The effect of age and sex selective harvest patterns for baleen whales

Abstract

The taking of calves and females accompanied by calves is prohibited under the original and current forms of the Schedule of the International Whaling Commission. Proposed regulations under the Revised Management Scheme would reduce catch limits according to the proportion of females in the landings when females make up more than one-half of the landings. The implications of regulations on age, sex and reproductive status were explored by examining the relative impacts of baleen whale harvests with different age, sex and female reproductive status selectivity patterns using a matrix population model. The effects of 11 harvest patterns with varying selectivity for females and calves were measured by computing the fraction of the population that would have to be killed to reduce the growth rate of the population model to zero and the corresponding fraction of the population that would be landed (harvest fraction). The harvest impact per whale landed was measured for each selectivity pattern by expressing the harvest fraction as a percentage of the value of that fraction for a harvest random across age, sex and reproductive status. The harvest impacts per whale landed of the 11 patterns ranged from 64% greater to 29% lower than a random harvest. The patterns with the lowest harvest impact per whale landed were the pattern consistent with the IWC Schedule of prohibiting harvest of calves and females accompanied by calves, and the pattern of harvesting only calves. The harvest selectivity patterns which increased the vulnerability of females had the greatest impact. Relative to the IWC’s Revised Management Procedure, this increased female vulnerability was roughly compensated for by the decrease in catch limits as the proportion of females in the catches increased.