No evidence for recovery in the population of sperm whale bulls off Western Australia, 30 years post-whaling

Abstract

The global sperm whale Physeter macrocephalus population has been protected from large-scale commercial whaling for >25 yr, yet there is no clear evidence of recovery in any heavily exploited stock. This may indicate that whaling has long-term demographic effects on this species or that other endogenous or exogenous processes are inhibiting population growth. This study investigates the status of mature sperm bulls off Albany, Western Australia, a population reduced through whaling by 74% between 1955 and 1978. We conducted an aerial survey designed as far as possible to provide an index of abundance comparable with that derived from the whale ‘spotter’ planes employed by the Albany whaling company from 1968 to 1978, using the number of sperm bulls seen on each morning flight as a comparative index between bulls seen historically and in 2009. The mean number of sperm bulls seen on transect in 2009 was 2.43 (95% percentile interval [0.96, 6.08]); this increased to 3.38 (95% percentile interval [1.30, 7.60]) when sightings off transect were included. Both 2009 point estimates were lower than the mean (±SE) number seen in any of the years between 1968 and 1978, which ranged from 6.30 (±1.18) in 1976 to 12.45 (±1.83) in 1968. The lack of recovery in the population of bull sperm whales off Albany, despite full protection, is of concern and adds weight to the growing body of evidence that suggests that sperm whales may not be recovering effectively from past exploitation.