Diet of the Skagerrak harbor seal: How has the energy content in the diet changed during the last decades?

Abstract

As a generalist top predator, the harbor seal is of crucial importance to the balance of the coastal ecosystem. The number of newborn pups in the Northern Skagerrak population has been declining since 2012, where food limitation is suggested as a potential cause. Decline in pregnancy rates is commonly observed in marine mammal populations as a response to collapsing prey stocks. This study aimed to investigate food limitation as a potential stressor by comparing the recent diet, with historic harbor seal diets. By analyzing fish otoliths in scat samples collected from the harbor seal population in the northern Skagerrak in 2019, the species composition and energy content in the diet was estimated. This diet was compared with historical diets from the same seal population. The recent diet had an intermediate level of energy content compared to the historic diets, which did not indicate a lower diet quality based on diet composition only. However, the diet composition was different. Atlantic herring, an energy dense and historically important species diet, whose energy contribution has declined notably from around 24 % down to 2,8 % in the recent diet. The diet has instead shifted towards flatfish species which constituted 46% of the energy content in the recent diet, compared to 5,7-18 % in the past. Based on the biomass the main species in the current diet was Atlantic cod, common dab, European flounder, European plaice, ballan wrasse, and lemon sole, amounting to 62 % of the biomass, and 60 % of the energy content in the diet. This study was limited due to constraints in the historic data. The harbor seal population in Skagerrak could still be facing nutritional stress. The energy content in the harbor seal diet needs further investigation to inform conservation action and avoid population declines resulting from decreased birth rates.