Body condition of phocid seals during a period of rapid environmental change in the Bering Sea and Aleutian Islands, Alaska

Abstract

A warming climate has driven rapid physical changes in the Arctic environment, particularly in the Bering Sea. Biological changes are also increasingly evident in the Bering Sea and adjacent waters. The ecological results have been profound and relatively well documented for fishes and lower trophic levels. Upper trophic predators such as marine mammals, however, have been underrepresented in ecosystem surveys, models, and efforts to practice ecosystem-based fisheries management. We used multiple linear regression to model body condition (mass/length) as a function of species, age class, sex and year for ribbon and spotted seals in the Bering Sea, and harbor seals in the Aleutian Islands, from 2007 to 2018, for evidence of recent trends that might reflect trophic or bottom-up changes in the ecosystem. Model-averaged coefficients (in kg cm−1, relative to the overall mean) indicated that body condition was lower for subadults (−0.063; 95% CI: −0.074 – −0.051) and pups (−0.120; 95% CI: −0.129 – −0.112) than for adults (0.183, the negative sum of the subadults and pups coefficients). Body condition for males (0.010; 95% CI: 0.002–0.019) was higher than for females (−0.010). Overall, body condition declined annually (−0.014 per year; 95% CI: −0.025 – −0.004), and there was an additive annual decline in body condition of seal pups across all species and sexes (−0.020; 95% CI: −0.030 – −0.011). An additive annual increase in body condition of spotted seals across all sexes and age classes (0.013; 95% CI: 0.004–0.022) mitigated the annual declines for this species. Model-averaged fitted values therefore indicated annual declines in body condition for ribbon and harbor seals of all sex and age classes, and for spotted seal pups. We relate these declines to the trend in Bering Sea ice extent and to recent, rapid changes brought on by the significant Northeast Pacific marine heat wave of 2014–2016 and its lingering effects through 2018 and 2019. The results indicate that these typically resilient, long-lived, generalist predators can be impacted by bottom-up forcing.