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

Factors in the life history of the edible crab (

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 spawn­ ing 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 mini­ mum 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, Eng­ land [tel: (+44) 1502 56 22 44, fax: (+44) 1502 5138 56],

Introduction addition, the landing of berried (ovigerous) or soft (recently moulted) crabs was also prohibited in 1877. The United Kingdom in recent years has been im­ The North Sea crab fishery was extensively studied in plementing a progressive change in the management of the early 1960s, and was followed by a research pro­ the edible crab (Cancer pagurus L.) fishery, taking gramme on the English Channel fishery in the period greater account of regional variations in crab biology. 1968-1976, which included the collection of catch and Research work, first in the North Sea and more recently effort and population structure data, tagging experi­ in the English Channel, has identified and quantified ments to determine growth and migrations, and general various crab life history features which have determined observations on crab biology. The English Channel the way in which the English fishing industry has devel­ study led to recommendations for changes in minimum oped and exploited crabs, and the approach scientists landing size regulations. We are currently reassessing have taken to model stocks and offer fisheries manage­ the English Channel crab stock, and as well as an analy­ ment advice. sis of the catch, effort, and size composition data, and The crab fishery in the English Channel has expanded stock modelling, emphasis is being placed upon improv­ in the last 20 years to become the major European crab ing our understanding of reproductive strategy and fishery, landing >10 0001 per annum. This increased interactions with other fisheries and seabed uses. exploitation called into question the effectiveness of the The intention of this article is to show how the North management regime which had essentially been deter­ Sea and English Channel fisheries, population model­ mined in the nineteenth century. In UK waters there had ling, and stock management are driven by the currently been, until 1986, a single national minimum landing size known aspects of the crab’s life history and to discuss which was introduce^ in 1877 at 108 mm (4.25 in) cara­ and identify the research currently underway or antici­ pace width, and increased to 115 mm (4.5 in) in 1951. In pated 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 the pleopods. Ovigerous crabs overwinter without feed­ management advice. ing, with a gelatinous plug in the hindgut and “poor” hepatopancreas condition (Howard, 1982). Fecundity is Reproductive cycle high, ranging from 0.25 to 3 million eggs, with larger crabs carrying the most eggs (Edwards, 1979; Le Foil, Direct observations on the development of the vas 1986). deferens, presence of spermatozoa, and biometric Hatching of the larvae takes place some 7-9 months analysis of allometric growth of chelae indicate that most after spawning. Larvae have been recorded in the plank­ male C. pagurus in the North Sea that exceed 110 mm ton from March to December, but the main hatching carapace width (CW) are mature (Edwards, 1979). Le period is May to July. The larval stages are well de­ Foil (1986) gave an estimate for 50% male maturity of scribed and larval development time in relation to ~100mm CW in the Bay of Biscay. Berried crabs are temperature has been studied (Nichols et al., 1982; rarely caught in traps. Samples of berried crabs are Thompson and Ayers, 1988). Larval surveys have been consequently small and infrequent. The records of the undertaken in the North Sea (Nichols et al. , 1982) and smallest berried crabs range from 115 mm CW (Pearson, historical data for the English Channel examined 1908, northern North Sea), through 129mm CW (Thompson and Ayers, 1987). Little is known about (Edwards, 1979, central North Sea), 133mm CW vertical distribution (Harding and Nichols, 1987) and (Brown and Bennett, 1980, English Channel), to nothing about settlement. 152 mm CW (Pearson, 1908, Irish Sea). Juvenile crabs are found intertidally and in shallow Oviduct plugs, which indicate that copulation has inshore waters (Latrouite and Le Foil, 1989) and the occurred, have been observed in crabs as small as mean size of trap-caught crabs (juvenile and adults) 107 mm CW (Edwards, 1979) and 105 mm CW (Brown increases with water depth (Brown and Bennett, 1980). and Bennett, 1980). The presence of plugs is not, how­ Very little is known about the behaviour, feeding, habi­ ever, necessarily an indication of sexual maturity. tat requirements, growth, mortality, predation, etc., of Analysis of ovary development showed that 13% of the juvenile C. pagurus. There are no data on stock and size group 115-126 mm CW had ripe gonads (Edwards, recruitment. Figure 1 summarizes some aspects of the 1979, off southwest Ireland). Le Foil (1986), using ovary reproductive cycle which are relatively well known, but development, estimated that in the Bay of Biscay 50% of there are still major gaps in our knowledge. females had reached sexual maturity at a size of —110 mm CW. These results show that size at maturity Growth for females is not well established, with considerable variation depending upon the reproductive features Growth data for C. pagurus are derived mainly from used to indicate maturity, and also perhaps on the area tagging studies using the persistent suture tag (Edwards, being studied. 1965). Moult frequency has been estimated using Han­ It is well established that mating occurs between a soft cock and Edwards’ (1967) anniversary technique. Moult recently moulted female and a hard male. Edwards increment observations are quite extensive, but the (1966) describes the pairing of premoult females and more critical moult frequency is not so well estimated. intermoult males for 3-21 days before the female moults While both sexes of juvenile C. pagurus have similar and mating occurs, and for 1-2 days afterwards. Phero- sized moult increments (Latrouite and Morizur, 1988a), monal identification and attraction to a premoult female there are other clear differences in the growth patterns by a potential mate has been postulated. of adults (Fig. 2); males have average moult increments Sperm are stored in the spermatheca pending spawn­ which are larger than those estimated for females ing. It has been suggested that spawning can occur the (Edwards, 1965; Hancock and Edwards, 1967; Bennett, same year that moulting and mating take place, or may 1974b; Latrouite and Morizur, 1988a). This difference in be delayed until the following year, and that one impreg­ growth pattern between the sexes could be explained by nation may result in multiple spawning using the one the partitioning of energy resources in mature females to batch of sperm stored in the spermatheca (Edwards, egg production, rather than growth (Bennett, 1974b). 1979). While there is some evidence to substantiate Within these growth patterns there is considerable varia­ these spawning strategies, there are no estimates of the bility in the size of moult increments (Bennett, 1974b). proportion of the spawning stock which undertake these In the English Channel, crabs moult at various times of various options. Spawning occurs in late autumn and the year, when factors controlling growth, such as food early winter. Laboratory studies (Edwards, 1979) indi­ supply and temperature, vary and may determine the cated that spawning females require a soft substratum of size of moult increments. Some of the observed varia­ sand or gravel in order to scoop a hollow in which to bility in the moult increments may be the result of limb lower the abdomen and ensure attachment of the eggs to loss. Bennett (1973) has shown that for C. pagurus limb ICES mar. Sei. Symp., 199 (1995) Factors in the life history o f the edible crab 91

ISUMMERl

Larvae planktonic for 2 months

YEAR 1 HATCHING

Multiple spawning? 1 or 2 year cycle ? Settlement where? YEAR 2 WINTER

GROWTH

YEAR 3

Soft substrate? Fasting. Reducing moult frequency Berried female ,------, 1 /4 to 3 million eggs |SUMMER| YEAR 4

Hard male Soft female

YEAR 5 MATING

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

loss and regeneration can reduce body growth and either English Channel is higher than that in the North Sea reduce or increase the intermoult period. (Fig. 3). Latrouite and Morizur (1988a) working in the In the English Channel moult frequency decreases southwest of the English Channel and further south in with increase in size, in females more so then in males the Bay of Biscay calculated annual female growth rates (Bennett, 1974b; Latrouite and Morizur, 1988a); the which, because of higher moult frequencies, exceeded reverse appears to be the case in the North Sea (Han­ those estimated by Bennett (1974b) in the northern part cock and Edwards, 1967). The female reproductive of the English Channel. cycle could be expected to result in a reduction in female Moult frequency estimations can be subject to bias moulting frequency, with ovigerous crabs in temporary resulting from the timing of releases and recaptures in anecdysis, and possibly spawning more than once from a relation to moulting periods, catchability changes single moulting and mating episode. depending upon the stage of the moult cycle, trap selec­ A comparison of the growth data collected in the tion bias, differential survival rates of moulted and non­ North Sea (Edwards, 1965; Hancock and Edwards, moulted crabs, tag loss during moulting, movements out 1967), in the English Channel (Bennett, 1974b; of the recapture area, and the reproductive cycle. Latrouite and Morizur, 1988a), and in the Bay of Biscay Latrouite and Morizur (1988a) suggest there may be (Latrouite and Morizur, 1988a) showed little variation considerable annual variation in moulting, with “good” in moult increments between areas (Fig. 2). There are, and “bad” years, which might also bias moult frequency however, major sexual and areal differences in moult estimates. It is also possible that there are genetic differ­ frequency which are reflected in differences in annual ences between the stocks in the North Sea and the growth. Bennett (1974b) noted that, (a) in the English English Channel, or that growth differences are environ­ Channel adult male crabs moult more frequently than mentally controlled or induced. The warmer mean water females, but the opposite was the case in the North Sea, temperatures in the western Channel, compared with and (b) that the moult frequency of adult males in the the North Sea, may be the reason for the prolonged 92 D. B. Bennett ICES mar. Sei. Symp., 199 (1995)

45 y moulting period there, resulting in an increased moult­ ing frequency (Bennett, 1974b).

s 40 - Movements It has been known since the early 1900s (Williamson, 1900; Meek 1913) that C. pagurus can move consider­ able 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- S 30 - sen, 1979; Hallbäck, 1969), the English Channel (Ben­ nett 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-dis­ 80 100 120 140 160 180 200 tance directed movements by mature female crabs, and Premoult Carapace Width (mm) essentially local random movements by males. Figure 2. Comparison of the relationship between premoult In the North Sea, local inshore/offshore seasonal mi­ size and moult increment for male and for female C. pagurus grations were observed (Edwards, 1979), but female tagged and recaptured in the English Channel (Eng (Bennett, crabs also made extensive movements northwards along 1974b); Fr (Latrouite and Morizur, 1988a)), Bay of Biscay the east coast of England to southeast Scotland (Fig. 4). (Latrouite and Morizur, 1988a), and the North Sea (Edwards, A study in the English Channel (Bennett and Brown, 1965; Hancock and Edwards, 1967). ■ = Male - Channel (Eng); □ = Female - Channel (Eng); • = Male - Channel 1983) also showed some crabs, particularly females, (Fr); O = Female - Channel (Fr); ♦ = Male - N. Sea; O = making extensive movements moving mainly in a wes­ Female - N. Sea; A = Female - Biscay. terly 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

1.4 y 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 1.2 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 o> . £ 0.8 - - (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 •K 0 -6 - nomadic with the larger males ranging over greater distances than smaller ones (Bennett and Brown, 0.4 -- 1983). In contrast, females showed more directed movements, which seem to be one-way, and can be 0.2 -- 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 95 115 135 155 175 195 currents. Premoult Carapace Width (mm) Figure 3. Comparison of the relationship between premoult size and frequency of moulting (per annum) of male and female Catchability C. pagurus. Sources as per Figure 2. ■ = Male - Channel; □ = Female - Channel ; O = Female - Channel/Biscay ; ♦ = Male - In the western English Channel, 85% of the crab N. Sea 1959-1963; A = M ale-N. Sea 1965-1966;O = Female landings are made in the 6 month period from June - N. Sea 1959-1963; A = Female - N. Sea 1965-1966. to November, with female landings-per-unit-effort ICES mar. Sei. Symp., 199 (1995) Factors in the life history of the edible crab 93

5° 0° 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. SCOTLAND in the summer could be related to mating behaviour, or NORTH SEA the result of intraspecific competition with the abundant 55° female crabs for both food and space during approach and entry to the traps (Bennett, 1974a).

IRISH SEA 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 y WALES ENGLAND 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 i 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 50° 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 FRANCE 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 200m BAY OF LaCtiapelle\ BISCAY the eastern Channel, catches were always dominated by Rank male crabs. Latrouite and Morizur (1988b), using princi­ Figure 4. Summary of the movements of C. Pagurus in the pal component analysis on samples collected from the North Sea (Edwards, 1979), English Channel (Bennett and English Channel and the Bay of Biscay, showed similar Brown, 1983; Latrouite and Le Foil, 1989), and Bay of Biscay heterogeneity in both size and sex ratio. They demon­ (Latrouite and Le Foil, 1989). strated that most of the size variation could be explained by spatial factors, rather than by annual variation. (l.p.u.e.) being five times greater in these peak months than in the rest of the year (Fig. 5). These dramatic Stock density changes in catchability seem to be related to the repro­ ductive cycle. Ovigerous females overwinter without The distribution of C. pagurus is extensive from Norway feeding and are rarely caught in baited traps, accounting in the north down to the north African coast and into the for the low l.p.u.e. from December to May. Hatching Mediterranean. The major fisheries, however, are occurs in May-July and post-ovigerous females, vora­ around the British Isles and adjacent French coasts, and cious after a long fasting period, are readily caught in off Norway and Sweden. baited traps in June and July (Fig. 5). In addition to The population structure in the North Sea and the these changes in catchability there is also some immi­ English Channel shows quite marked differences, with gration by mature females onto the western mid-Chan- crabs in the English Channel having a much greater nel grounds, increasing the density. Female crabs caught maximum size. A distinctive feature of the English in the autumn have well-developed ovaries and high Channel fishery is the presence of very large male meat yields (Brown and Bennett, 1980). As the females “cock” crabs - up to 267 mm CW (Brown and Bennett, spawn in November/December l.p.u.e. falls dramati­ 1980). Male crabs over 180 mm are rare in the catches in cally as ovigerous crabs enter the overwintering non­ the North Sea. This difference in size also applies to feeding stage of their life cycle. female crabs, with average sizes in the English Channel Male l.p.u.e. is considerably lower than for females being between 160 and 175 mm, compared to averages 94 94 Figure 5.Figure top, Females = bottom. = Females top, . . Bennett B. D. C.pagurus

landings per unit effort from the logbook of a western Channel fisherman for the period 1976-1985. Males = = Males 1976-1985. for period fisherman the of Channel alogbook from the western unit effort per landings LPUE (KG/100 POTS) LPUE (KG/100 POTS' 200 250- 300- 400-1 350- 100 150- 100-, - 0 4 20 30- - 0 7 - 0 8 90- 10 A FEB JAN - - - - ' \ ' i 91 1982 o - 1981 —i H t- 1976 1976 t- H 91 o 1982 -o- 1981 96 x 17 -- 1978 -V- 1977 -x- 1976 MAR x 17 -- 1978 -V- 1977 -x- P MY U JL U SP C NV DEC NOV OCT SEP AUG JUL JUN MAY APR 93 f- 1984 -fr- 1983 1983 MONTH o 17 -A- 1979 •o- o- ■o - o - S, 94 fl 1985 -ffl~ 1984 1979 1979

50° Modelling The catch and effort statistics in the crab fisheries of Europe are poor, despite their long history and biologi­

Limit of cal study. Surplus yield modelling requires much better larvae survey statistics than those currently available in the UK. The Larvae >27m'J inability to age large crustaceans directly has made the estimation of mortality parameters difficult and has sti­ Crab fishing grounds fled 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/long­ Figure 6. Crab (C. pagurus) fishing grounds in the North Sea, term losses/gains in landings from a change in minimum English Channel, and Bay of Biscay, along with areas where larvae (>27 numbers m (zoea stages I-V and megalopae)) landing size (MLS)? The modelling approach taken with have been found. C. pagurus has therefore been relatively simplistic. 96 D. B. Bennett ICES mar. Sei. Symp., 199 (1995)

Hancock (1965, 1975) evaluated the MLS in the In an assessment of the North Sea crab fisheries, North Sea fisheries using the method of Gulland Addison and Bennett (1992) used the length cohort (1961). Landings and effort for the Norfolk (southern analysis (LCA) model of Jones (1974,1981). This model North Sea) fishery were examined by Hancock (1965), estimates population numbers from a length compo­ who showed how catchability varied seasonally in re­ sition of the catch using von Bertalanffy growth par­ sponse to behaviour associated with moulting and ameters. F at length can be calculated and yield-per- breeding. In using mark-recapture studies to estimate recruit (Y/R) and biomass-per-recruit (B/R) predictions population size and rate of exploitation he also high­ made for changes in fishing effort and/or minimum land­ lighted those life history factors which could bias recap­ ing size. This model is sensitive to the life history factors ture rates. Differences in moult frequency between the which may bias catch composition samples and growth sexes pointed up the need to consider males and estimates, and to natural mortality (Addison, 1989). females separately. The model used by Hancock esti­ The available knowledge of size at maturity and fec­ mated the survival of crabs between the old and the undity was used in a stock and recruitment model new minimum landing, sized and calculated the change (Addison and Bennett, 1992). In the absence of time in yield, which depended upon the relative values of series information on stock and recruitment the ration­ fishing (F) and natural (M) mortality. Attempts were ale of Bannister and Addison (1986) was adopted, using made to estimate these independently from catch com­ a range of parameter values and different assumptions positions and from tagging, but Hancock (1975) con­ about the likely form of the stock-recruitment curve. sidered that various factors of the life history, such as The results suggested that unless the stock-recruitment catchability changes, the complex growth, and emi­ relationship is of an overcompensatory nature, the Y/R gration made estimates of F and M unreliable. The approach may underestimate the benefits of an increase model was therefore run with a range of both par­ in MLS. ameters to evaluate relative changes in yield from in­ This assessment cautioned against unequivocal ac­ creases in minimum landing size. ceptance of the results, where there were some life For the English Channel, Bennett (1979) also history factors which were inadequately estimated. In attempted to estimate mortality rates from catch curves the Norfolk (southern North Sea) fishery the nature of and tagging. Annual growth estimates were used to split the seabed substrate is very different from that further population size frequency distributions into annual in­ north (Howard, 1980) and this may be a major factor tervals to produce “age” frequency distributions from determining the characteristically small size compo­ which total mortality (Z) could be estimated. While size sition of crabs and lobsters (Homarus gammarus (L.)) composition samples could be biased because smaller in this area. The LCA depends upon the assumption size groups were underrepresented, either because of that the sampled size compositions reflect the current partial recruitment to the fishable stock or because of a level and pattern of exploitation and it has been sug­ lower catchability of smaller crabs, the main problem of gested (e.g., Addison, 1986) that, in a situation like potential bias in the English Channel comes from the that off Norfolk, crustacean size compositions may not complex population structure and the question of stock respond to changes in the level or pattern of exploi­ identity. Bennett (1979) showed how different estimates tation in the same manner as those of finfish popu­ of Z could be calculated, depending upon the spatial and lations. temporal classification of the size composition samples. For example, a bias is introduced by the increased Management catchability of mature females after hatching and the immigration of large females onto the offshore western The European fishery for C. pagurus is currently Channel grounds, which gave a lower mortality in the managed on a national basis. The European Community autumn than in the spring, the opposite to that expected. does allow for a minimum landing size in its technical As it was not possible to quantify recruitment to the measures, but has not so far determined what the size fishable stock, a yield-per-recruit (Y/R) model was used should be. There are no effort controls, direct or in­ (Bennett, 1979). This calculated Y/R as the sum of catch direct, except in a minor way with local Sea Fishery weights at each age for a given recruitment from the Committee by-laws in England and Wales. recruit age to maximum age with exploitation patterns As a result of the studies in the English Channel varied to achieve changes in MLS. The approach was still (Bennett, 1979) and the reassessment in the North Sea dependent upon likely ranges of F and M estimated (Addison and Bennett, 1992) there have been major independently. However, the results from this assess­ changes in UK crab management. The estimation of ment generated management advice which changed the crab growth rates in the English Channel, which were UK approach to MLS management (see section on man­ higher than those in the North Sea and showed a agement). greater growth for males than females, coupled with the ICES mar. Sri. Symp., 199 (1995) Factors in the life history o f the edible crab 97

knowledge of the population structure and the Y/R protect the spawning stock, and reduce the risks of assessment, resulted in two fundamental changes in recruitment failure, this regulation for crabs seems irrel­ management by MLS. Legislation was enacted to allow evant. The mid-Channel autumn fishery for prespawning for both regional size limits, and a size limit for each sex. mature females, which has increased considerably in In 1986 the MLS in the eastern Channel was raised recent years, must pose some threat of recruitment over­ from 115 mm CW to 140mm CW for both sexes, and in fishing. The crabs seem to congregate in certain areas the western Channel the male limit was raised a further and to be highly catchable before they spawn. Such 20 mm to 160 mm CW. In the English Channel fishery, behaviour could leave the spawning stock very vulner­ large (>160 mm CW) male “cock” crabs are sold at a able to overfishing. However, this risk might be balanced premium price. In addition to justifying the larger MLS by the very low catchability of over-wintering ovigerous for males in terms of the higher Y/R resulting from their females (but this would be dependent upon the level of F faster allometric growth, there was also a higher econ­ on prespawners), and the suggestion from the recent omic return to be gained from preventing the landing of larval surveys (Thompson et al., 1995) that the spawning males >140 <160 mm CW at the lower value “hen” (all grounds may be somewhat more extensive than the females -I- small males) price (Bennett, 1979). existing fishing grounds. Recent discoveries of pres­ It has been assumed that the MLSs in force in the pawning aggregations offshore in the North Sea and English Channel exceed the 50% maturity size. This their exploitation are inevitably giving rise to fears of assumption was dependent on North Sea maturity data recruitment overfishing and the demise of traditional and needed to be checked. Current research on crab inshore fisheries which might be dependent upon recruit­ reproduction in the English Channel is aimed at estimat­ ment by larval drift inshore or by adult immigration. ing both size at maturity and fecundity. The MLS has There is considerable concern that crabs, particularly recently (February 1990) been increased to 125 mm CW over-wintering ovigerous females, are also vulnerable to around the rest of the UK coast (130 mm CW in South other fishing methods, e.g. beam trawling and scallop Wales). An exception was made for local areas in the dredging, and to other seabed uses, e.g. aggregate southern North Sea and the Irish Sea, where uncertain­ extraction and sewage disposal. A better understanding ties about certain aspects of the life history of crabs of the distribution of ovigerous crabs and their habitat justified leaving the MLS at 115 mm CW (Addison and requirements, behaviour, and vulnerability to capture, Bennett, 1992). Consideration is being given to the use damage, or disturbance with possible egg abortion is of escape gaps to enhance the efficacy of the MLS regu­ essential to an assessment of the risks of recruitment lations (Brown, 1982; Lovewell and Addison, 1989). failure from a combination of directed fishing and other The other regulations applied to C. pagurus in the UK activities. are the prohibition of the landing of ovigerous (berried) There are clearly gaps in our knowledge of the life and soft (recently moulted) crabs. Soft crabs have a poor history of C. pagurus. We are currently attempting to meat yield but, when they are a high proportion of the improve our understanding of the temporal and spatial catch during the moulting period, fishermen are tempted variability in population structure resulting from to land them. In the North Sea the moulting period is changes in catchability, which the moulting and repro­ reasonably well defined, but females moult before the ductive cycles impose. Tagging studies and larval males, and in the English Channel soft crabs can be surveys are investigating stock relationships and the found in most months of the year. Thus a closed season possible links between inshore and offshore fishing to prevent the catching of soft crabs seems inappro­ grounds through movements of mature females and lar­ priate. There is at present no practical objective test to val drift. Fecundity and size at maturity are being esti­ indicate what is a soft crab, and the regulation is often mated to ensure optimization of the MLSs in relation to flouted to the economic detriment of fishermen, mer­ the reproductive cycle. Fishing effort and power con­ chants, and consumers. This seems to be a problem tinue to increase in the UK pot fisheries, negating the which would be best solved by the industry itself, i.e. benefits of recent MLS increases which have been set to stop selling and buying soft crabs. take account of the spatial and sexual variation in crab The continuation of the ban on the landing of berried biology. The scope for further optimization of technical crabs seems a little incongruous, given that the catcha­ controls like MLSs is limited. Future management of C. bility of ovigerous crabs is low and, particularly, that a pagurus stocks is likely to depend upon fishermen and similar ban on the landing of berried lobsters was res­ managers grasping the nettle of direct effort regulation. cinded in 1966. This was done because the ban could not be justified as being of benefit to lobster recruitment, References since nothing was known about the stock-recruitment Addison, J. T. 1986. Density-dependent mortality and the relationship, and was in any case difficult to enforce relationship between size composition and fishing effort in (Bennett and Edwards, 1981). In terms of attempting to lobster populations. Can. J. Fish, aquat. Sei., 43:2360-2367. 98 D. B. Bennett ICES m ar. Sei. Sym p., 199 (1995)

Addison, J. T. 1989. Sensitivity of length cohort analysis to Hancock, D. A., and Edwards, E. 1967. Estimation of annual errors in input parameters. Annex 2 in Report of the Work­ growth in the edible crab (Cancerpagurus L.). J. Cons. int. ing Group on Nephrops Stocks. ICES CM 1989/Assess: 18. Explor. Mer, 31: 246-264. 163 pp. Harding, D. H., and Nichols, J. H. 1987. Plankton surveys off Addison, J. T., and Bennett, D. B. 1992. Assessment of mini­ the north east coast of England in 1976: an introductory mum landing sizes of Cancer pagurus L. on the east coast of report and summary of the results. Fish. Res. Tech. Rep., England. Fish. Res., 13: 67-88. MAFF Direct. Fish. Res., Lowestoft, 86: 1-56. Bannister, R. C. A ., and Addison, J. T. 1986. Effect of assump­ Howard, A. E. 1980. Substrate controls on the size composition tions about the stock-recruitment relationship on a lobster of lobster (Homarusgammarus L.) populations. J. Cons. int. (Homarus gammarus) stock assessment. Can. J. 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