Research Document - 1999/158

An analytical assessment of the porbeagle shark (Lamna nasus) population in the northwest Atlantic

By S. Campana, L. Marks, W. Joyce, P. Hurley, M. Showell, and D. Kulka

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. Catches of 1000-2000t throughout the 1990s appear to have reduced the population again, despite the introduction of a 1000t TAC between 1997-1999. 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. This research program led to the development of a confirmed growth model, established the presence of a single stock in the NW Atlantic, provided preliminary maturity ogives by length and age, and resulted in a credible estimate for natural mortality rate (=0.10). A standardized catch rate analysis indicated that the relative abundance of porbeagle in 1998 was about 50% of its 1991 level, while the standardized catch rate of mature porbeagle declined to 30% of its 1992 level. Based on Peterson analysis of tag recaptures, recent stock abundance is about 15-20% of the size of the virgin population that was present in the 1960s. Yield per recruit analysis produced an F_0.1 target fishing mortality of 0.083, but indicated that Spawning Stock Biomass (SSB) is sensitive to even lower levels of F. The reference point at which the spawning population is maintained at 30% of its original level (a level at which the spawning population is more likely to be sustained) is F₃₀ = 0.067. Independent measures of fishing mortality based on catch curve analysis, Peterson analysis and mean weight in the catch all suggest that F has been around 0.11 since 1996. Such a level of F is about 33% higher than F_0.1, and based on a mean annual catch of 1130t per year since 1996, suggests a F_0.1 yield on the order of 850t. An independent calculation of minimum replacement mortality rate estimated from life table analysis indicates that a fishing mortality of less than about 0.07 is required if the spawning numbers are to be maintained. Porbeagle have a low pup production rate and mature considerably after the age they first appear in the fishery. In light of the very low numbers of mature females now found in the population, it is important to protect them, possibly by restricting access to areas and/or seasons where large females are present.

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