Long-term feeding ecology of harbour seals: Implication for sustainable ecosystem-based management

This research project analyses more than 2,000 diet samples collected over the past two decades to improve understanding of the seals’ ecological role and to support sustainable, ecosystem-based management.

The harbour seal population in the Skagerrak declined sharply during the 20th century due to overexploitation and environmental pollutants, reaching a minimum of around 2,000 individuals in the 1970s. Since then, the population has recovered substantially as a result of targeted conservation measures.

As seal numbers increased, interactions and conflicts with fisheries also intensified. Since 2009, seal hunting has therefore been reintroduced as a management tool, with regional quotas of up to 900 seals per year. However, fundamental knowledge about harbour seal diets and their impact on fish stocks is still lacking.

This project addresses these knowledge gaps by analysing more than 2,000 harbour seal diet samples collected along the Swedish Skagerrak coast between 1999 and 2024. By combining long-term diet data with fish stock assessments and advanced statistical models, the project aims to clarify the role of harbour seals in the marine ecosystem and their interactions with fish populations.

Research questions and objectives

The overarching aim of the project is to provide scientifically robust knowledge to support ecosystem-based and adaptive management of harbour seals and fisheries. Key questions include:

• How does the diet of harbour seals vary between areas, seasons and years?
• To what extent is seal diet shaped by local prey availability, and are seals selective or opportunistic feeders?
• How do sex and age influence dietary preferences and foraging behaviour?
• How does total prey consumption by seals vary over time, and how does seal predation compare to fishing mortality?

Long-term data and modern methods

The study is based on a unique long-term dataset of more than 2,000 diet samples collected along the Swedish Skagerrak coast between 1999 and 2024. The samples include both faeces and stomach contents from hunted seals.

Fish remains are identified by analysing otoliths and other hard parts to determine prey species, number of individuals and fish size. For species that are difficult to distinguish, machine learning methods are used to support identification.

To analyse patterns across space and time, the project applies statistical models that link seal diet to environmental conditions and data from coastal fish monitoring programmes.

Supporting better management

By improving knowledge about harbour seal feeding behaviour and its effects on fish stocks, the project provides a stronger scientific basis for ecosystem-based management. In the long term, this knowledge can help reduce conflicts between stakeholders, support healthy seal populations and promote more sustainable coexistence between fisheries and seal conservation in the Skagerrak.

Funding

The project is funded by the Swedish Environmental Protection Agency’s environmental research programme, in collaboration with Formas.