Fisheries ecology of the brown crab (Cancer pagurus L.) in the Isle of Man

The brown crab or European edible crab Cancer pagurus is one of the most economically important crab species in European waters with landings of 49 263 t in 2013. C. pagurus is also one of the most important commercial species in the United Kingdom with landings of 28 778 t in 2013 worth approximately £33.5M at first sale. The global capture production of brown crab has increased markedly during the last two decades. Despite this, the crab fishery is relatively data poor in terms of the ecology and catch characteristics of this species, particularly in the Irish Sea. The aims of this study were to provide the information to fill science gaps needed to inform sustainable brown crab fisheries in the Isle of Man. This was done by studying the population structure, reproductive ecology and catch characteristics of the brown crab fishery, and by examining the impacts of scallop fisheries on brown crabs, as well as assessing the by-catch composition of the brown crab pot fishery. A combination of sampling techniques was used to collect data which included: baited pots between Spring 2012 and Summer 2013, dredge, trawl and shore surveys. The catch per unit effort (CPUE) of crabs caught in traps was strongly influenced by environmental (i.e., temperature, season, area) and fishery specific factors (i.e., pot volume). This study compared fishery dependent data (FDD) and fishery independent data (FID) to assess the CPUE estimates. Both of these data sources gave similar results which indicated that fisher-based surveys provide good quality information for future monitoring of the population provided that environmental and gear effects are accounted for. This study also determined the total exploitation rate of brown crab using both commercial and recreational fisheries data. The latter contributed only a small percentage of total mortality which is largely due to strict controls on total recreational fishing effort. Observations of the distribution of brown crabs around the Isle of Man indicated the presence of a reproductive migration. Ovigerous crabs were generally found in offshore areas. These egg carrying females varied in size from 134 to 215 mm CW and each individual carried an estimated 0.4-3.0 million eggs. However, examination of behavioural maturity indicated that the earliest presence of sperm plugs was at 110 mm CW while functional maturity was 134 mm CW. There appears to be a predictable trend across Europe in the minimum size of egg bearing which appears to be related to water temperature.Based on quantitative data collected throughout the dredge surveys the potential impact of scallop dredging on the brown crab population was estimated in terms of by-catch, damage and mortality estimates. This study indicated that female crabs were particularly vulnerable to scallop dredging around the North-West coast of the island in November when fishing effort was highest in this area. Estimates of the potential mortality associated with scallop dredging led to a lower and higher estimate of a potential annual crab by-catch mortality of between 11 t and 17 t respectively (assuming 45% mortality of the crab by-catch), which represented 2.2 - 3.4% of the commercial landings of brown crab for the Isle of Man. Thus at present levels of fishing (up until 2014) this would seem to contribute a relatively small proportion of total mortality to the crab population. Nevertheless, an extension of the scallop dredging closed season until the end of November in the area of the Targets fishing ground might be a useful conservation measure if scallop dredging activity were to increase in the future. By-catch in pot fisheries is poorly studied in general. A total of 43 by-catch species were found in crustacean pots lifted around the Isle of Man and the velvet swimming crab Necora puber was the most abundant by-catch species. This study identified significant spatial differences in the CPUE of by-catch species; the highest by-catch CPUE was recorded around the west coast of the Isle of Man. However, seasonal changes in by-catch were less important. There was a significant negative relationship between the two target species (brown crab and European common lobster). However, there was no significant relationship between by-catch composition and target species (a combination of crab and lobster) compositio