the ‘puzzling paucity’ of feeding territories in freshwater fish. This pattern may merely reflect a paucity of behavioral observations, however, since BARLOW @. 166) stresses that ‘even the numerous ichthyologists who work on freshwater fishes in the temperate zones have been slow to recognize the value of entering the aquatic realm to observe fishes directly.’

In an earlier paper (WILSON et al. 1993), we showed that juvenile pumpkinseed sunfish (1~pomi.rgzbbo.ru.r) maintained home ranges of % 10 m of shoreline and that the fish returned to these sites when displaced from them. In this paper we document home range size with greater precision and try to distinguish among three possible ecological and behavioral determinants of the small home ranges: 1. intraspecific aggression; 2. predator avoidance; and 3. feeding efficiency. In the first part of the study, we observed marked fish in their natural habitat to see how they behaved as a function of distance from the center of their home range. In the second half of the study, we manipulated the presence and absence of an established home range by capturing and marking juveniles from two similar and adjacent ponds and releasing them all into one of the ponds. The behavior of the ‘foreigners’, who were permanently displaced from their home ranges, was compared with the behavior of the ‘residents’, who were free to return to their established home ranges.

Methods

Study Site

We conducted the study at Cornell Utuversity’s experimental pond facility in Ithaca, NY. The ponds are 30 x 30 m with a bottom that slopes gradually to 2 m in depth. The ponds we utilized contained adult and juvenile pumpkinseed sunfish (Ixpomis gibbosus) and fathead minnows (Pimpbales pmmelus). Aquatic vegetation consisted primarily of Eurasian water milfoil (Myriopby//xm ~picutum). Predatory fish were not present but other predators such as water snakes (Nutrix sp.), great blue heron (Ardeu hemdiar), and belted kingfishers (Mepceryfe u/gon) were cnmmon.

Observational Methods

In our previous study (WIISON et al. 1993), fish were observed from the shore through binoculars and from the water with a mask and snorkel. Both methods had problems; visibility from the shore is variable and depends on weather conditions, while behavior patterns of fish are altered by the presence of the observer in the water. We therefore designed a glass-bottomed observation vessel to facilitate observations.

902 K. COLEMAN & D. S. WILSON

The vessel was a 2.43 x 1.22 x 0.61 m plywood box with a 0.61 x 1.22 m plate glass window located on one end o f the bottom. The inside of the vessel was painted black and the top was covered, allowng light tn penetrate only through the bottom window and making the human observer relatively invisible to the fish.

The vessel was maneuvered around the pond by two methods. In the first part of our study. two transects were established along the west side of pond 225, 1 and 3 m from the shore, respectively. A 100 m tape measure was stretched along the bottom o f the pond and a wire cable was stretched above the pond along each transect. The vessel was attached to the wire cable, which ran through guide holes under the top cover, allowing the observer within to pull the vessel along the transect by pulling on the cable. The tape measure along the bottom o f each transect was visible from the vessel, allowing the position o f the fish to be recorded.

For the second part o f our study, a wire cable was stretched ughtly around four posts at each corner o f pond 225. Two lines crossed the pond in perpendicular drections and were attached to the cables by pulleys. The vessel was located at the intersection o f the cross lines. The observer could maneuver the vessel around the entire pond by pulling on the cross lines, which were marked at 0.5 m intervals, providing an x-y coordinate system that allowed the location of the vessel to he recorded at any time.

Fish appeared to behave naturally when observed from the vessel and if anything were attracted t o i t as a large physical object. They seemed oblivious to the observer and often foraged within a few inches from the glass window. We dci not claim that the vessel had no effect on the behavior of the fish, hut it almost certainly had a smaller effect than an ohserver in the water and enabled us t o observe the fish in much hveater detail than possible through toinoculars from the shore.

Part 1. Observational Study

On 20 JuI. 1992, six unhaited cylindrical wire minnow traps (6 mm mesh size, 44 cm long and 23 cm in diameter, with openings 3.5 cm in diameter) were placed on the west side of pond 225. The traps were placed at 5 m intervals, = 1 m from shore in 0.5 m of water. The traps were retrieved after 5 min, the fish were removed, and the traps were replaced for another 5 min interval. Then the entire side of the pond was seined to capture the fish that did not enter the traps. ( M y juvenile pumpkinseed sunfish between 40 and 60 mm standard lenqh were used for this study and other fish were returned unmarked t o the pond. The pupose o f this sampling procedure was to study individual differences in shyness and boldnesb. Bold individuals are more likely t o enter the traps (see WIISON et al. 1993, 1004). However, shy and bold individuals do not differ in home range size (D. S. WllsoN et al. 1993, unpubl. data), s o for the purposes of this paper all tish will toe treated as a single ~ v o u p

A total o f 160 fish were measured (standard length) and individually tagged with a unique color comloination of three 3 mm diameter glass beads on a rubber thread. The thread was similar tn that used in commercially available ‘spaghetti string’ tags (e.g. F l o y Tags, Seattle). The thread was passed with a needle through the dorsal musculature just antenor to the dorsal tin and ued into a loop. This is a miniaturized version o f a t a d n g procedure that is commonly used t o mark larger tish. The measuring and taggng procedure required % 1 min per fish and 6 h total, after which the fish were returned to the side of the pond from which they were captured.

Fish were censused and observed between 1030 and 1530 h a t = 2-d intervals over a 35-d period, for a total of 17 observation sessions. For each session, starting at one end o f the pond, marked tish were observed for a 3-min penod with the vessel in a stationary position. The vessel was then moved 6 m along the transect and held stationary for another 3 min observation period, and s o on. until both transects were completed. Visibility along the transcct from the vessel was zz 6 m, and s o this method allowed the entire transect to be observed. The rest of the pond was censuxd from the water with a mask and snorkel or from the shore with binoculars six times during the study period to confirm that the maiotity of marked tish do not stray far from their original capture site (WIISON et al. 1903).

For every marked tish that was observed during a 3 min interval, we recorded: 1. thc prosition o f the fish along the transect; 2. the vertical location of the fish (’bottom, midwater, surface); and attempted to record 3. feeding events; 4. the location of prey (water column, sediment, vegetation); 5. the fish’s proximity t o other fish (subjectively esumated as number of body lenghs from the nearest neighbor); and 6 . aggvessive interactions (nips, chases and submissive postures as described by HI:A(:HAM 1988). Ptmr visibility cxcasionally prevented us from observing all behavior patterns for each fish. While we were able to visualize tags, fish could be dfficult t o see in certain light conditions o r in dense vegetation.

All marked fish tended to remain in shallow water and were observed more frequently along the 1 m than the 3 m transect. Their home ranges can therefore be described by their position along the shore. For


every fish that was observed at least four times, its median position was calculated and defined as the center of its home range. The distance from the median position was then used as the independent variable in regression analyses with feeding events, location in the water column, and aggressive interactions as the dependent variables.

Part 2. Displacement Experiment

O n 16 Sep. 1992,20 sunfish (‘the residents’) were collected with minnow traps from the north side of pond 225 and an additional 20 sunfish (‘the foreigners’) were collected from another pond (pond 221) that was similar in species composition, density and physical structure. All fish were measured and individually marked as described above and released on the north side o f pond 225.

The fish were observed during and immediately after their release from the glass-bottomed vessel and from the shore using binoculars. Thereafter, focal observations were conducted between 1030 and 1530 h on 14 d during a 29-d period. On each day, as many marked fish as possible were located and followed for a 3-min period, dunng which the behavior patterns listed above were recorded by speaking into a tape recorder. The x-v coordinate position o f the fish was recorded at the beginning and end of the focal observation period, allowing both the location in the pond and net movement during the 3 min interval to be measured. No fish was observed more than once on a given day.

Unlike the observational study, in which all marked fish could reliably be observed on one side o f the pond, the displacement experiment required the entire pond to be searched to locate the foreipers, who could not be expected to stay close to the release site. To cover the larger area, focal observations from the vessel were supplemented with censuses that were taken from the water with a mask and snorkel on 14 d during the 28-d period. Plastic flags placed at 3-m intervals around the 900 mz pond allowed the observer t o hft his or her head out of the water bnefly after identifying a fish to locate and mark its position in the pond on a map printed on a waterproof tablet. N o behavioral observations were taken during the censuses, which were intended solely to measure location. When censuses from the water and focal observations from the vessel were conducted on the same day, they were separated by at least 3 h.

Statistical Analysis

In the obsenwional study, distance from center of home range was treated as an independent variable and the various behavior patterns were treated as dependent variables for regression analyses. Flowever, if multiple obsewations are conducted within a short period o f time on a single individual, they are likely to be correlated with each other and should not he treated as independent events. For example, if a single fish takes three or four bites in quick succession during a census, this should not be treated as four separate feeding events at a single distance from the center of the home range. Accordingly, all observations o f single fish at a single location on a given day were treated as a single data point in the regression analyses (e.g. feed VR. not feed).

Occasionally, single fish were observed at different distances from their home range during a single census. For example, a fish might follow the vessel as it moved or might be observed along the 3-m transect in addition to the 1-m transect. Behavior patterns of a single fish at different locations on a given day were treated as separate data points. For example, if a single fish is observed near the center of its home range but does not feed and later is observed feeding near the edge of its home range, these are counted as two independent data points in the regression analysis. This seems reasonable, since the basic purpose of the study is to relate behavior to &stance from center of home range. Therefore, we report statistics on the number of fish sightings for a particular behavior pattern, rather than the number of fish, unless otherwise stated.

In the displacement experiment, whose purpose is to compare differences between two treatment groups (residents vs. foreigners), it is appropriate t o include individual fish as a level in the statistical analysis. However, standard repeated measures tests require an equal number of observations on each individual, which was not possible in our experiment because every fish could not be located on every day that focal observations were taken. We therefore programmed our own randomization test (SOKAI. & Ro1iI.F 1981; P(YWIN & ROFF 1993) in which individual fish are randomly assigned t o two treatments many times to generate a null distribution of the test statistic against which the value of the experimental results can be evaluated. For example, 18 residents and 20 foreigners were observed for variable numbers of times in the drsplacement experiment. The randomization test first calculates the Mann-Whitney U statistic for the actual data. It then randomly allocates the 38 individuals into groups of size 18 and 20 and calculates the


M a n n - n t n e y U statistic for the comparison of these random groupings. This procedure is repeated 10 OOO times and the experimental results are judged s ipt icant if the null value of the statistic exceeds the experimental value less than 5% of the time. Individual fish are included as a level in this analysis because all the observations on a single fish are moved as a block to one treatment or the other. The level of significance is thetefore always less than when single observations are treated as independent events. We employed the randomization test in the dtsplacement experiment only when significant results were found with the Mam-Whitney U-test by itself.

Results

Observational Study

Of the 169 marked fish, 118 (69.82%) were observed at least once during the experiment and 80 (47.34Y0) were observed at least four times. Tlurteen marked fish (0.08%) were observed elsewhere in the pond in adhtion to the side on which they were captured. Only one fish was observed elsewhere and not observed on the side of capture. While 51 (30.18O/o) of the fish were not seen on the side of capture, they were not seen on any other side of the pond either, and no marked fish were ever noted more than 3 m from the shore during the six censuses of the entire pond. Thus, the majority of tagged fish that were observed during the study remained close to their capture site over a 35-d period, as in our previous study.

There are a number of reasons why some fish were observed infrequently or not at all. Some tagged fish developed fungus infections around the tags which may have increased mortality. In addition, tagged fish may have been more conspicuous to avian predators such as herons and kingfishers. It is difficult to estimate these artificially induced mortality rates without knowing the mortality rate of untagged fish. Some of the tags became untied and were found at the bottom of the pond. Finally, even tagged fish that remained in the study site may have been concealed by vegetation and missed during a census. The problem is aggravated by individual differences along the shy-bold continuum, which cause some individuals to be observed much less often than others (WIISON et al. 1993).

Figure l a shows the distribution of census points for all fish observed at least four times, expressed as distance from center of home range. When home range is defined as the distance between the two most extreme census points, the average home range size was 11.46 m (n = 80 fish, SD = 5.72). As might be expected, there is a positive but fairly weak relationship between home range size and the number of times a fish was observed (r2 = 0.144, p < 0.01).

It is obvious from the shape of the distribution in Fig. l a that pumpkinseeds do not utilize their home range equally but spend the majority of their time near the center. Over half the sightings (59.12%) are within 2 m of the median and 80% of the sighting are within 4 m of the median. This findmg is in agreement with other studies which found that animals such as juvenile lemon sharks (MORRI~SEY & GRUBER 1993) and moles (LOY et al. 1994) tend to remain in one area of their home range. Additionally, the home ranges are distributed relatively evenly along the shore, such that the edge of one individual’s home range is at the center of another’s (Fig. 2). We now examine the three factors that might contribute to the maintenance of these small home ranges.

Pumpkinseeds exhibit a high rate of aggression in aquaria and have been used as


4 1 0 20 3 0 2 4 0 ( 0 1 0 20 3 0 2 4 0

Distance from Median (m) Distance from Median (m)

F&, 1; a. Frequency distnbuuon of observed distances of fish from the median of their home ranges for the observational study (n = 80 fish); b. Frequency distribution of observed distances of resident fish from the

median of their home ranges for the displacement expenment (n = 18 fish)

model organisms to study dominance hierarchies in the laboratory (BEACHAM 1988; DUGATKIN & OHLSIN 1990). In contrast, few intraspecific aggressive interactions were observed in the natural population that we studied. Only two chases and one submissive posture were observed during the censuses (= 314 fish sightings; 0.19 interactions/h). Observable ongoing aggression is apparently rare in this system. The question of whether aggression influences the initial establishment of home ranges will be addressed below.

0 30 Distance Along Shoreline (m)

F& 2: Examples of the home ranges o f 25 fish from the observational study. Each line represents all points in which a fish was seen. The solid circle represents the median of the home range


If pumpkinseeds center their home range on a refuge from predation, we would expect them to be more clustered around protective objects than they are in Fig. 2. In addtion, we might expect individuals to exhibit increasingly cautious behavior at the edge of their home range. One common antipredator behavior is to avoid the open water and maintain proximity to physical structures such as vegetation or the benthos. We therefore divided the censused fish into two classes based on their positjon in the water column (top or midwater vs. bottom or vegetation). The average distances from the center of home range for these classes are 3.57 m (n = 209) and 2.46 m (n = 68), respectively, which is contrary to our prediction and statistically sipficant (Mann-Whitney U = 8308, p = 0.04). The open water classifications are considered to be more risky for the fish, while the benthos or vegetation would provide more shelter from potential predators. However, this interpretation must be viewed with caution. Fish observed in the benthos or vegetation may have been foraging and not seeking refuge at all. To exclude this possibility, we compared the position of the fish when no feeding event was observed. Nonfeedng fish seen in the open water were an average of 3.59 m from the center of their home range (n = 201), while those in the vegetation/benthos were 3.02 m (n = 42) (Mann-Whitney U = 4512.5, p = 0.48).

Another common antipredator behavior in fish is to seek the proximity of conspecifics (PITCHER 1986; LIMA & DII.I. 1990; WOOTTON 1990). Proximity to nearest neighbor was recorded for the censused fish. Fish that were within three body lengths of each other were considered to be in an aggregation or cluster (WIISON et al.

1993). The average distance from center of home range for fish in a cluster (3.78 m, n = 120) did not differ significantly from those not clustered (3.07 m, n = 72; Mann-Whitney U = 4803.5, p = 0.19). Thus, fish do not appear to increase grouping tendencies as they move away from the center of their home ranges.

Another possible explanation for small home ranges is that familiarity with the habitat increases foraging success. If so, then feeding rate should be greatest near the center of the home range. During the censuses, fish that fed during a focal period (n = 37) were an average distance of 1.85 m from the center of their home range, compared to an average distance of 3.49 m for fish that did not feed (n = 253). The difference is statistically sipficant (Mann-Whitney U = 5848, p = 0.01) suggesting that foraging success is greatest near the center of the home range. As a more stringent test of the hypothesis, we can confine our analysis to fish that were observed to feed at least once during the censuses, by comparing the average distance from the center of their home range when they were observed fee&ng with the average &stance when they were not observed feeding. The result of this pairwise analysis is also statistically significant (Wilcoxon Paired Sign Ranks 2 = -2.3738, p = 0.02, n = 19 fish).

Displacement Experiment

As expected, the residents remained primarily at the north side of the pond for the duration of the experiment, although exceptions occurred that will be discussed below. In contrast, the foreigners quickly dispersed to all four sides of the pond (x2 = 120.93, df = 3, p < 0.01; Table 1).


7uble 1: Total number of times residents and foreigners were seen on each side of the pond during observations from the observation vessel and water. Residents were taken from the north side and all fish

were released there

I No. of fish sightings Side of pond Resident Foreiper I

~ ~~

North 115 25 East 17 67 South 2 34 West 21 41

The residents had an average home range size of 50.47 m ( n = 18 fish, SD = 21.50), which is much larger than was found in the observational study. The home range was calculated as before for fish seen at least four times and includes every coordinate in which a fish was seen. The fish were assumed to travel along the shoreline; therefore, i fa fish was seen on two sides of the pond, the home range was considered to be the distance o f the circumference between the points, and not the shortest line between them. As in the previous experiment, the resident fish spent a majority of their time close to the center of their home range (Fig. 1 b); 64% within 5 m and SO'%) within 10 m o f the median.

It was possible that the foreigners initially dispersed throughout the pond but soon adopted home range sizes similar to the residents. To examine this possibility, we estimated movement separately for each week of the 4 wk experiment. For every fish that was seen at least two times, we calculated the minimum distance between adjacent

F&. 3: Movement patterns of fish from the displacement experiment. The points represent the average distance between census locations of foreigners (solid points) and residents (open circles) in the pond


7able 2; Distance traveled between censuses in the dlsplacement experiment, including the Mann-Whitney U probability values and those from the randomization test (see text)

Mean distance moved (m) Resident Foreigner

Week Mean n Mean n

All 2.42 114 4.31 102 1 2.83 29 5.11 28 2 2.06 33 4.53 34 3 2.21 28 4.50 20 4 2.67 24 2.65 20

Statistical analysis Mam-Whitney u Randomization U P P

4170.5 0.0003 0.0009 278 0.04 0.08 311.5 0.002 0.01 186 0.05 0.09 237 0.94 NP'

'Randomization test not performed because data were not s ipf icant with the Mann-Whitney U test

locations, again assuming that the fish travelled along the shoreline. As shown in Fig. 3, foreigners travelled more than residents for the first 3 wk of the experiment but seemed to adopt more restricted home ranges during the fourth week (see Table 2). Figure 4 provides an example of the movements of one foreigner and one resident over the course of the experiment.

In addition to movement over a period of days, the coordinate positions at the beginning and end of each focal observation allowed us to estimate movement over a 3 min period. Foreigners travelled more at this time scale also (Mann-Whttney U = 2521.5, p = 0.04; randomization p = 0.04).

No aggression was observed between the foreigners and residents (marked or unmarked) immediately after their release. During the entire experiment, only eight aggressive acts involving foreigners were observed during 97 focal samples (= 291 min; 1.65 interactions/h) compared to four aggressive acts involving residents during 76

Resident Foreigner F2. 4; Examples of the movement of one resident and one foreigner over the course of the displacement experiment. Each number corresponds to the week in which the fish was observed at that particular

location in the pond


Tuhle 3: Number o f bites taken per 3-min focal sample in the dsplacement experiment, including the Mann-Whitney U probability values and those from the randomization tests (see text)

Mean distance moved (m) Statistical analysis Resident Foreigner Mann-Whitney U Randomization

n n Week Mean Mean U P P

All 1.25 72 0.66 86 3820 0.004 0.02 1 I .04 27 0.70 40 618 (1.22 NP' 2 2.40 10 0.47 17 139 0.003 0.009 3 1.33 15 1 .00 12 96 0.75 NP' 4 0.00 20 0.53 17 21 1 0.14 NP'

'Randomization test not performed because data were not significant with the Mann-Whitney U test

focal samples (= 228 min; 1.05 interactions/h). These rates are not significantly different from each other (x2 = 0.69, df = 1, p > 0.7) or from the first experiment (x2 = 0.72, df = 1, p > 0.7).

There was no difference between residents and foreigners in their use of the top or midwater vs. bottom habitats. The residents were seen near the benthos or in the vegetation 52.310/0 (n = 63 sightings) of the time, compared to 54.03% (n = 87) for the foreigners (sign test p = 0.48). There was also no difference between foreigners and residents in their propensity to be clustered. The residents were found in aggregations 56.45Oh of the time (n = 62), while foreigners were clustered in 58.57% of sightings (n = 70) (sign test p = 0.53).

Residents fed significantly more than foreigners over the course of the entire experiment (Mann-Whitney U = 3820, p = 0.004; randomization p = 0.02; 1.25 bites/ focal observation for residents n = 72; 0.66 bites/focal observation for foreigners, n = 86). When each week was analyzed separately, residents fed more during each week but the difference is only significant during week 2 (see Table 3).

Discussion

Previous mark and recapture studies have revealed that adult pumpkinseeds (SHOF,MAKER 1952; KLDRNA 1967; REED 1971) and other freshwater fish (GUNNING & SHOOP 1963; LEWIS & FLICKINGER 1967; BERRA & GUNNING 1972; BERRA 1973; FISH & SAVrTL 1983; HILL & GROSSMAN 1987; MUNDAHL & INGERSOLL 1989) are site specific in lakes, ponds and streams. In the first part of the current study, we have shown that juvenile pumpkinseeds establish home ranges of M 11 m2 in a small pond, which is consistent with our earlier findtngs (WILSON et al. 1993). However, in the chsplacement experiment, the average home range size of residents was larger. Several possible explanations may account for this dtfference. It may be an artifact of the dtfferent methods used in the two studies. In the first study, home ranges were only measured on one side of the pond, and only observations from the vessel were included in the determination of home range size. Fish that had home ranges near the corners presumably moved onto the adjacent sides that were outside our transect. Also, observations taken during the censuses from the water were not included in the

calculations o f home range size in the first study, while they were in the second. Therefore, home range size may have been underestimated in the observational study. Alternatively, the increased home range size in the second study may reflect a temporal change in spacing patterns. While both studies were conducted in the same pond, the displacement study was conducted later in the season, when food sources may have been declining. The larger home ranges may therefore reflect increased movements to find food (FISH & SAVlTZ 1983). Densities of fish in the pond may also have decreased, although we were not able to measure this. Finally, the observation vessel may have had an unintended influence on home range size. Several days after the beginning of the second study, a number of marked residents whose home ranges were on the north-east comer o f the pond were observed around the research vessel, which was kept at the south-east corner. These fish returned to their original home ranges when the vessel was relocated to the south-west comer but another set of marked residents from the north- west corner was attracted t o the vessel at its new location. Thus, the fish closest to the vessel appeared to extend their home ranges to include the vessel. This problem was not evident during the first experiment because the vessel was kept on the edge of the transect and was therefore within the existing home range o f many of the fish.

The small home ranges observed in this study are remarkable because they can be traversed in only a few minutes at a normal swimming speed and the fish are not restricted to their home ranges by either intraspecific aggession or by predators. Individuals also return to their home range when displaced (RI:.I:.D 1971; W i i s o ~ et al. 1993). There is clearly something important about a home range that compels fish to choose and remain within a very small area, yet home ranges are not defended and overlap a p-eat deal.

The behavior of fish that were transplanted from one pond to another, and therefore were permanently displaced from their home range, is also remarkable. If home ranges are easily and somewhat arbitrarily chosen, we would expect the foreigners to adopt them quickly, but instead they roamed the entire pond for a period of 3 wk. Finding the right home range must be important to warrant such a large investment of time at the expense of feeding. I t is possible that the foreigners did not quickly adopt a new home range because they were searching for their old home range, but this just brings us to the same conclusion, that the right home range must be important to warrant such a large investment of time to relocate it.

Despite the fact that pumplunseeds are highly agressive in aquaria, we observed a relatively low rate of aggessive interactions in our study population. Low rates of aggression have also been found in natural populations o f territorial species such as Atlantic salmon and other salmonids (Ki:i:im & GRANT 1995). Lack o f apgession sometimes merely signifies that dominance relations have already been established based on past aggressive interactions. We therefore expected to observe a burst of aggressive interactions among residents and the foreigners when they were introduced into the observation pond. This was not the case, suggesting that aggressive interactions were truly a rate event. It is easy to cause aggression in natural populations of pumpkinseeds by providing a concentrated resource, but such resources are evidently sufficiently rare that aggTession did not have an important effect on home range size in our study.

Home Range Size in Pumpkinseed Sunfish 91 1

Our two measures of predator avoidance were: 1. Proximity to physical structure such as vegetation or the bottom; and 2. Proximity to other fish. The first is ambiguous because physical structure can serve as a feedmg site in addition to a refuge from predators. The second is more satisfactory because there is little reason for juveniles to seek the presence of conspecifics, other than to avoid risk. If individuals centered their home range on a refuge from predators, we would expect them to cluster with other fish at the edge of their home range, but this was not the case. We would also expect the home ranges themselves to be clustered around obvious physical structures that provide refuge from predators, yet this was not the case either. Since foreigners did not know the terrain of their new pond, we would expect them to cluster more than the residents, but they did not. Finally, in our previous study of the shy-bold continuum in juvenile pumpkinseeds (WIISON et al. 1993), shy individuals clustered more than bold individuals yet did not have a smaller home range size. All of these lines of evidence indicate that predator avoidance was not an important determinant of home range size in our study population.

Fish predators such as largemouth bass were absent from our study population, but avian predators such as great blue heron and belted kingfishers were common. Additionally, the fish are descendants from populations that did have fish predators, and so predator responses should be a part of the repertoire of the sunfish even in the absence of actual predators (CURIO 1993; MAGURRAN & SI~GHI:RS 1994). Predator avoidance behavior has been shown to have a genetic component in a few fish species, including pppies (SEGHERS 1974) and sticklebacks (GILES & HUNTINGFORD 1984). It is entirely possible that the introduction o f fish predators might have dramatic effects on home range size and utilization in juvenile pumpkinseeds. However, our study demonstrates that home ranges remain small even in the absence of fish predators.

Feeding rate does appear to be related to home range size in juvenile pumpkinseeds. Fish feed more near the center of their home range than on the periphery. Foreigners that roamed the pond fed at a slower rate than the residents with their established home ranges.

Feeding is often cited as an important determinant of home range size in other species. FISH & S~vrTz (1983) suggested that variation in the home range size of centrarchid species within a single lake may be due to variation in prey density. Thus, piscivorous adult yellow perch (Percu Juvescens) and largemouth bass (Micmptems sulmoides) have larger home ranges than the largely molluscivorous adult pumpkinseed sunfish. We do not think that these broad resource categorizations can explain the very small home range size and details of home range utilization that we have observed. Sedentary benthic prey may cause a forager to move through its habitat at a slower rate than planktivorous prey, but they do not necessarily cause the forager to turn back after it has gone a few meters, or to return to a certain area when dtsplaced, or to forage less successfully when permanently displaced. To explain these patterns it seems necessary to invoke familiarity with an area as an important determinant of feeding success. Perhaps the littoral zone habitat consists of certain ‘hot spots’ of resource abundance that must be learned. This implies that resources regenerate faster in some areas than in others, or else the ‘hot spots’ would not be worth visiting again. Benthic invertebrate prey are seldom regarded as renewable resources in this sense. It is not


obvious why one patch searched for snails or chironomid larvae would regenerate faster than another, yet the behavior of the pumpkinseeds suggests that they do. One possibility is that predator avoidance behavior, rather than reproduction, causes the prey of juvenile pumpkinseeds to be a renewable resource over the short term. If the majority of prey in a patch hide, or otherwise successfully evade predation by a pumpkinseed that visits the patch, then the patch may well be worth visiting agam after the prey have resumed foraging.

Behavioral ecology has become such a conceptually oriented science that species are often used to test specific hypotheses without first checking if the assumptions of the hypotheses are appropriate for the species. Thus, optimal foraging models assume that the forager cruises through a limitless number of patches, habitat choice models assume that the forager samples among a wide array of habitats and models of aggression assume that aggression plays an important role in the social life of the species. It appears that none of these assumptions are warranted for the juverule pumpkinseeds in our study population, who seldom interact aggressively and forage repeatedly over a very small area. It will be interesting to use these results to construct a more appropriate set of models, which can be tested on pumpkinseeds in the future.

Acknowledgements

We are very grateful to B o b J O H N S o N for the use of the ponds as well as helpful suggestions. We thank Jeff A R ~ N D T for help with the randomization tests, Gary SLUIVAN for help with field work, and Anne B. CLARK, Ted DI~.ARSTY"., Sum WILCOX, and the EEB group at Binghamton University for useful discussion. We also thank James GRANT, Jane BROCKMANN. and an anonymous reviewer for useful comments.

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