Population dynamics of porbeagle in the northwest Atlantic, with an assessment of status to 2009 and projections for recovery

A forward projecting, age- and sex-structured life history model, fit to catch-at-length and catch per unit effort data to the end of 2008, was used to evaluate porbeagle (Lamna nasus) population dynamics in the northwest Atlantic. Four variants of the population model were evaluated, 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 population models predict recovery to SSN20% before 2014 if the human-induced mortality rate is kept at or below 4% of the vulnerable biomass. Under the low productivity model, recovery to SSNMSY is predicted to take over 100 years at exploitation rates of 4% of the vulnerable biomass. All models except that with the lowest productivity 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 SSNMSY on the order of decades.