Kappa, F., T.Härkönen, C.Freitas, D.Carroll, and Okay. C.Harding. 2026. “Shifting Harbor Seal (Phoca vitulina) Diet May Reflect Ecosystem Changes in Skagerrak.” Marine Mammal Science 42, no. 1: e70069. https://doi.org/10.1111/mms.70069.
Information diet
Harbor seals (Phoca vitulina) are ready to feed on heaps of completely different species. Being a “generalist predator”, as that is feeding strategy is thought, signifies that they’ve the power to change their diet if needed – if one prey species turns into more durable to discover, they’ll feed on one thing which is more available. This additionally means, that by finding out their diet, scientists can get info on the abundance of completely different prey species. In short, scientists are ready to use seals to present info on changes in fish communities.
It’s a poop job…
Harbour seals recurrently come ashore (haul out) on seashores to relaxation. When they’re on land, they do what all animals do… and their poop (referred to as “scat”) samples can be utilized to examine what they’ve been eating. These scat samples will be collected off the seashore, popped into a plastic bag and frozen, prepared for evaluation. Tiny ear bones (otoliths) from the consumed fish will be discovered within the scat samples. As completely different fish species have completely different formed otoliths, this may be a great source of info.
Otolith from Atlantic cod – Gadus morhua – by Gunter Tschuch, by way of Wikimedia commons
A new examine by Foteini Kappa and their group has been seal diet utilizing these tiny fish ear bones. The group collected 73 harbor seal scats from an space within the northern Skagerrak, Norway. They took the scat samples from three completely different places the place seals recurrently haul out. Otoliths discovered within the samples had been assigned to species and counted, permitting scientists to work out the proportion of every fish species within the particular person samples.
Kappa and their group had been then ready to discover changes in diet over time, by evaluating diet information from 1977, 1978, 1989 and the current samples from 2020. They definitely discovered some changes had taken place over time (Figure 1). In the more current pattern, the very best quantity of otoliths got here from sandeel (39.6%), then Norway pout (21.8%), then herring / pollack / saithe (grouped collectively)(12.8%) and blue whiting (7.9%). The gadid household, which incorporates Norway pout, whiting and the group herring / pollack / saithe represented 50.3% of the diet.
In the historic information set, the diet was dominated by Norway pout (34.7%), sandeel (11.7%) and herring (10.3%).
What does this imply?
There is a lot of selection of species in harbor seal diet discovered on this examine – which helps the very fact they’re generalist predators and highlights how extremely adaptable they are often. In the current information, 19 species of fish had been represented within the diet. Compared with earlier research and the historic information used on this examine, the quantity of cod within the diet of harbor seals on this space has decreased. The contribution of herring to the diet has additionally decreased dramatically.
Why is that this important? Not all prey species are created equal. Some species have a increased power content material, and these are important for predators as they’ll get more power for much less effort when selecting these prey species. Herring are thought-about to be high high quality prey, and the current information show fewer of these within the diet. This causes some concern about whether or not seals are ready to get enough power. They could also be catching enough fish – but when the fish are the unsuitable species to be fulfilling the seals dietary wants, the seals could show indicators of “nutritional stress”. This finally reveals up within the population as smaller animals and fewer seal pups.
This might also give details about the fish communities over the time period. Fish populations do fluctuate yearly, however seeing a large-scale shift in diet in seals means there has possible been some ecosystem changes, which shifts in distribution or abundance of some of the prey species.
The examine reveals the significance of long-term monitoring packages, to higher perceive each the seals themselves, but additionally wider environmental change.
Cover photograph – A harbor seal (Phoca vitulina) hauled out on a rock. by Charles J. Sharp by way of Wikimedia Commons
I’m a post-doctoral researcher on the University of Hawaiʻi at Mānoa. My analysis pursuits cowl many facets of the ecology of marine mammals, specifically the components that drive distribution and abundance of whales and dolphins, in addition to the impacts of human actions on these animals. I’ve spent a lot of time within the discipline, conducting surveys (very fortunate). When back at my desk, I take advantage of ecological modelling techniques to examine relationships between animals and their setting.
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