Pacific walrus (Odobenus rosmarus divergens) reproductive capacity changes in three time frames during 1975–2010

Abstract

Reductions in sea ice and increases in air and seawater temperatures have been documented in the Arctic, making female Pacific walruses (Odobenus rosmarus divergens) vulnerable to changes in foraging, energy budgets, and reproductive fitness. The aim of the present study was to assess how walrus reproductive capacity has changed over a span of 35 years analyzing ovaries from three distinct time frames: 1975, 1994 to 1999 and 2008 to 2010. Ovarian weights and volumes, corpora lutea diameter, total number of corpora lutea and albicantia, and the percent of females ovulating in their current cycle were used to evaluate reproductive capacity. Ovaries were collected from walruses hunted by Alaska Native communities for subsistence purposes. There were no differences in ovarian weights or percent of quiescent females between 1975 and 2008 to 2010. Ovaries from 1994 to 1999 were significantly heavier, exhibited more corpora, and all females from this time frame were ovulating at the time of harvest. Reproductive capacity was limited during 1975, due to known density-dependent stressors; reproductive capacity increased during 1994–1999, as harvests increased and more resources became available, and in 2008–2010, females were as reproductively limited as those of 1975. The cause for this reduction in reproductive capacity is unknown, but maybe a result of multiple factors, including an increase in population size coincident with a decrease in carrying capacity, and cumulative stressors relating to sea ice loss, contaminants, and anthropogenic impacts.