A molecular method to quantify sex-specific consumption of Chinook salmon (Oncorhynchus tshawytscha) by Pacific harbor seals …

Sex-biased predation may potentially skew sex ratios in adult populations, which may affect reproduction. Sex-biased predation by pinnipeds is of particular interest as it may impact fish populations of conservation and commercial interest such as salmon. However, sex-biased predation is difficult to measure in the wild and this is particularly true for marine mammals, since predation events in open water are often hidden from direct observation. Molecular scatology (genetic analyses of scat) has been used to non-invasively determine the proportion of prey items consumed in the diet, and it may be possible to determine sex-specific consumption of prey items using a similar approach. In this study, I develop a molecular method to measure the proportions of male and female Chinook salmon (Oncorhynchus tshawytscha) consumed by harbor seals (Phoca vitulina richardii) employing scat. By using QPCR, I established that the proportions of male and female Chinook DNA could be determined in a controlled mixed sample by measuring a y-linked marker, GHp-Y, in the sample in relation to a male control sample. I then applied the assay to harbor seal scat samples from haul-outs in the Strait of Georgia, Canada. Although the assay amplified in 83% of scat samples, 30% of scat samples quantified had an estimated male proportion > 1. The lack of robustness of the assay might have been a result of contaminants in scat DNA extractions, which differentially impacted target genes. Lastly, using whole body tissue mixtures of males and females, I constructed a calibration curve to relate the DNA measurements of the assay to biomass proportions. The calibration curve was skewed by high male DNA density (presumably due to differences in gonad mass between sexes) precluding my ability to infer sex-specific consumption. Chinook populations return to rivers at different stages of reproductive development, and the tissue DNA density bias observed in this study may only apply to certain prey populations in the field. Despite the DNA density male bias, the median estimated male proportion in scat samples was 0.31, which suggests that harbor seals are eating low amounts of male salmon and may have a bias towards females. However, further development of this approach is needed to make strong inferences about sex-specific consumption.