Estimating historical eastern North Pacific blue whale catches using spatial calling patterns.

Yulia V Ivashchenko,Trevor A Branch,Cole C Monnahan,Isabelle Charrier,Kathleen M Stafford,Erin M Oleson

doi:10.1371/journal.pone.0098974

Abstract

Blue whales (Balaenoptera musculus) 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 existing abundance estimates for the ENP population, it is difficult to estimate pre-exploitation abundance levels and gauge their recovery because historical catches of the ENP population are difficult to separate from catches of other populations in the North Pacific. We collated previously unreported Soviet catches and combined these with known catches to form the most current estimates of North Pacific blue whale catches. We split these conflated catches using recorded acoustic calls from throughout the North Pacific, the knowledge that the ENP population produces a different call than blue whales in the western North Pacific (WNP). 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, which totaled 9,773, we estimate that ENP blue whale catches totaled 3,411 (95% range 2,593 to 4,114) from 1905–1971, and amounted to 35% (95% range 27% to 42%) of all catches in the North Pacific. Thus most catches in the North Pacific were for WNP blue whales, totaling 6,362 (95% range 5,659 to 7,180). The uncertainty in the acoustic data influence the results substantially more than uncertainty in catch locations and dates, but the results are fairly insensitive to the ecological assumptions made in the analysis. The results of this study provide information for future studies investigating the recovery of these populations and the impact of continuing and future sources of anthropogenic mortality.

Highlights

The blue whale (Balaenoptera musculus) is an endangered species with three widely recognized subspecies found in the Southern Hemisphere (B.m. intermedia), Indian Ocean (B.m. brevicauda) and in the North Pacific and North Atlantic (B.m. musculus) [1,2]
At present a large population known as the eastern North Pacific (ENP) population feeds each summer within 30 miles of the California coast and is believed to range from the equator to the Gulf of Alaska [4]
The full acoustic models were selected for both ENP and western North Pacific (WNP) populations and used throughout the rest of the analysis. These model predictions depended on the month, but generally classified parts of the Gulf of Alaska, the west coast of the US, and the eastern tropical Pacific as being predominantly ENP (Figures 6 and 7)

Summary

Introduction

The blue whale (Balaenoptera musculus) is an endangered species with three widely recognized subspecies found in the Southern Hemisphere (B.m. intermedia), Indian Ocean (B.m. brevicauda) and in the North Pacific and North Atlantic (B.m. musculus) [1,2]. In the North Pacific, blue whales were hunted from 1905–1971 and despite decades without commercial catches the status of their recovery is unknown [3]. At present a large population known as the eastern North Pacific (ENP) population feeds each summer within 30 miles of the California coast and is believed to range from the equator to the Gulf of Alaska [4]. A formal assessment of their current abundance relative to historical levels has not been conducted, largely because of the difficulty in separating historical catches of the ENP population from catches from other populations in the western North Pacific (WNP)

Methods

Results

Conclusion