Effects of temporal closures and gear modifications on the population of dusky sharks in the northwestern Atlantic Ocean.

摘要

My objectives in this study were two-fold. First, I built an age-structured model to assess the effects of fishing on population trends for dusky shark. This model included sensitivity analyses to assess the effects of time/area closures, reduced mortality as a result of reduced soak times for the bottom longline fishery, full selectivity of age-zero animals, removal of Catch Per Unit Effort (CPUE) series and changes to other model parameters on overall population sizes. My second objective was to build a spatial model that evaluated the effects time/area closures and changes to other model parameters had on the population size of three life-stages (juvenile, subadult and adult) of the dusky shark.;Results showed that the impacts of fishing already imposed on the dusky shark would be difficult to overcome even with the implementation of time/area closures, gear modifications and/or catch and discards being reduced for another 20 years. Results of the base case, all scenarios and sensitivity analyses except for one (increasing virgin biomass) of the age-structured model indicated that the population of dusky sharks in the northwestern Atlantic Ocean is at less than 60% of virgin biomass. Recent work has shown that the Maximum Sustainable Yield (MSY) for dusky sharks may be well above 50% of the carrying capacity. Fisheries managers must determine whether the high depletion rates reported in these models suggest this species is overfished, and would therefore require long-term targets for population recovery to sustainable levels.;The base case version of the spatial model illustrated that the majority of total density (numbers) for all three stages (juvenile, subadult and adult) occurred in the closed region of the model, but the effects of fishing were still seen in the boundaries between open and closed areas. Sensitivity analyses showed that parameter values affected the densities and the redistribution of fishing effort for all three stages. However, all of the models (except for the open and closed versions) indicated that the highest densities were found in the closed portion of the model and that the redistribution of fishing effort was concentrated into a very small area on either side of the closures. Neonate densities in all models generally followed the same trends as total density, and recruitment densities varied very little throughout individual models.;Results of these models suggest that dusky shark populations have been heavily reduced and that research into the effects of time/area closures on dusky shark populations should continue. Fisheries managers should investigate new management options that will reduce the fishing mortality of dusky sharks caught as bycatch. Managers should also increase observer effort in fisheries that catch dusky sharks as bycatch and improve on recording and reporting by fishers and dealers. Future research should be aimed at determining ways to reduce fishing mortality rates for the dusky shark and improving our knowledge of parameters used in spatial modeling.