Date of Award
Doctor of Philosophy in Biological and Environmental Sciences
David A. Bengtson
Red crab (Chaceon spp.) fishery resources exist on both sides on the North Atlantic, and the fisheries that harvest these resources seek to maintain their sustainability. To be able to conduct fishery assessments with less uncertainty, resource managers need a better understanding of the life history characteristics of the species, more recent information on the abundance and distribution of the resource, and finally reliable estimates of the levels of exploitation and the effects of harvesting of the resources. This study contributes to the body of knowledge of the resource in the western Atlantic resource and red crab fishery off the Northeast US coast, and of the resource in the eastern Atlantic and emerging red crab fishery in the Cape Verde Islands off West Africa.
One aspect of this study assessed the characteristics of sea-sampled catches in the western and eastern Atlantic as indicators of population characteristics. In the western Atlantic, I investigated the past and current status of the population, changes in the size distribution of the red crab population due to fishing and the effects of discarding. I used data from a tagging study initiated in 2010 and 2004-2005 trawl survey by Dr. Richard Wahle of the University of Maine. I have supplemented these data with National Marine Fisheries Service trawl-survey data from 1974, and data that I collected in the summer of 2012 and 2013 aboard red crab commercial fishing vessels. In the eastern Atlantic, I used data collected during four exploratory fishing trips on a virgin resource located in the Cape Verde Islands.
In general, the life history characteristics of red crab resources are similar in the western and eastern Atlantic in terms of carapace width and size-frequency iii distribution. Males are always larger than females. In the western Atlantic I found that there has been a size increase in the harvested male population between 2010 and 2012, and I hypothesize that this could be due to a decrease in fishing effort overall, or is a response to an effort to return the smaller males to the sea. I also documented that there is a discard mortality of male and female red crabs, but the level of this mortality is small, and interestingly, corresponds to a level reported by a previous researcher based on “hotel experiments.” The fishery in the western Atlantic, although certified sustainable by the Marine Stewardship Council, experiences the effects of moderate fishing intensity on males, and is considered fully exploited. I observed a change in the selectivity of fishermen when sorting the red crab catch from the trap, as the discard probability function has become steeper and resembles a more knife-edge selection process. In the northern area, the proportion of total discards to landings decreased from 1.35 in 2002 to 0.27 in 2012, and females became the dominant portion of the discards. In the southern area, the proportion of total discards to landings decreased from 0.82 in 2010 to 0.10 in 2012, with females representing the dominant portion of the discards. The discard mortality study for females suggests that the fishing effects have been negligible on the female red crabs, as discard mortality is estimated to be minimal at 5%.
As for male red crab, the result of a length-based catch curve analysis (LCCA) indicates evidence of higher fishing mortality (F) in recent years (2012) for the northern area (F = 0.47) compared to the southern area (F = 0.20). Although this finding reflects the current fishery status, the estimated F value might be overestimated due to the more selective nature of trap surveys (the catch is aggregated toward the mean carapace width) and these values are valid only for the localized areas (600-800 meters) where the fishing operations occur. Although there has been concern regarding mating success of females due to lack of large males, I found no evidence to support this hypothesis. I found that the probability of successful mating in the logistic model increased as carapace width increased. I also made a comparison of mating success from the 1974 to the 2004-05 surveys for the southern area, and that indicates a higher probability of mating success in the more recent years.
I separated the red crab population in the northwest Atlantic into two sub-stocks, with Hudson Canyon as a bio-geographic barrier dividing the population along the upper continental slope off New England (north) and the mid-Atlantic (south), so as to be able to differentiate effects of fishing on the older mature fishery in the north and east from the newer fishery in the south. The results of a biomass dynamic model used to estimate maximum sustainable yield (MSY) found that the total MSY is estimated to be 1797 mt; this level is slightly higher than the current Total Allowable Landing (TAL) that was set at 1775 mt. The average fishing mortality in the northern area (0.13) was lower than that in the southern area (0.17), and both values are lower than the values estimated from the LCCA analysis. The F/FMSY and B/BMSY ratios from north and south areas indicate that the red crab resource is neither overfished nor is overfishing occurring in the most recent year (2012).
In the eastern Atlantic, I noted a low CPUE for red crabs in Cape Verde, and that indicates a low fishable biomass (377 MT). This estimated biomass will support a MSY of only 28 MT or 18% higher (with the addition of the area around Brava and v Fogo Islands) and this combined with the low CPUE will not support an economically viable and biologically sustainable fishery.
Syuhada, Imam, "THE BIOLOGY AND POPULATION ECOLOGY OF DEEP-SEA RED CRABS, CHACEON SPP. IN THE NORTH ATLANTIC OCEAN" (2014). Open Access Dissertations. Paper 245.