Epsilon Archive for Student Projects

Abstract

This study compared the diet of harbour seals (Phoca vitulina) and grey seals (Halichoerus grypus) at Måkläppen, southern Sweden, using scat samples collected in August and September 2025. Måkläppen is a unique place in Sweden where both grey seals and harbour seals can be found regularly in large numbers. The population development of the two species at Måkläppen is different, where the number of grey seals seems to increase, whereas the number of harbour seals does not seem to change. Basic knowledge of the diet of grey seals and harbour seals at Måkläppen is missing, and further studies are needed.

In total, 12 grey seal diet samples and 74 harbour seal diet samples were collected. Out of these, only 5 grey seal samples and 52 harbour seal samples contained otoliths, which allowed estimation of prey biomass and were therefore included in the analyses. Diet composition was quantified using frequency of occurrence, relative numerical abundance, relative biomass, and prey size estimated from otolith size – fish size regression equations.

The results showed differences in diet composition between the two seal species. Harbour seals showed a broader and more mixed diet, whereas grey seals relied on a small number of species. Harbour seals had more prey families in their diet than grey seals, with 10 prey families for harbour seals and 5 prey families for grey seals. All prey species that were found in the diet of grey seals were also found in the diet of harbour seals. The three prey families that dominated the diet biomass of harbour seals are Zoarcidae (eelpout), Pleuronectidae (flatfish), and Clupeidae (herring). The diet biomass of grey seals was dominated by two prey families, Belonidae (garfish) and Gadidae (codfish). Harbour seals caught most prey of the prey families Zoarcidae (eelpout), Pleuronectidae (flatfish), and Ammodytidae (sand eel). The most common prey family caught by grey seals was Ammodytidae (sand eel).

The study found no significant differences in prey size, likely due to the small number of grey seal samples. The study also found that grey seals ate more of the larger sized prey families Belonidae (garfish) and Gadidae (codfish). This study suggests that there are differences between the grey seal and harbour seal diets at Måkläppen. The differences in diet can be one way for the seals to reduce competition between them. However, because only five grey seal samples contained quantifiable prey biomass, the results should be interpreted cautiously and verified with larger sample sizes in future studies. This study also needs to be followed up on to better understand the seals' diet around Måkläppen, how much their diets overlap, and whether differences in diet can explain why the seal populations are developing differently.