Effects of sea ice on growth rates of an endangered population of gray whales

G Gailey,O Tyurneva,P Van Der Wolf,K Drozdov,I Zhmaev,Y Yakovlev,O Sychenko,V Vertyankin

doi:10.1038/s41598-020-58435-3

Abstract

The western gray whale population is endangered with approximately 175 individuals and 33 known reproductive females. Photo-identification studies were conducted from 2002–2017 during the gray whale foraging season off northeastern Sakhalin Island, Russia. Despite abundant prey resources, significant variation in whales’ body condition, inter-birth intervals and calf survival have been documented with limited understanding of factors that account for the observed variability. We examine sea ice concentrations at their known foraging grounds to define the maximum duration of a “foraging season”. We explore the relationship between foraging season length during a female’s pregnancy and post-weaning calf survival and reproduction. Approximately 77% of the variation in calf survival, which ranged annually from 10–80%, was associated with the duration of the feeding season while the mother was pregnant. Poor body conditions and prolonged inter-birth intervals of western gray whales have also been documented to coincide with shorter duration in feeding seasons found in this study. These results imply that shorter foraging seasons are associated with reduced energy intake by physically limiting the number of days gray whales can forage, and thus sea ice conditions may be one limiting factor affecting growth rates of this endangered population of baleen whales.

Highlights

The western gray whale population is endangered with approximately 175 individuals and 33 known reproductive females
We examine the variability of sea ice conditions on the western gray whales foraging grounds and explore potential associations with calf recruitment and, growth rates of this population
Given a pool of 33 known reproductive females, a median of 54% of the calves consistently returned to the Sakhalin feeding area in the subsequent 4 years and contributed to the population size of western gray whales

Summary

Introduction

The western gray whale population is endangered with approximately 175 individuals and 33 known reproductive females. Sea ice extent in the Bering Sea has been suggested to limit calf production in eastern gray whales with a significant decrease in number of calves during shorter feeding seasons when pregnant females were foraging. 70% of the inter-annual variation in the estimated number of northbound calves of eastern gray whales were explained by the average sea ice condition in the Bering Sea in the year prior to birth[19,31]. This suggests that existing pregnancies were impacted during poor foraging conditions compared to suppression of ovulation as suggested by Rice and Wolman[32]. We examine the variability of sea ice conditions on the western gray whales foraging grounds and explore potential associations with calf recruitment and, growth rates of this population

Methods

Results

Conclusion