ICES mar. Sei. Symp., 199: 89-98. 1995

Factors in the life history of the edible crab (<Cancer pagurus L.) that influence modelling and management

David B. Bennett

Bennett, D. B. 1995. Factors in the life history of the edible crab (Cancer pagurus L.) that influence modelling and management. - ICES mar. Sei. Symp., 199: 89-98.

The United Kingdom has implemented progressive changes in the management regulations of the edible crab (Cancerpagurus). Various aspects of crab life history are reviewed to show how they have influenced the fisheries for this species and stock management. Aspects of the reproductive cycle, including size at maturity and spawn­ing strategy and the spatial distribution of larvae and juveniles are poorly known. Males grow faster than females, and there appear to be real differences in growth rates. Tagging studies indicate that extensive movements are made by mature females. Catch rates of crabs are influenced by moulting and breeding cycles. The population structure is complex with seasonal and spatial variation in both size composition and sex ratio. Stock relationships are difficult to interpret. Yield-per-recruit modelling has been used to investigate management options. Models used have included various life history factors, explicitly or implicitly, with some data assumptions. Nevertheless, a pragmatic approach to fishery management has resulted in several changes to mini­mum landing size regulations in the UK. These consider sexual and regional variation of biological factors such as growth. There exists a risk of recruitment overfishing arising from the exploitation of prespawners, and interactions with other fishing gears and seabed uses like aggregate dredging.

David B. Bennett: Ministry o f Agriculture, Fisheries and Food, Directorate o f Fisheries Research, Fisheries Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 OHT, Eng­land [tel: (+44) 1502 56 22 44, fax: (+44) 1502 5138 56],

Introduction

The United Kingdom in recent years has been im­plementing a progressive change in the management of the edible crab (Cancer pagurus L.) fishery, taking greater account of regional variations in crab biology. Research work, first in the North Sea and more recently in the English Channel, has identified and quantified various crab life history features which have determined the way in which the English fishing industry has devel­oped and exploited crabs, and the approach scientists have taken to model stocks and offer fisheries manage­ment advice.

The crab fishery in the English Channel has expanded in the last 20 years to become the major European crab fishery, landing >10 0001 per annum. This increased exploitation called into question the effectiveness of the management regime which had essentially been deter­mined in the nineteenth century. In UK waters there had been, until 1986, a single national minimum landing size which was introduce^ in 1877 at 108 mm (4.25 in) cara­pace width, and increased to 115 mm (4.5 in) in 1951. In

addition, the landing of berried (ovigerous) or soft (recently moulted) crabs was also prohibited in 1877.

The North Sea crab fishery was extensively studied in the early 1960s, and was followed by a research pro­gramme on the English Channel fishery in the period 1968-1976, which included the collection of catch and effort and population structure data, tagging experi­ments to determine growth and migrations, and general observations on crab biology. The English Channel study led to recommendations for changes in minimum landing size regulations. We are currently reassessing the English Channel crab stock, and as well as an analy­sis of the catch, effort, and size composition data, and stock modelling, emphasis is being placed upon improv­ing our understanding of reproductive strategy and interactions with other fisheries and seabed uses.

The intention of this article is to show how the North Sea and English Channel fisheries, population model­ling, and stock management are driven by the currently known aspects of the crab’s life history and to discuss and identify the research currently underway or antici­pated as essential to provide the additional understand­

90 D. B. Bennett ICES mar. Sei. Symp., 199(1995)

ing needed to improve stock assessments and fisheries management advice.

Reproductive cycle

Direct observations on the development of the vas deferens, presence of spermatozoa, and biometric analysis of allometric growth of chelae indicate that most male C. pagurus in the North Sea that exceed 110 mm carapace width (CW) are mature (Edwards, 1979). Le Foil (1986) gave an estimate for 50% male maturity of ~100m m CW in the Bay of Biscay. Berried crabs are rarely caught in traps. Samples of berried crabs are consequently small and infrequent. The records of the smallest berried crabs range from 115 mm CW (Pearson, 1908, northern North Sea), through 129mm CW (Edwards, 1979, central North Sea), 133mm CW (Brown and Bennett, 1980, English Channel), to 152 mm CW (Pearson, 1908, Irish Sea).

Oviduct plugs, which indicate that copulation has occurred, have been observed in crabs as small as 107 mm CW (Edwards, 1979) and 105 mm CW (Brown and Bennett, 1980). The presence of plugs is not, how­ever, necessarily an indication of sexual maturity. Analysis of ovary development showed that 13% of the size group 115-126 mm CW had ripe gonads (Edwards, 1979, off southwest Ireland). Le Foil (1986), using ovary development, estimated that in the Bay of Biscay 50% of females had reached sexual maturity at a size of —110 mm CW. These results show that size at maturity for females is not well established, with considerable variation depending upon the reproductive features used to indicate maturity, and also perhaps on the area being studied.

It is well established that mating occurs between a soft recently moulted female and a hard male. Edwards (1966) describes the pairing of premoult females and intermoult males for 3-21 days before the female moults and mating occurs, and for 1-2 days afterwards. Phero- monal identification and attraction to a premoult female by a potential mate has been postulated.

Sperm are stored in the spermatheca pending spawn­ing. It has been suggested that spawning can occur the same year that moulting and mating take place, or may be delayed until the following year, and that one impreg­nation may result in multiple spawning using the one batch of sperm stored in the spermatheca (Edwards,1979). While there is some evidence to substantiate these spawning strategies, there are no estimates of the proportion of the spawning stock which undertake these various options. Spawning occurs in late autumn and early winter. Laboratory studies (Edwards, 1979) indi­cated that spawning females require a soft substratum of sand or gravel in order to scoop a hollow in which to lower the abdomen and ensure attachment of the eggs to

the pleopods. Ovigerous crabs overwinter without feed­ing, with a gelatinous plug in the hindgut and “poor” hepatopancreas condition (Howard, 1982). Fecundity is high, ranging from 0.25 to 3 million eggs, with larger crabs carrying the most eggs (Edwards, 1979; Le Foil, 1986).

Hatching of the larvae takes place some 7 -9 months after spawning. Larvae have been recorded in the plank­ton from March to December, but the main hatching period is May to July. The larval stages are well de­scribed and larval development time in relation to tem perature has been studied (Nichols et al., 1982; Thompson and Ayers, 1988). Larval surveys have been undertaken in the North Sea (Nichols et al. , 1982) and historical data for the English Channel examined (Thompson and Ayers, 1987). Little is known about vertical distribution (Harding and Nichols, 1987) and nothing about settlement.

Juvenile crabs are found intertidally and in shallow inshore waters (Latrouite and Le Foil, 1989) and the mean size of trap-caught crabs (juvenile and adults) increases with water depth (Brown and Bennett, 1980). Very little is known about the behaviour, feeding, habi­tat requirements, growth, mortality, predation, etc., of juvenile C. pagurus. There are no data on stock and recruitment. Figure 1 summarizes some aspects of the reproductive cycle which are relatively well known, but there are still major gaps in our knowledge.

Growth

Growth data for C. pagurus are derived mainly from tagging studies using the persistent suture tag (Edwards, 1965). Moult frequency has been estimated using H an­cock and Edwards’ (1967) anniversary technique. Moult increment observations are quite extensive, but the more critical moult frequency is not so well estimated.

While both sexes of juvenile C. pagurus have similar sized moult increments (Latrouite and Morizur, 1988a), there are other clear differences in the growth patterns of adults (Fig. 2); males have average moult increments which are larger than those estimated for females (Edwards, 1965; Hancock and Edwards, 1967; Bennett, 1974b; Latrouite and Morizur, 1988a). This difference in growth pattern between the sexes could be explained by the partitioning of energy resources in mature females to egg production, rather than growth (Bennett, 1974b). Within these growth patterns there is considerable varia­bility in the size of moult increments (Bennett, 1974b). In the English Channel, crabs moult at various times of the year, when factors controlling growth, such as food supply and temperature, vary and may determine the size of moult increments. Some of the observed varia­bility in the moult increments may be the result of limb loss. Bennett (1973) has shown that for C. pagurus limb

ICES mar. Sei. Sym p., 199 (1995) Factors in the life history o f the edible crab 91

ISUMMERl

Larvae planktonic for

2 months

HATCHINGYEAR 1

Multiple spawning? 1 or 2 year cycle ?

Settlementwhere?

WINTERYEAR 2

GROWTH

YEAR 3

Soft substrate? Fasting.

Reducing moult frequency Berried female ,------------------ ,

1 /4 to 3 million eggs |SUMMER|YEAR 4

Hard male Soft female

MATINGYEAR 5

FEMALE EMIGRATION

Figure 1. Summary of the main life cycle features of C. pagurus.

loss and regeneration can reduce body growth and either reduce or increase the intermoult period.

In the English C hannel m oult frequency decreases

with increase in size, in fem ales m ore so then in males (B ennett, 1974b; Latrouite and M orizur, 1988a); the reverse appears to be the case in the N orth Sea (H an ­cock and E dw ards, 1967). T he fem ale reproductive cycle could be expected to result in a reduction in fem ale m oulting frequency, with ovigerous crabs in tem porary anecdysis, and possibly spawning m ore than once from a

single moulting and mating episode.A comparison of the growth data collected in the

North Sea (Edwards, 1965; Hancock and Edwards, 1967), in the English Channel (Bennett, 1974b; Latrouite and Morizur, 1988a), and in the Bay of Biscay (Latrouite and Morizur, 1988a) showed little variation in moult increments between areas (Fig. 2). There are, however, major sexual and areal differences in moult frequency which are reflected in differences in annual growth. Bennett (1974b) noted that, (a) in the English Channel adult male crabs moult more frequently than females, but the opposite was the case in the North Sea, and (b) that the moult frequency of adult males in the

English Channel is higher than that in the North Sea (Fig. 3). Latrouite and Morizur (1988a) working in the southwest of the English Channel and further south in the Bay of Biscay calculated annual female growth rates which, because of higher moult frequencies, exceeded those estimated by Bennett (1974b) in the northern part of the English Channel.

Moult frequency estimations can be subject to bias resulting from the timing of releases and recaptures in relation to moulting periods, catchability changes depending upon the stage of the moult cycle, trap selec­tion bias, differential survival rates of moulted and non­moulted crabs, tag loss during moulting, movements out of the recapture area, and the reproductive cycle. Latrouite and Morizur (1988a) suggest there may be considerable annual variation in moulting, with “good” and “bad” years, which might also bias moult frequency estimates. It is also possible that there are genetic differ­ences between the stocks in the North Sea and the English Channel, or that growth differences are environ­mentally controlled or induced. The warmer mean water temperatures in the western Channel, compared with the North Sea, may be the reason for the prolonged

92 D. B. Bennett

45 y

s 40 -

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80 100 120 140 160 180 200

Premoult Carapace Width (mm)

Figure 2. Comparison of the relationship between premoult size and moult increment for male and for female C. pagurus tagged and recaptured in the English Channel (Eng (Bennett, 1974b); Fr (Latrouite and Morizur, 1988a)), Bay of Biscay (Latrouite and Morizur, 1988a), and the North Sea (Edwards, 1965; Hancock and Edwards, 1967). ■ = Male - Channel (Eng); □ = Female - Channel (Eng); • = Male - Channel (Fr); O = Female - Channel (Fr); ♦ = Male - N. Sea; O = Female - N. Sea; A = Female - Biscay.

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95 115 135 155 175 195

Premoult Carapace Width (mm)

Figure 3. Comparison of the relationship between premoult size and frequency of moulting (per annum) of male and female C. pagurus. Sources as per Figure 2. ■ = Male - Channel; □ = Female - Channel ; O = Female - Channel/Biscay ; ♦ = Male - N. Sea 1959-1963; A = M ale-N . Sea 1965-1966;O = Female - N. Sea 1959-1963; A = Female - N. Sea 1965-1966.

ICES mar. Sei. Sym p., 199 (1995)

moulting period there, resulting in an increased moult­ing frequency (Bennett, 1974b).

Movements

It has been known since the early 1900s (Williamson, 1900; Meek 1913) that C. pagurus can move consider­able distances. The subsequent development of the persistent suture tag (Edwards, 1965) has permitted the study of long-term movements during experiments in the North Sea (Edwards, 1965, 1971; Hancock and Edwards, 1967; Mason, 1965), Scandinavia (Gunder- sen, 1979; Hallbäck, 1969), the English Channel (Ben­nett and Brown, 1983), and the southwestern English Channel and Bay of Biscay (Latrouite and Le Foil, 1989). All these studies, including the early ones using tags which were lost at ecdysis, have shown long-dis­tance directed movements by mature female crabs, and essentially local random movements by males.

In the North Sea, local inshore/offshore seasonal mi­grations were observed (Edwards, 1979), but female crabs also made extensive movements northwards along the east coast of England to southeast Scotland (Fig. 4). A study in the English Channel (Bennett and Brown, 1983) also showed some crabs, particularly females, making extensive movements moving mainly in a wes­terly direction down the Channel (Fig. 4). Some of the female crabs tagged at the western end of the Channel were recaptured to the south, off the French coast. Recaptures from the French studies (Latrouite and Le Foil, 1989) showed movements in a westerly direction in the southwestern Channel, while in the Bay of Biscay movements were southwest or south, some well offshore into deep water (up to 200 m) on La Chapelle Bank (Fig.4).

While the majority of male movements observed in these studies were small and non-directed, a few larger (mean size >180 mm CW) males tagged offshore in the English Channel were recaptured west of their release positions. It seems likely that male C. pagurus are nomadic with the larger males ranging over greater distances than smaller ones (Bennett and Brown, 1983). In contrast, females showed more directed movements, which seem to be one-way, and can be over considerable distances. These emigrations may be related to the breeding behaviour, ensuring a suitable seabed substrate for overwintering ovigerous crabs and allowing for distribution of the larval phase on residual currents.

Catchability

In the western English Channel, 85% of the crab landings are made in the 6 month period from June to November, with female landings-per-unit-effort

ICES mar. Sei. Symp., 199 (1995) Factors in the life history o f the edible crab 93

5° 0°

SCOTLANDNORTH SEA

55°

IRISH SEA

ENGLANDy WALES

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50°

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200m

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Figure 4. Summary of the movements of C. Pagurus in the North Sea (Edwards, 1979), English Channel (Bennett and Brown, 1983; Latrouite and Le Foil, 1989), and Bay of Biscay (Latrouite and Le Foil, 1989).

(l.p .u .e.) being five times greater in these peak months than in the rest of the year (Fig. 5). These dramatic changes in catchability seem to be related to the repro­ductive cycle. Ovigerous females overwinter without feeding and are rarely caught in baited traps, accounting for the low l.p.u.e. from December to May. Hatching occurs in M ay-July and post-ovigerous females, vora­cious after a long fasting period, are readily caught in baited traps in June and July (Fig. 5). In addition to these changes in catchability there is also some immi­gration by mature females onto the western mid-Chan- nel grounds, increasing the density. Female crabs caught in the autumn have well-developed ovaries and high meat yields (Brown and Bennett, 1980). As the females spawn in November/December l.p.u.e. falls dramati­cally as ovigerous crabs enter the overwintering non­feeding stage of their life cycle.

Male l.p.u.e. is considerably lower than for females

during most of the year (Fig. 5). There is a negative relationship between male and female catch rates (Brown and Bennett, 1980), and the lower male l.p.u.e. in the summer could be related to mating behaviour, or the result of intraspecific competition with the abundant female crabs for both food and space during approach and entry to the traps (Bennett, 1974a).

Population structure

There are marked spatial differences in size composition and sex ratio of crabs, which are overlaid by seasonal variations (Brown and Bennett, 1980; Latrouite and Morizur, 1988b). In a well-sampled area in the western Channel (Brown and Bennett, 1980), the mean size of females was fairly stable through the first half of the year, at about 160 mm, and rose significantly in August to around 175 mm. Mean size of male crabs seemed to be highest in the spring, but standard errors of the means were high. In this same area seasonal changes in sex ratio were also observed, with a sex ratio approaching 1:1 in the spring, but by August up to 96% of the catch was female. The increase in mean size and change in the sex ratio coincides with the increase in catchability and density already discussed.

Brown and Bennett (1980) showed that the mean size of males in particular, but also of females, increases with water depth. They also showed that while in the western Channel sex ratios are approximately 1:1 in the first half of the year, females predominated in the second half; in the eastern Channel, catches were always dominated by male crabs. Latrouite and Morizur (1988b), using princi­pal component analysis on samples collected from the English Channel and the Bay of Biscay, showed similar heterogeneity in both size and sex ratio. They demon­strated that most of the size variation could be explained by spatial factors, rather than by annual variation.

Stock density

The distribution of C. pagurus is extensive from Norway in the north down to the north African coast and into the Mediterranean. The major fisheries, however, are around the British Isles and adjacent French coasts, and off Norway and Sweden.

The population structure in the North Sea and the English Channel shows quite marked differences, with crabs in the English Channel having a much greater maximum size. A distinctive feature of the English Channel fishery is the presence of very large male “cock” crabs - up to 267 mm CW (Brown and Bennett,1980). Male crabs over 180 mm are rare in the catches in the North Sea. This difference in size also applies to female crabs, with average sizes in the English Channel being between 160 and 175 mm, compared to averages

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94 D. B. Bennett ICES m ar. Sei. Sym p., 199 (1995)

100-,H t- 1976 —i 1981 - o 1982

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Figure 5. C. pagurus landings per unit effort from the logbook of a western Channel fisherman for the period 1976-1985. Males = top, Females = bottom.

ICES m ar. Sei. Symp., 199 (1995) Factors in the life history o f the edible crab 95

of 128 mm in the southern North Sea (Brown, 1975), and 146 mm to 151 mm further north (Edwards, 1967). Even in the newly exploited fishery offshore in the North Sea the mean size of both males and females is below that observed in the English Channel, suggesting that these size differences are not the result of different exploi­tation rates.

Growth differences between crabs in the English Channel and the North Sea have already been noted above. While moult increments were similar, there were considerable differences in moult frequency and, consequently, annual growth. The data on reproductive features, such as size at maturity and fecundity (see above), are not adequate for an areal comparison.

It can be seen from the larval distribution in the North Sea (Nichols et al., 1982) that the distribution of larvae is much wider than the present fishing grounds (Fig. 6). However, the Thames estuary area, between

5P 0°

50°

Limit of larvae survey

Larvae >27m'J

Crab fishing grounds

Figure 6. Crab (C. pagurus) fishing grounds in the North Sea, English Channel, and Bay of Biscay, along with areas where larvae (>27 numbers m (zoea stages I-V and megalopae)) have been found.

the North Sea and the English Channel, has a soft substrate with few crabs, and it would be quite reason­able to conclude that crabs in the English Channel are separate from the North Sea. In conjunction with a recent larval survey in the English Channel (Thompson et al. , 1995), hydrographic modelling predicted larval drift from the eastern Channel through the Dover Strait into the southern North Sea. However, the presence of several larval stages together in high abundance at one location suggested there was little drift through the Dover Straits prior to the survey. The results so far are thus equivocal and it is not clear whether there are separate stocks in these two areas.

In the English Channel and its western approaches, there is crab fishing throughout much of this area as far west as the edge of the continental shelf. The fishing grounds are extensive (Fig. 6) and larval surveys (Thompson et al., 1995) indicate a wide distribution. The hydrography of the English Channel is complex, and still not fully understood. The residual current drift is believed to be eastward up the Channel (Lee and Ramster, 1981; Pingree, 1980). The westward move­ments of mature females down the English Channel may be contranatent behaviour, with the larvae drifting back up the Channel on the easterly residual currents (Bennett and Brown, 1983). A t the western end of the Channel tagging studies suggested a link between the Channel and the Bay of Biscay, with some female crabs moving south (section on growth). The French experi­ments have shown movements between the inshore grounds and the deepwater area on the continental shelf.

Clearly, larval and adult dispersal is quite complex and not yet fully understood. The source of recruitment in most areas is not clear; larval distribution is extensive offshore in deep water, yet the juveniles are observed mainly inshore in shallow waters. All these extensive movements by females, together with the evolving but as yet incomplete picture of larval distribution, makes it difficult to interpret the underlying stock relationships.

Modelling

The catch and effort statistics in the crab fisheries of Europe are poor, despite their long history and biologi­cal study. Surplus yield modelling requires much better statistics than those currently available in the UK. The inability to age large crustaceans directly has made the estimation of mortality parameters difficult and has sti­fled the use of age-based assessment models, developed originally for finfish. All the assessments to date have had one question in mind - what would be the short/long­term losses/gains in landings from a change in minimum landing size (MLS)? The modelling approach taken with C. pagurus has therefore been relatively simplistic.

96 D. B. Bennett ICES m ar. Sei. Sym p., 199 (1995)

Hancock (1965, 1975) evaluated the MLS in the N orth Sea fisheries using the method of Gulland (1961). Landings and effort for the Norfolk (southern North Sea) fishery were examined by Hancock (1965), who showed how catchability varied seasonally in re­sponse to behaviour associated with moulting and breeding. In using m ark-recapture studies to estimate population size and rate of exploitation he also high­lighted those life history factors which could bias recap­ture rates. Differences in moult frequency between the sexes pointed up the need to consider males and females separately. The model used by Hancock esti­m ated the survival of crabs between the old and the new minimum landing, sized and calculated the change in yield, which depended upon the relative values of fishing (F) and natural (M) mortality. A ttempts were made to estimate these independently from catch com­positions and from tagging, but Hancock (1975) con­sidered that various factors of the life history, such as catchability changes, the complex growth, and emi­gration made estimates of F and M unreliable. The model was therefore run with a range of both par­ameters to evaluate relative changes in yield from in­creases in minimum landing size.

For the English Channel, Bennett (1979) also attem pted to estimate mortality rates from catch curves and tagging. Annual growth estimates were used to split population size frequency distributions into annual in­tervals to produce “age” frequency distributions from which total mortality (Z) could be estimated. While size composition samples could be biased because smaller size groups were underrepresented, either because of partial recruitment to the fishable stock or because of a lower catchability of smaller crabs, the main problem of potential bias in the English Channel comes from the complex population structure and the question of stock identity. Bennett (1979) showed how different estimates of Z could be calculated, depending upon the spatial and temporal classification of the size composition samples. For example, a bias is introduced by the increased catchability of mature females after hatching and the immigration of large females onto the offshore western Channel grounds, which gave a lower mortality in the autumn than in the spring, the opposite to that expected.

As it was not possible to quantify recruitment to the fishable stock, a yield-per-recruit (Y/R) model was used (Bennett, 1979). This calculated Y/R as the sum of catch weights at each age for a given recruitment from the recruit age to maximum age with exploitation patterns varied to achieve changes in MLS. The approach was still dependent upon likely ranges of F and M estimated independently. However, the results from this assess­ment generated management advice which changed the UK approach to MLS management (see section on man­agement).

In an assessment of the North Sea crab fisheries, Addison and Bennett (1992) used the length cohort analysis (LCA) model of Jones (1974,1981). This model estimates population numbers from a length compo­sition of the catch using von Bertalanffy growth par­ameters. F at length can be calculated and yield-per- recruit (Y/R) and biomass-per-recruit (B/R) predictions made for changes in fishing effort and/or minimum land­ing size. This model is sensitive to the life history factors which may bias catch composition samples and growth estimates, and to natural mortality (Addison, 1989).

The available knowledge of size at maturity and fec­undity was used in a stock and recruitment model (Addison and Bennett, 1992). In the absence of time series information on stock and recruitment the ration­ale of Bannister and Addison (1986) was adopted, using a range of param eter values and different assumptions about the likely form of the stock-recruitm ent curve. The results suggested that unless the stock-recruitm ent relationship is of an overcompensatory nature, the Y/R approach may underestimate the benefits of an increase in MLS.

This assessment cautioned against unequivocal ac­ceptance of the results, where there were some life history factors which were inadequately estimated. In the Norfolk (southern North Sea) fishery the nature of the seabed substrate is very different from that further north (Howard, 1980) and this may be a major factor determining the characteristically small size compo­sition of crabs and lobsters (Homarus gammarus (L.)) in this area. The LCA depends upon the assumption that the sampled size compositions reflect the current level and pattern of exploitation and it has been sug­gested (e.g., Addison, 1986) that, in a situation like that off Norfolk, crustacean size compositions may not respond to changes in the level or pattern of exploi­tation in the same manner as those of finfish popu­lations.

Management

The European fishery for C. pagurus is currently managed on a national basis. The European Community does allow for a minimum landing size in its technical measures, but has not so far determined what the size should be. There are no effort controls, direct or in­direct, except in a minor way with local Sea Fishery Committee by-laws in England and Wales.

As a result of the studies in the English Channel (Bennett, 1979) and the reassessment in the North Sea (Addison and Bennett, 1992) there have been major changes in UK crab management. The estimation of crab growth rates in the English Channel, which were higher than those in the North Sea and showed a greater growth for males than females, coupled with the

ICES m ar. Sri. Symp., 199 (1995) Factors in the life history o f the edible crab 97

knowledge of the population structure and the Y/R assessment, resulted in two fundamental changes in management by MLS. Legislation was enacted to allow for both regional size limits, and a size limit for each sex.

In 1986 the MLS in the eastern Channel was raised from 115 mm CW to 140mm CW for both sexes, and in the western Channel the male limit was raised a further 20 mm to 160 mm CW. In the English Channel fishery, large (>160 mm CW) male “cock” crabs are sold at a premium price. In addition to justifying the larger MLS for males in terms of the higher Y/R resulting from their faster allometric growth, there was also a higher econ­omic return to be gained from preventing the landing of males >140 <160 mm CW at the lower value “hen” (all females -I- small males) price (Bennett, 1979).

It has been assumed that the MLSs in force in the English Channel exceed the 50% maturity size. This assumption was dependent on North Sea maturity data and needed to be checked. Current research on crab reproduction in the English Channel is aimed at estimat­ing both size at maturity and fecundity. The MLS has recently (February 1990) been increased to 125 mm CW around the rest of the UK coast (130 mm CW in South Wales). An exception was made for local areas in the southern North Sea and the Irish Sea, where uncertain­ties about certain aspects of the life history of crabs justified leaving the MLS at 115 mm CW (Addison and Bennett, 1992). Consideration is being given to the use of escape gaps to enhance the efficacy of the MLS regu­lations (Brown, 1982; Lovewell and Addison, 1989).

The other regulations applied to C. pagurus in the UK are the prohibition of the landing of ovigerous (berried) and soft (recently moulted) crabs. Soft crabs have a poor meat yield but, when they are a high proportion of the catch during the moulting period, fishermen are tempted to land them. In the North Sea the moulting period is reasonably well defined, but females moult before the males, and in the English Channel soft crabs can be found in most months of the year. Thus a closed season to prevent the catching of soft crabs seems inappro­priate. There is at present no practical objective test to indicate what is a soft crab, and the regulation is often flouted to the economic detriment of fishermen, mer­chants, and consumers. This seems to be a problem which would be best solved by the industry itself, i.e. stop selling and buying soft crabs.

The continuation of the ban on the landing of berried crabs seems a little incongruous, given that the catcha­bility of ovigerous crabs is low and, particularly, that a similar ban on the landing of berried lobsters was res­cinded in 1966. This was done because the ban could not be justified as being of benefit to lobster recruitment, since nothing was known about the stock-recruitment relationship, and was in any case difficult to enforce (Bennett and Edwards, 1981). In terms of attempting to

protect the spawning stock, and reduce the risks of recruitment failure, this regulation for crabs seems irrel­evant. The mid-Channel autumn fishery for prespawning mature females, which has increased considerably in recent years, must pose some threat of recruitment over­fishing. The crabs seem to congregate in certain areas and to be highly catchable before they spawn. Such behaviour could leave the spawning stock very vulner­able to overfishing. However, this risk might be balanced by the very low catchability of over-wintering ovigerous females (but this would be dependent upon the level of F on prespawners), and the suggestion from the recent larval surveys (Thompson et a l., 1995) that the spawning grounds may be somewhat more extensive than the existing fishing grounds. Recent discoveries of pres­pawning aggregations offshore in the North Sea and their exploitation are inevitably giving rise to fears of recruitment overfishing and the demise of traditional inshore fisheries which might be dependent upon recruit­ment by larval drift inshore or by adult immigration.

There is considerable concern that crabs, particularly over-wintering ovigerous females, are also vulnerable to other fishing methods, e.g. beam trawling and scallop dredging, and to other seabed uses, e.g. aggregate extraction and sewage disposal. A better understanding of the distribution of ovigerous crabs and their habitat requirements, behaviour, and vulnerability to capture, damage, or disturbance with possible egg abortion is essential to an assessment of the risks of recruitment failure from a combination of directed fishing and other activities.

There are clearly gaps in our knowledge of the life history of C. pagurus. We are currently attempting to improve our understanding of the temporal and spatial variability in population structure resulting from changes in catchability, which the moulting and repro­ductive cycles impose. Tagging studies and larval surveys are investigating stock relationships and the possible links between inshore and offshore fishing grounds through movements of mature females and lar­val drift. Fecundity and size at maturity are being esti­mated to ensure optimization of the MLSs in relation to the reproductive cycle. Fishing effort and power con­tinue to increase in the UK pot fisheries, negating the benefits of recent MLS increases which have been set to take account of the spatial and sexual variation in crab biology. The scope for further optimization of technical controls like MLSs is limited. Future management of C. pagurus stocks is likely to depend upon fishermen and managers grasping the nettle of direct effort regulation.

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