Population Dynamics of the Porbeagle in the Northwest Atlantic Ocean

Abstract

A virgin population of porbeagles Lamna nasus in the northwest Atlantic Ocean supported annual catches of up to 9,000 metric tons (mt) in the early 1960s before the fishery collapsed in 1967. Low and apparently sustainable catches of about 350 mt in the 1970s and 1980s allowed the stock to partially rebuild before a new fishery arose in the early 1990s. The response of the population to this renewed fishing pressure has been unclear until now. However, a new population dynamics analysis suggests that population abundance has once again declined. On the basis of more than 140,000 length measurements, an extensive catch rate index, a confirmed growth model, and a catch-at-age matrix, it appears that at least 90% of the sexually mature population has been lost as fishing mortality has increased. Independent measures of fishing mortality (F) based on Petersen analysis of tag-recaptures, Paloheimo Zs, and a population model all suggest that fishing mortality was about 0.20 in 2000. Biological reference points based on life table analysis indicate that fishing at F _0.1 = 0.18 will result in population collapse, that F = 0.08 corresponds to zero population growth, and that fishing mortality at maximum sustainable yield is about 0.04. Porbeagles have a low pup production rate and mature considerably after the age at which they first appear in the fishery. In light of the very low numbers of mature females now found in the population, it is unlikely that even the strict quota management now in place will allow the population to rebuild quickly. However, the shark fishing industry has actively supported scientific research and conservation practices in recent years, suggesting that long-term sustainability may still be possible.