Determining local sex ratios and sex-specific predation by harbor seals

Investigators: Dietmar Schwarz and Alejandro Acevedo-Gutiérrez (WWU), and Austen Thomas (Smith-Root Inc.)

Harbor seals appear to have large impacts on Chinook and coho, with predation on juvenile salmon being of the greatest concern. Consumption models that assess the impact of seal predation assume that all seal individuals consume prey in equal proportions. However, in a recent study conducted by Western Washington University (WWU) labs that combined molecular seal sex-ID and prey DNA metabarcoding, determined that male seals consumed significantly higher proportions of salmon than female seals at two different estuarine haul-outs. In addition, haul-out locations had distinct seasonal changes in sex ratio that were repeated in two consecutive sampling years. The combination of a male bias for salmon with the concentration of higher numbers of males at locations and times crucial for outmigrating salmon may result in higher than estimated impacts of harbor seals on salmon populations. Prey consumption estimation models may need to account for sex-specific predation if local sex ratios are consistently skewed within the modelled region.

This project expands the molecular sex-ID of scat to all haul-out sites throughout the Salish Sea of Washington for which scat prey DNA metabarcoding data have already been obtained. Six of these sites have been sampled across multiple months and years and are especially valuable for studying the spatiotemporal variability of sex ratio and sex-specific predation by harbor seals. This study provides valuable data for models that seek to incorporate sex-specific predation to obtain more realistic estimates of harbor seals’ impact on salmon populations of concern.

Harbor seals hauled out on a log boom.