Food habits of harbor seals (Phoca vitulina) in two estuaries in northern Puget Sound, Washington

Abstract

There is a long-held belief that marine mammals are a threat to fishery resources. In Puget Sound, there is particular concern about the potential impacts of pinniped predation on depleted or recovering populations of rocky reef bottomfish. To understand the potential effects of pinnipeds on fish stocks, it is necessary first to describe the types of prey that they consume. The goal of this study was to describe the seasonal diet composition of the Pacific harbor seal (Phoca vitulina) in two estuaries, Padilla Bay and Drayton Harbor. Fecal samples (“scats”) were collected from haul-out sites during pre-pupping (May – June) and pupping (July – September) seasons in 2006. Otoliths and other diagnostic skeletal structures were used to identify prey to the lowest possible taxon. Frequency of occurrence (% FO) was calculated for all prey taxa, and occurrences of the top (> 25% FO) prey species were compared between seasons (Drayton Harbor pre-pupping and pupping), years (Drayton Harbor 1992 and 2006), and sites (Padilla Bay and Drayton Harbor). I also compared seal diet from Padilla Bay and Drayton Harbor with that from non-estuarine haul-out sites in the San Juan Islands. Overall, 40 prey taxa, representing at least 26 taxonomic families, were identified in 198 harbor seal scats from the estuaries. In Padilla Bay, the most common prey were gunnel (family Pholidae), snake prickleback (Lumpenus sagitta), Pacific staghorn sculpin (Leptocottus armatus), and shiner perch (Cymatogaster aggregata). Threespine v stickleback (Gasterosteus aculeatus) and Pacific herring (Clupea pallasi) were the most frequently consumed species in Drayton Harbor, and shiner perch, snake prickleback, Pacific staghorn sculpin, mammal, and flatfish also each occurred in more than 50% of samples from at least one season. The majority (> 85%) of samples contained demersal and benthopelagic taxa; pelagic prey were also common in Drayton Harbor diet. Occurrences of top prey taxa varied by season, year, and site. Most top prey species were consumed more frequently during pupping season in Drayton Harbor. The diversity of Drayton Harbor pupping season diet (9.3 ± 2.99 prey taxa/sample) was also significantly higher than pre-pupping season (6.1 ± 2.82 prey taxa/sample) and Padilla Bay pupping season (4.0 ± 1.68 prey taxa/sample) diets. All top prey taxa differed significantly between estuarine and non-estuarine haul-out sites. Diet composition suggested that harbor seals in Padilla Bay and Drayton Harbor foraged primarily within estuarine habitats, such as those found near the haul-out sites, and some Drayton Harbor seals occasionally fed in other habitats (e.g., freshwater). Temporal and spatial variations in diet appeared to reflect differences in the availability of prey taxa, but this was not always the case (e.g., increased predation on Pacific herring between 1992 and 2006). Drayton Harbor represents the first account of mammals as harbor seal prey. Considering the proximity of some northern Puget Sound estuaries to rocky habitats, including the candidate marine reserves in Skagit County, it is necessary to monitor the food habits of harbor seals in various habitats near marine reserves to assess more accurately the degree of predation on depressed fish stocks.