Quantifying seal diet is an important component of our research portfolio for two main reasons. Firstly, harbour seals and grey seals are perceived to negatively impact commercially important species of fish, leading to conflict between fisheries (including recreational anglers) and seals. The main reason for hunting harbor seals in Iceland today is to reduce the potential effects that many people believe seal predation can have on salmonid populations and hence indirectly on the salmon and trout angling industry.
Secondly, persistent shifts in environmental conditions can precipitate major changes in plankton communities, drive shifts in species abundance and phenology and may lead to bottom-up forcing on higher trophic levels, including species such as harbour seals and grey seals. Hence, environmental changes may also be contributing to seal population declines and knowledge of diet provides important insights into how seals respond to changes in resource availability.
The importance of salmonids in the diet of harbor seals has however never been investigated in Iceland before, and scarce knowledge on the subject exist internationally. The aim of this project is therefore to investigate the importance of salmonids in the diet of harbour seals that haul-out in a estuary areas in Northwestern Iceland.
We use several different methods of dietary analysis, including hard-part analysis (analysis of otoliths and bones) and prey-DNA meta barcoding analysis on seal feces. Stable isotope analysis of seal hair and muscle has also been carried out. Data from salmon anglers has been collected to investigate seal injuries on caught fish and further, monitoring of haul-out behaviour in an estuary area and tagging of seals with telemetry equipment has been conducted.
Results from hard-part analysis and prey-DNA analysis show that the main prey of harbour seals were sand eels, flatfishes, gadoids, herring and capelin, while salmonids were not an important prey for harbour seals in the area. The survey of salmon anglers where the amount of caught fish with seal injuries (bite and claw marks) was investigated support these results and showed a low rate of seal injuries. Stable isotope analysis has also been carried out on muscle and hair tissue from harbour seals. Data analysis and manuscript preparation is currently underway. This project will for example integrate variation in spatial and a temporal variation in harbour seal diet. Given different tissues have different turnover times, the results will provide insights into diet and habitat use during different periods over the annual cycle. Samples from seals by-caught in different parts of Iceland will be compared and differences in diet depending on factors like age, sex etc. will also be investigated.