Harbor seal consumption of steelhead upon marine entry

• Main Author: Moore, Megan
• Format: Text
• Language: English
• Published: Western CEDAR 2022
• Subjects: Pacific; harbor seal
• Online Access: https://cedar.wwu.edu/ssec/2022ssec/allsessions/143; https://cedar.wwu.edu/context/ssec/article/3305/type/native/viewcontent/Harbor_20seal_20consumption_20SSEC.pptx_AWSAccessKeyId_AKIAYVUS7KB2I6J5NAUO_Signature_z_2FuN_2B57wmYqUyQEZssYsnEsLYXw_3D_Expires_1690453762

Predation by harbor seals may be threatening the recovery of ESA-listed steelhead. After freshwater rearing, steelhead smolts experience high mortality during their rapid migration from river mouth to the Pacific Ocean. Previous work indicates that harbor seal predation is a primary source of early marine mortality in the Salish Sea, but the level of impact exerted by harbor seals on steelhead has not been quantitatively assessed. We calculated the percentage of Nisqually River steelhead migrants consumed by harbor seals in 2016, 2017, and 2018 from estimates of (1) the proportion of harbor seal diet comprised of steelhead, (2) the number of smolt outmigrants in the Nisqually River, (3) the downstream survival of smolts to the estuary, and (4) the number of harbor seals residing in South Puget Sound (SPS). Metabarcoding of DNA in harbor seal fecal samples collected in SPS identified small fractions of harbor seal diet (maximum monthly average = 1.8%) comprised of steelhead smolts. Those small fractions, however, translated into substantial mortality on Nisqually steelhead cohorts when multiplied by the estimated energetic demand of the SPS harbor seal population. Model results indicate that between 10% (2017) and 36% (2018) of outmigrating steelhead were eaten by harbor seals during the 24-kilometer migration through SPS, and that harbor seal predation accounted for 28% (2017) to 76% (2018) of the total mortality incurred over that same distance. Independent estimates of steelhead smolt mortality during the same years, quantified using behavioral metrics of acoustic tagged Nisqually smolts, indicate that most of the mortality within SPS occurs in or near the estuary. This study demonstrates the capacity of marine mammal predators to substantially impact survival rates and potential productivity of small prey populations, and provides information to managers tasked with balancing the impacts of one protected species with the recovery of another.