Population Trends of the Eastern North Pacific Blue Whale

Abstract

Blue whales (<italic>Balaenoptera musculus<italic>) were exploited extensively around the world and remain endangered. In the North Pacific their population structure is unclear and current status unknown, with the exception of a well-studied eastern North Pacific (ENP) population. Despite decades without whaling, recent abundance estimates for ENP blue whales indicate a slowed recovery, leading to speculation that their recovery may be hindered by fatal collisions with large vessels. I test this hypothesis by first estimating historical catches and then assessing the population trends with Bayesian population dynamics models under basic assumptions about historical and future ship strikes. I collated previously unreported Soviet catches and combined these with known catches to form the most current estimates of North Pacific blue whale catches. I used the knowledge that ENP whales produce a different song call than blue whales in the western North Pacific to split the conflated catches. The catches were split by estimating spatiotemporal occurrence of blue whales with generalized additive models fitted to acoustic call patterns, which predict the probability a catch belonged to the ENP population based on the proportion of calls of each population recorded by latitude, longitude, and month. When applied to the conflated historical catches, I estimate that ENP blue whale catches totaled 2,969 (95% credible interval 2,301 to 3,516) from 1905-1971, and amounted to 35% (95% credible interval 27% to 41%) of all catches in the North Pacific. The uncertainty in the acoustic data influence the results substantially more than uncertainty in catch locations and dates. These estimated catch series were then combined with abundance estimates and used to assess the recovery of ENP blue whales. I found the population has recovered to a median of 97% of pre-exploitation levels (credible interval 62% to 99%) and thus density dependence is primarily responsible for their slowed recovery. Further, current levels of ship strikes are not an immediate risk and thus ship strike mitigation would impact the population minimally. Indeed, vessel traffic would need to increase 3-5 times the current levels before the population is at risk. The recovery of ENP blue whale is a conservation success story and demonstrates that endangered cetacean populations can recover under careful management.