Research Document - 2005/053
Status and Recovery Potential of Porbeagle Shark in the Northwest Atlantic
By Gibson, A.J.F., and S.E. Campana
Abundance of porbeagle shark (Lamna nasus) in the Northwest Atlantic has declined and, in 2004, the population was listed as endangered by COSEWIC with the recommendation that it be listed under Schedule 1 of Canada’s Species at Risk Act. If listed, activities that would harm the species would be prohibited and a recovery plan would be required. Before the listing decision is made, decisions on permitting incidental harm and in support of recovery planning need to be made. In support of this process, this document was prepared to evaluate the present status of porbeagle and to assess expected time frames for recovery under different management scenarios. Towards this objective, we used a forward projecting, age- and sex-structured life history model, fit to catch-at-length and catch per unit effort (CPUE) data to the end of 2004, to evaluate porbeagle population dynamics.
Four variants of the model are presented. One of the models is fit to standardized CPUE series for mature and immature porbeagle, whereas the other three models are fit to the un-standardized CPUE expressed as weight per hook. These later three models differ in their assumed productivity. The models place the present abundance in the range of 12.2% to 24.3% its size in 1961, and female spawner abundance at about 7% to 15% of its 1961 level. Estimated female spawner abundance in 2005 ranges between 77% and 92% of the female spawner abundance in 2002. The three models using the un-standardized CPUE indicate that total abundance is roughly unchanged since 2002, with corresponding increases in the abundance of immature fish.
Reproductive rates could not be reliably estimated with the models, so model scenarios with lifetime maximum reproductive rates in the range of 1.6 to 2.6 spawners/spawner, thought to be appropriate for this population, were investigated. All models indicate that this population can recover if levels of human-induced mortality are kept low. Although recovery rates vary among models, time scales are on the order of decades. Using deterministic population projections, the integrated CPUE models predict recovery to 20% of the unfished equilibrium biomass to take about one to three decades if human-induced mortality is kept to about 2% to 4% of the vulnerable biomass. Recovery to the level that produces MSY is expected to take much longer. Presently, a 5-year management (recovery) plan is in effect to the end of 2007 with a quota of 250t. Simulations based on the most pessimistic model (lowest starting abundance) indicate that managing under this plan until the end of 2007 will only slightly delay recovery in comparison with fishery closures implemented in 2005. This strategy would provide an opportunity for uncertainties in the estimates of population size and productivity to be resolved, including recent increases in CPUE that are not well predicted by the model, and discrepancies between the model and tagging data, the later suggesting higher survival rates than those predicted by the model.
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