Factors affecting the survival of harbor (Phoca vitulina) and gray seal (Halichoerus grypus) juveniles admitted for rehabilitation in the UK and Ireland

Abstract

The UK shores are home to approximately 40% of the world's population of gray seals (Halichoerus grypus) and 40% of Europe's harbor seals (Phoca vitulina). Stranded juvenile seals of both species are frequently rescued and admitted for rehabilitation. This study investigates the causes of P. vitulina and H. grypus admittance to rehabilitation centers in the UK and Ireland and identifies factors that can affect juvenile seal survival. Rehabilitation records for 1,435 P. vitulina and 2,691 H. grypus were used from five rehabilitation centers from 1988 through 2020. The most common nonexclusive reasons for seal admission to rehabilitation centers included malnourishment (37%), injuries (37%), maternal abandonment (15%), lethargy (12%), and parasite infections (8%). A mixed effects logistic regression model showed that H. grypus had 4.55 times higher survival odds than P. vitulina and that the odds of survival to release multiplied by 1.07 for every kilogram over their age-predicted weight. This weight-dependent survival could be attributed to the importance of fat in thermoregulation, hydration, and buoyancy during foraging. We recommend that seal rehabilitators pay special attention to the weight of admitted juvenile seals during triage and treatment to enhance their odds of survival and consequent release to the wild.