Catch and Stock Status of Porbeagle Shark (Lamna nasus) in the Northwest Atlantic to 2007

The abundance of porbeagle shark (Lamna nasus) in the Northwest Atlantic has declined substantially since the fishery began in 1961. In 2004, the population was designated as Endangered by Canada's COSEWIC, and in 2007, the species was proposed for listing under Appendix 2 of CITES. Although most of the porbeagle population resides in Atlantic Canadian waters, some porbeagle are caught outside of Canada's EEZ. Thus it is possible that porbeagle catches in unrestricted international waters could jeopardize Canadian attempts to allow recovery of the population. Therefore, the NAFO Fisheries Commission requested a review of historical and current catches and bycatches in both the NAFO Convention Area and the NRA, information on distribution and abundance, and identification of fishery areas or exclusion zones which might reduce incidental bycatch. Porbeagle landings since 1961 have been as high as 9000t, but have averaged less than 500t annually since the introduction of restrictive Canadian catch quotas in 2001. Most of the Canadian catch has been by pelagic longline. Landings in the NRA were reported differently to NAFO and ICCAT, and have been small and sporadic with the exception of 2005 and 2006, when they exceeded Canadian landings. The accuracy of the 2005 and 2006 NAFO statistics for porbeagle has been questioned, but in general, they probably under-report actual porbeagle catches. The current recovery plan for porbeagle places strict and monitored catch quotas of 185t on Canadian vessels at levels that are less than the MSY catch of 250t. If NRA catches (either reported or unreported) are substantial (>100t), then total porbeagle catches (including the Canadian catch) would put the porbeagle exploitation rate at unsustainable levels. Population projections indicate that the population would crash at catch levels exceeding about 300t. A forward-projecting age- and sex-structured population dynamics model was used to model the abundance and biomass of the population. A population viability analysis was used to project population recovery under various scenarios. Model variants place the present abundance at about 22% its size in 1961, and female spawner abundance at about 14% of its 1961 level. All models indicate that the population can recover if levels of human- induced mortality are kept below about a 4% exploitation rate, corresponding to a total catch of 185t. Although recovery rates vary among models, time scales are on the order of decades. Porbeagle are a cold-water temperate shark species, with well defined temperature limits. Therefore, porbeagle exclusion zones in the northwest Atlantic could be defined as latitudes between 38-48 °N and temperatures at depth of 2-14°C. Particularly sensitive areas for porbeagle are those associated with mating off southern Newfoundland, suggesting that the NRA near the Grand Banks is also a mating area. The fisheries most likely to catch porbeagle are pelagic and bottom longline gear, as well as gillnets.