Fisheries biology of the edible crab (Cancer pagurus) in the Kattegat and the Skagerrak - implications for sustainable management

Abstract: The European edible crab (Cancer pagurus) is exploited to a varying degree in its area of distribution. Landings in United Kingdom and Ireland may have peaked around year 2000, reaching 25 000 and 13 000 tonnes, respectively but have slightly decreased since. On the other hand, landings in Norway have quadrupled in a decade, now reaching 8000 tonnes, which is due to industrial and governmental funding and subsidies, and an increased northward crab distribution. Official commercial landings in Sweden are approx. 150 tonnes. Compared to countries such as UK and Norway the Swedish landings are low but the crab fishery plays an important role for coastal fishermen along the Swedish west coast earning a living from several alternative fisheries through out the year. The research covered in this thesis is focused on management prerequisites for sustainable exploitation of the edible crab in the Kattegat and in the Skagerrak. I have studied if the present management strategy (escape gaps) is adequate for a sustainable recruitment, and I have also investigated key knowledge for management of commercial stocks, the stock structure. I have also estimated to what degree the stock is exploited and I have performed two independent stock abundance assessments. I present data (paper I) that describes at what size female and male edible crab reach sexual maturity, based on characters as sperm presence, gonad development and morphometric analyses of abdomen or chela. These data indicated that the present indirect minimum landing size maintained by escape gaps size is not adequate. A minimum landing size for females need to be set to 132 mm to allow 50 % of the individuals to be physiologically mature before being fished. Considering indications that even larger sizes are necessary for spawning, 140 mm CW is a more appropriate MLS to strengthen the spawning stock. Males were shown to mature at smaller sizes as 50 % of the males were mature at 120 mm CW. Furthermore small males, despite being physiologically mature, might not have the same chance for copulation as larger males. For practical purposes, in addition to the copulation advantage of larger males, 140 mm CW is the recommended MLS also for males. Therefore the escape gap size needs to be increased to 90 mm, from present 75 mm. Together with colleagues in paper II I have looked at the migration potential, using markrecapture technique of the crab along the Swedish west coast, as an indication of stock structure. The potential for crabs to move between geographical areas, is one component of the connectivity between areas, and a high migration potential indicates high connectivity which may lead to wide stock distributions and no distinct stock structure. Our study showed a high propensity for migration among females which in many cases moved distances of 100 km, or even as far as 228 km whereas males most often were found within shorter distances from the release point also after several years. The direction of the long-distance female migrations >20 km was predominately towards the south. This southward migration of females may compensate for the northward larval dispersal with the prevailing coastal current. In Paper III I investigated together with colleagues the genetic population structure of edible crab stock in Swedish waters using microsatellite DNA. In this study we found a a lack of spatial and temporal genetic differentiation between crab stocks in Kattegat, Skagerrak and even up to Ålesund, Norway – a waterborne distance of 1300 km. Local management of the edible crab can be considered and implemented whereby stakeholders take a precautionary approach such as implementing landing size restrictions, not fishing below a certain local biomass or above a defined fishing mortality. In paper IV we present an estimate of the total abundance of crabs on the Swedish west coast, based on a combination of experimental fisheries to get effective fishing area and catch per unit effort data, and GIS modelling of available crab habitat. In a mark-recapture experiment in the 6 Koster fiord area we estimated the effective fishing area around crab pots targeting edible crabs i.e. the area from where the crabs are captured if the pot was 100% effective at catching crabs, to 2293 ± 1137 m2 (mean ± 95% confidence interval), corresponding to a circle with a radius of 26.6 ± 6.3 m. In a separate experimental fishery in the Fjällbacka archipelago (35 km south) we estimated catch per unit effort at two depths strata (15-18 m and 25-30m) at 7 locations during two seasons. Using the estimated effective fishing areas around the string we calculated an average density of 0.0038 ± 0.0015 crabs/m2. The area of suitable crab habitat in the Swedish part of the Skagerrak and Kattegat, 10 and 40 meters water depth, and with a bottom consisting of bedrock, stone, gravel or sand, was estimated to 4142 km2. This suitable crab habitat combined with the density estimates from Fjällbacka would indicate that the catchable population of crabs on the Swedish west coast would be approximately 10-22 million (95 % confidence interval) of crabs. In paper V we assessed stock indicators such as fishing mortality, stock abundance and egg production. The assessments were based on estimations of growth parameters and resampling of parameter values in a length cohort analysis (LCA). The fishing mortality of females was larger than for males both in Kattegat and in Skagerrak. This pattern can be explained by the higher market price for females, so that fishermen choose female dominated grounds and/or land more females. Another possibility is that the catchablity of females in general seems to be higher during the main landing autumn period. The fishing mortality in the investigated area seems to be low. The stock biomass of edible crabs available for the Swedish fishermen in Kattegat and Skagerrak is estimated to 4-8 million edible crab or 1600-2600 tonnes. This stock estimate is based on official data on the Swedish commercial landings and estimated recreational landings, i.e. approx. 400 tonnes in total, and is highly dependent on the total landings and on the input values of growth parameter and natural mortality. The status of the edible crab stock in the Kattegat and the Skagerrak appears to be good. The fishing mortality is low and no trend (decreasing or increasing) in logbook landing per unit effort (LPUE) can been seen over the last 13 years. Since 2004, using landing and effort data from logbooks for vessels > 10 m, the LPUE is on average around 2.0-2.5 kg/pot. This thesis provides new and increased knowledge concerning the sexual maturity, migration potential, and genetic population structure of the edible crab in Kattegat and Skagerrak and provides the first estimates on the total stock based on LCA modelling as well as an stock estimate using crab density and suitable crab habitat.