Research Document - 2003/007

Status of the Porbeagle Shark (Lamna nasus) Population in the Northwest Atlantic in the Context of Species at Risk

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

Abstract

Porbeagle sharks produce few offspring and mature at a late age compared to the age of first capture. This combination of life history characteristics makes porbeagle highly susceptible to over-exploitation. The ESU for porbeagle in the NW Atlantic is represented by a single, largely Canadian population inhabiting the area from Georges Bank/Gulf of Maine to Newfoundland and the Gulf of St. Lawrence.

The virgin porbeagle population 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 population slowly recovered during the 1970s and 1980s when annual landings averaged 350t. 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. Based on an extensive reconstruction of porbeagle shark abundance, all indicators of population size have declined substantially since 1961. Current population size is estimated to be 10-20% of that of the virgin 1961 population. All lines of evidence indicate that fishing mortality is largely or solely responsible for the decline in population abundance since 1961.

The most recent stock assessment indicates that catches averaging 1000t per year in the 1990s have resulted in an F of about 0.20. Life table analysis indicates that a fishing mortality above 0.08 will cause the population to decline. The current catch quota of 200-250t corresponds to a fishing mortality at or below MSY (F of 0.04-0.05), and will allow population recovery. Thus the population decline has ceased, is reversible, and will increase in numbers at a rate of about 2.5% per year. The maximum rate of increase in an unfished population is about 5% per year.

The current area of occupancy varies seasonally due to large-scale migrations. Summing across the yearly distributional range (Gulf of Maine, Scotian Shelf, southern NF, Gulf of St. Lawrence) gives a total area of about 425,000 km2. There does not appear to have been any change in area of occupancy or degree of fragmentation since 1961.

The current estimate of mature females in the population is 6075, which is about 10% that of the virgin population 3 generations earlier.

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