Harbour seals are generalist and opportunistic predators. They prey on locally available fish species; however, their diet is often dominated by a few key species and varies seasonally and geographically. In Skagerrak, harbour seals hunt over soft sea floors with little or no vegetation at moderate depths (above 30 m), however they have been observed foraging deeper (more than 120 m). They usually target small fish (less than 30 cm in length) from different species. The diet is dominated by species which live near the sea floor, but also includes a number of pelagic fish, which live in the open ocean but visit the seabed during their daily migrations. Some of the fish species that harbour seals prey on are sandeel, blue whiting, whiting, haddock, pollack, saithe, poorcod, cod, Norway pout, herring, ballan wrasse, vahl’s eelpout, and many more! Cephalopods, like squid, can also occasionally be seen in harbour seal diets.
To investigate seal’s diet, fish otoliths are collected from seal’s droppings. Otoliths, commonly known as "earstones," are hard, calcium carbonate structures located in the head of all teleost fishes. They serve as a balance organ and aid in hearing. Otoliths have unique shapes for each fish species, and they can give information about diet composition as well as an estimation of prey size and biomass.
Above: How do we know what seals eat?
Exploration of the feeding behavior of seals is important because it indicates environmental changes. The reconstruction of predators' diets helps understand predator-prey interactions and assess harbor seals' ecological role in marine ecosystem.
Above: Samples of seal poo are collected in Kosterhavet National Park (top left). Fish otoliths (top right) are collected from poo and cleaned. Fish species have unique shapes and sizes which can be used to reconstruct seal diets (bottom).