Research Document - 2001/067

Analytical Assessment of the Porbeagle Shark (Lamna nasus) Population in the Northwest Atlantic, with Estimates of Long-term Sustainable Yield

By Campana, S., Marks, L., Joyce, W., Harley, S.

Abstract

A virgin population of porbeagle in the NW Atlantic was fished intensively at catch levels of about 4500t per year in the early 1960s before the fishery collapsed 6 years later. The fishery appeared sustainable during the 1970s and 1980s when annual landings averaged 350t, and the population slowly recovered.. Catches of 1000-2000t throughout the 1990s appear to have once again reduced population abundance, resulting in very low catch rates and disturbingly low numbers of mature females. In 1998, an intensive research program on porbeagle was initiated with the support and funding of the shark fishing industry, and in collaboration with the Apex Predator Program of NMFS. Research to date has led to the development of a confirmed growth model, established the presence of a single stock in the NW Atlantic, suggested size- and sex-specific migration patterns, determined fecundity and maturity ogives by length and age, revealed highly specific temperature and depth associations, determined diet, and resulted in credible estimates for natural mortality rate (=0.10) which increase after sexual maturity (to 0.2 in females). The TAC of 850t introduced in 1999, based on scientific information available to that point, resulted in preliminary estimates of F0.1 yield, mortality and stock abundance. Nevertheless, it was acknowledged at the time that the F0.1 yield was probably not sustainable. The current assessment confirms the unsustainability of fishing at F0.1 for porbeagle, and indicates that a fishing mortality above 0.08 will cause the population to decline. A fishing mortality of 0.04-0.05 is required if the population is to be allowed to recover. Independent estimates of recent fishing mortality based on Petersen analysis of tag recaptures, Paloheimo Zs, and an age- and sex-structured population model all suggest that F is now about 0.20. A standardized catch rate analysis indicated that the relative abundance of young porbeagle in 2000 was 30% of its 1991 level, while the standardized catch rate of mature porbeagle declined to 10% of its 1992 level. Current population size appears to be at 10-20% of virgin levels. The 850t TAC of the past two years is close to the MSY of a healthy population. However, the current population is seriously depleted and will require a greatly reduced fishing mortality if recovery is to occur.

The major recommendations from this paper are:

1. More extensive baseline coverage of interior Fraser coho, especially in non-Thompson tributaries upstream of the Fraser-Thompson confluence are required to aid in the delineation of populations and Conservation Units, and provide more precise estimates of the distribution and numbers of interior Fraser coho in catches.
2. Rates of genetic exchange between generations and among populations need to be determined.
3. To enable more effective fisheries management, coho encounter and DNA based stock composition information should be used to develop a model of coho marine distribution and migratory timing.
4. Although benefits can be seen from the extreme fishery management measures taken in recent years, these measures should remain in place to permit populations the opportunity to rebuild.

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