CSG 15 Research and Development Final Project Report (Not to Be Used for LINK Projects)

DEPARTMENT for ENVIRONMENT, FOOD and RURAL AFFAIRS CSG 15 Research and Development Final Project Report (Not to be used for LINK projects) Two hard copies of this form should be returned to: Research Policy and International Division, Final Reports Unit DEFRA, Area 301 Cromwell House, Dean Stanley Street, London, SW1P 3JH. An electronic version should be e-mailed to [email protected] Project title Edible crab spawning grounds in the English Channel DEFRA project code MF0227 Contractor organisation CEFAS and location Pakefield Road Lowestoft Suffolk NR33 0HT Total DEFRA project costs £ . Project start date 01/04/02 Project end date 31/03/2004 Executive summary (maximum 2 sides A4) The most important edible crab (Cancer pagurus) fishery in Europe is in the English Channel, worth at least £11 million annually, of which more than 70% is landed currently into England and Wales. The main area of the crab fishery extends westwards from the Isle of Wight along the south coast, with the most productive grounds for English and Channel Islands vessels being located around the south Devon peninsula out into mid- Channel. Although edible crab fisheries are managed primarily by regional minimum landing sizes, stock structure in the English Channel is still poorly understood. This limits the applicability of standard stock assessment methodology and hinders scientific advice on the appropriate scale for managing crab fisheries. Information on stock structure is also essential to provide an objective evaluation of the impacts on crab spawning stock of proposed aggregate extractions from the fishery area. The aims of this project were to describe the spatial distribution of larvae in the English Channel in 2002 and 2003, to compare these distributions with the results from previous surveys, to determine whether spawning female crabs are distributed in the same areas from year to year and evaluate the implications of these results for stock structure of crabs in the Channel, in conjunction with information on seabed characteristics, hydrographic information, and new information on genetic variation in edible crab populations. On the first research vessel survey in early May 2002, a total of 110 stations was successfully completed covering the western English Channel and eastern Celtic Sea area. On the second survey in May 2003 a total of 163 plankton stations was completed again covering the main crab fishing grounds in the western Channel together with the eastern Channel and the Southern Bight of the North Sea. Plankton samples were preserved in 4% buffered formalin, and Cancer pagurus larvae were identified and allocated to one of six development stages (five zoea and one megalopa). CSG 15 (9/01) 1 Project Edible crab spawning grounds in the English Channel DEFRA MF0227 title project code The surveys showed that there are two main concentrations of zoeae 1 larvae in the Channel which have been consistent features in crab larvae surveys over a period of 22 years, indicating the presence of important edible crab spawning grounds: (i) an extensive area in the mid-western Channel centred around 4ºW, 49º30′N and extending towards the English coast (ii) from the Isles of Scilly extending in a north-easterly direction off the north Cornish coast between 50º and 51ºN, roughly coinciding with the area known as the Trevose Grounds. Spawning also occurs south of the survey area, off the Brittany peninsula and in Biscay, and there may be some movement of larvae from these areas into the western Channel grounds. Larvae distributions in the survey area appear to be determined by a number of factors. In general crab larvae were aggregated in areas where there are significant fisheries, which in turn are based mainly on mature female crabs. Evidence suggests that spawning female crabs show a preference for coarse sands and gravels in which to incubate their eggs, but the distribution of crab larvae appears to be determined by local hydrographic features as much as the distribution of such substrates. Historical tagging data from both British and French studies suggest that mature female crabs in the Channel area undertake long distance movements in a westerly direction against the prevailing residual currents, but larvae appear to be retained within the western Channel region and there is no evidence of significant movement of larvae eastwards. There appears to be a central Channel zone, roughly between 0º and 2ºW, where there is little evidence of larvae production despite the fact that there are extensive crab fisheries in the area. Despite a series of nine plankton surveys over many years in the eastern Channel between early April and mid-June the picture remains unclear. Hatching in the eastern Channel is clearly later by several weeks than in the west, and the 2003 survey was too early to sample this. A small concentration of zoeae seen in the Dover Straits area in 2003 is not conclusive support for the observation of an area of high production at the eastern end of the Channel as found in a previous major Cancer pagurus survey in June 1989. In summary, local hydrographic features suggest a separation of the spawning stocks in the eastern and western Channel and it is unlikely that the major spawning grounds in the west provide larval recruitment to the east, mainly because of the distances required for the larvae to be transported before this can occur. This raises the question of whether the western and eastern Channel crab populations are genetically distinct. Results emanating from a recent Defra contract, Population genetics of shellfish in British waters, suggest that there is significantly greater genetic variation between local areas than might have been predicted from historical tagging studies of movements of crabs between areas. CSG 15 (9/01) 2 Project Edible crab spawning grounds in the English Channel DEFRA MF0227 title project code Scientific report (maximum 20 sides A4) Introduction Edible crab (Cancer pagurus) fisheries occur around all coasts of the British Isles and are one of the most valuable fisheries in the United Kingdom (Lee, 2003). The most important crab fishery in Europe is in the English Channel, fished primarily by vessels from England and Wales, France and the Channel Islands. It is worth at least £11 million annually, of which more than 70% is landed currently into England and Wales. The main area of the crab fishery extends westwards from the Isle of Wight, along the south coast, with the most productive grounds for English and Channel Islands vessels being located around the south Devon peninsula out into mid-Channel (Figure 1a). Landings by vessels from France confirm this pattern of distribution of the main stocks (Figure 1b). Edible crab fisheries are managed primarily by regional minimum landing sizes through both European Union and UK national legislation. Whilst much is known about aspects of crab biology which underlie these regional minimum sizes (Bennett, 1995), edible crab stock structure in the English Channel is still poorly understood. This limits the applicability of standard stock assessment methodology, and hinders scientific advice on the appropriate scale for managing crab fisheries. Information on stock structure is also essential to provide an objective evaluation of the impacts on crab spawning stock of proposed aggregate extractions from the fishery area. The aim of this project was to describe the spatial distribution of larvae in the English Channel, compare this distribution with the results from previous surveys, especially a large scale larvae survey undertaken in 1989, and so determine whether spawning female crabs are distributed in the same areas from year to year. In particular, it was important to try and ascertain whether the high density of early stage larvae found in 1989 in the Eastern Channel (Thompson et al., 1995) was a regular feature, as this result appeared to conflict with known information on edible crab migrations. Previous tagging studies had shown that whilst male crabs make only local movements, throughout most of the English Channel mature female crabs moved from east to west, these movements taking on a more southerly orientation in the extreme west (Figure 2). Notwithstanding the usual problems with tagging studies, which often describe the distribution of fishing effort as much as the movements of tagged animals, the tagging returns had been interpreted as part of the spawning strategy of the crab, allowing the planktonic larvae to drift back in the prevailing currents to the areas of maternal origin (Bennett and Brown, 1983; Cuillandre et al., 1984; Latrouite and LeFoll, 1989). In a previous similar study on the east coast of England (Defra contract MF0224, Edible crab spawning grounds off eastern England), the main crab spawning areas were consistent across years but the crab larvae distributions appeared to be driven primarily by hydrographic features rather than the distribution of suitable seabed habitats for mature egg-carrying females (Eaton et al., 2003). The results of contract MF0224 shed doubt on previous interpretations of stock structure based on analysis of movements of tagged mature female crabs, and it was important therefore to carry out similar investigations into the distribution of crab larvae in the English Channel and the factors that influence that distribution. The specific scientific objectives agreed for this project were: 1. To describe the distribution and abundance of larvae of the edible crab (Cancer pagurus) in the English

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