Research Document - 2012/096

Population dynamics of Northwest Atlantic porbeagle (Lamna nasus), with an assessment of status and projections for recovery

By S.E. Campana, A.J.F. Gibson, M. Fowler, A. Dorey, and W. Joyce

Abstract

As part of the International Commission for the Conservation of Atlantic Tunas (ICCAT) Shark Stock Assessment Meeting in Copenhagen, Denmark, in 2009, a stock assessment for Northwest Atlantic porbeagle shark (Lamna nasus) was prepared. A forward projecting, age- and sex-structured life history model was used, which was fit to catch-at-length and catch per unit effort data to the end of 2008, to evaluate porbeagle population dynamics. The assessment was also intended to assess the expected time frames for recovery under different management scenarios. This assessment was tabled at Fisheries and Oceans Canada’s 2012 pre-COSEWIC (Committee on the Status of Endangered Wildlife in Canada) review of porbeagle shark, although catch, discards and certain research results have been updated to the end of 2011.

Four variants of the population model are presented, all of which differ in their assumed productivity. The total population size is currently estimated to be about 22% to 27% of its size in 1961 and about 95% to 103% its size in 2001. The estimated number of mature females in 2009 is in the range of 11,000 to 14,000 individuals, or 12% to 16% of its 1961 level and 83% to 103% of its 2001 value.

All analyses indicate that this porbeagle population can recover at modest fishing mortalities, but that the time horizon for recovery is sensitive to the amount of human-induced mortality. All population models predict recovery to 20% of spawning stock numbers (SSN_20%) before 2014 if the human-induced mortality rate is kept at or below 4% of the vulnerable biomass. The model with the lowest assumed productivity predicts that recovery will occur if human-induced mortality is less than 4% the vulnerable biomass, but not at 8%. All other models predict recovery under higher exploitation rates. Under the low productivity model, recovery to spawning stock numbers at maximum sustainable yield (SSN_MSY) is predicted to take over 100 years at exploitation rates of 4% of the vulnerable biomass. These estimates are conditional on the assumed selectivity. Assuming the Shelf-Edge selectivity, Models 1, 3 and 4 (all of which fit better than low-productivity Model 2) predict that keeping the rate of human-induced mortality to less than 4% of the vulnerable biomass would be precautionary and would keep expected recovery times to SSN_MSY on the order of decades.

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