Increased presence of mammal-eating killer whales in the Salish Sea with implications for predator-prey dynamics

A long postreproductive life span is a shared trait among genetically distinct killer whale populations

The extended female postreproductive life span found in humans and some toothed whales remains an evolutionary puzzle. Theory predicts demographic patterns resulting in increased female relatedness with age (kinship dynamics) can select for a prolonged postreproductive life span due to the combined costs of intergenerational reproductive conflict and benefits of late-life helping. Here, we test this prediction using >40 years of longitudinal demographic data from the sympatric yet genetically distinct killer whale ecotypes: resident and Bigg's killer whales. The female relatedness with age is predicted to increase in both ecotypes, but with a less steep increase in Bigg's due to their different social structure. Here, we show that there is a significant postreproductive life span in both ecotypes with >30% of adult female years being lived as postreproductive, supporting the general prediction that an increase in local relatedness with age predisposes the evolution of a postreproductive life span. Differences in the magnitude of kinship dynamics however did not influence the timing or duration of the postreproductive life span with females in both ecotypes terminating reproduction before their mid-40s followed by an expected postreproductive period of about 20 years. Our results highlight the important role of kinship dynamics in the evolution of a long postreproductive life span in long-lived mammals, while further implying that the timing of menopause may be a robust trait that is persistent despite substantial variation in demographic patterns among populations.

Estimating the stock size of harbor seals (Phoca vitulina richardii) in the inland waters of Washington State using line-transect methods

Harbor seals (Phoca vitulina richardii) in the inland waters of Washington were reduced by predator control programs in the twentieth century, but stocks have rebounded since being protected in the 1970s. Three management stocks are recognized, but there is little information on their current abundance. We conducted 38,431 km of aerial line-transect surveys throughout the range of these stocks in 2013–2016, sighting a total of 4,678 groups of harbor seals. Line-transect analysis with Beaufort sea state as a covariate provided estimates of the number of seals in the water. We then incorporated tagging data from 15 instrumented seals to develop correction factors, both for seals missed in the water while diving, and those that were on shore. Tagging data were modeled with generalized linear mixed models to provide estimates of the proportions diving and hauled out. After applying these correction factors, we estimated that the Hood Canal stock contained 1,368 seals (CV = 16.8%), the Southern Puget Sound stock contained 1,976 seals (CV = 20.5%), and the Washington Northern Inland Waters stock contained 7,513 seals (CV = 11.5%). This study presents a non-traditional approach to estimating the size of Washington inland waters harbor seal stocks, which may also be applicable to other species for which survey and tagging data are available.