Research Document - 2008/071

Assessment of information used to develop a Recovery Potential assessment for basking shark Cetorhinus maximus (Pacific population) in Canada

By G. McFarlane, J. King, K. Leask
and L.B. Christensen

Abstract

Basking sharks (Canadian Pacific population) are now suggested for listing as Endangered under the Species at Risk Act. We assessed recovery potential for basking sharks in Canadian Pacific waters by considering current status, potential sources of human-induced mortality, and various strategies to mitigate harm and promote recovery. We used a simulation model to evaluate scenarios that span the range of plausible human activities that cause mortality. Basking sharks in Canadian Pacific waters are considered to be part of a North American Pacific coast population which migrates into Canadian waters in spring and summer and winters off California. We therefore assess scenarios for the whole Pacific coast.

Best estimates of current abundance range from 321 to 535 individuals. It is estimated that the decline from pre-exploited numbers exceeds 90%. It is believed that the bycatch of basking sharks in commercial fisheries limits current abundance. Other threats to the population (collisions with marine traffic, coastal development, ecotourism, etc) were identified, and mitigation proposals examined.

Specified recovery objectives that could be assessed through simulation modelling include a) rebuild to 1000 breeding pairs; b) attain 30, 40, 50, and 99% of carrying capacity (assumed equal to pre-exploitation numbers), and c) attain 30, 40, 50,and 99% of initial biomass (assumed to be biomass prior to exploitation). Recovery potential was estimated as the number of years required to attain the recovery objectives under four levels of human-induced mortality and evaluated using two plausible catch histories.

Using the best estimates of current abundance and stock decline, production model projections suggest that if a breeding population currently exists in the northeast Pacific Ocean, and no further human-induced mortality and changes to existing habitat occurs, that approximately 200 years are needed before population numbers will return to their unexploited states (Appendix C). If these animals are afforded complete protection, it will still take hundreds of years for the population to recover to 1000 breeding pairs. Recovery to 30% of the original biomass could happen within 45 years, if complete protection is afforded.  The fishing mortality that the population can sustain without suffering further decline from the 2007 population ranges from 10 to 17 individuals annually coast wide including Canadian and US waters.

Basking shark is a long lived species with a low rate of increase (i.e., Generation time of 22-33 years). The uncertainties in the projections of this report increases with time. To make progress in rehabilitating the basking shark population, will require government agencies to promote research and management activities for decades.

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