Science Advisory Report 2008/036
Status of Basking Sharks in Atlantic Canada
- The life history characteristics of basking sharks are inadequately known, and key parameters such as growth rate, natural mortality and fecundity are assumed rather than measured. However, there is little doubt that the species is relatively unproductive and incapable of sustaining even modest mortality rates.
- Basking shark distribution appears to be restricted to temperatures between 6-16 °C, which implies that observations of basking sharks north of Newfoundland and in cold waters elsewhere are likely to be misidentifications of Greenland sharks.
- There is no directed fishery for basking sharks in Canadian waters. Observed bycatch in foreign fisheries peaked in the 1980s and early 1990s at about 150 mt per year, but has averaged only a few metric tons annually since 2000. Basking sharks are caught incidentally in domestic fisheries, with most observed bycatches having occurred in groundfish and redfish trawl fisheries. When scaled to total landings, total estimated bycatch has averaged 164 mt annually (corresponding to 164 basking sharks) since 1986.
- It is possible that bycatch is somewhat larger than estimated, since there has been little in the way of observer coverage of inshore fishing gear such as gill nets and cod traps.
- Estimates of absolute basking shark abundance from aerial surveys of whales in the Bay of Fundy, the Scotian Shelf/Gulf of St. Lawrence and Newfoundland waters suggest numbers of 4,200, 5,340, and 560, respectively, for a total of 10,100 in the summer of 2007. These estimates are uncertain due to the number of assumptions that were invoked.
- A life table analysis indicated that the intrinsic rate of basking shark population growth (r) in an unfished population is 0.040, which is near the maximum sustainable bycatch mortality. With Fcrit= 0.043, and the annual mean number of discards being 164, and assuming 100% mortality of discards, this would suggest that the average population size which could support the estimated number of discards Ncrit would be about 4,800. The best available estimate of population size for 2007 is above Ncrit.
- A Monte Carlo analysis based on the life history of basking sharks and discard data was used to evaluate recent trends in abundance. The results of this population model, which are consistent with the results of the life table analysis, suggest a 23% probability (about a 1-in-5 chance) that the population is decreasing, although the uncertainty associated with the model inputs is large. This result is more or less consistent with SPUE indices in U.S. waters that show no evidence of a decline since 1979.
- Given the life history characteristics of the basking shark, high discard mortality associated with bycatch could lead to population collapse. Therefore it is important that basking shark bycatch continue to be monitored. Measures to improve species identification accuracy in the observer program, record the numbers of individuals and sex in the bycatch, and to reduce discard mortality would be useful.
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