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 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.