ASSESSING THE ABUNDANCE OF BASKING SHARKS (CETORHINUS MAXIMUS) IN ATLANTIC CANADIAN WATERS

Abstract

The Atlantic Canadian population of basking sharks (Cetorhinus maximus) is designated Special Concern by COSEWIC. Little is known about their population size or life history, though they are considered to have long life histories and be inherently unproductive. Estimating population size from aerial surveys gave mean abundances ranging from 3,600 to 7,146 in 2007 and from 13,544 to 27,682 in 2016, depending on assumptions. The estimated median intrinsic rate of population increase was 0.0197, and potential population growth became negative if historical bycatch was accounted for. These population growth rates cannot account for the increase in abundance predicted from aerial survey data. Substantial migration into Canadian waters and/or a much more productive life history would be needed in conjunction. These results highlight the need for continued monitoring, potential improvement in methods to estimate abundance, and the development of non-lethal sampling for determining life history parameters.