Research Document - 2008/004
Status of Basking Sharks in Atlantic Canada
By Campana, S.E., J. Gibson, J. Brazner, L. Marks, W. Joyce, J.-F. Gosselin, R.D. Kenney, P. Shelton, M. Simpson and J. Lawson
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 and 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 mt annually (i.e., a few individual fish) 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.
None of the existing fish surveys provide an abundance index for basking sharks. An annual index derived from surveys for right whales in the Bay of Fundy indicated a sharp increase in abundance in the 1990s, followed by an equally abrupt decline to 2000. The apparent change in abundance was likely due to changes in distribution due to oceanographic factors, rather than mortality. 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, 5367 and 558 respectively, for a total of 10,125 in the summer of 2007. These estimates are uncertain due to the number of assumptions that were invoked, but particularly that associated with the proportion of time at the surface.
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. The Monte Carlo simulation model results indicate a median value of r of 0.032, of Fcrit of 0.035 and of Ncrit of about 5900 sharks, the latter value still being less than the 2007 population size estimate. 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 and kept to a minimum. 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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