The grey seal, the cormorant and fisheries

The grey seal and cormorant populations in coastal areas have increased during the last decades. Grey seals cause problems to net and trap-net fishing – for professional fishermen in particular – because they eat fish from the nets, break the nets, and possibly impact the behaviour and movement of fish shoals. Cormorants are also believed to have a negative impact on pikeperch and perch populations in coastal areas.

Fish catch damaged by a grey seal.  Photo: Mervi Kunnasranta

In recent years, it has been observed that the number of large perch on the southern coast is decreasing, and the size of pikeperch catches are no longer increasing, particularly in coastal areas. It has been suggested that the reason for this is the growing cormorant population eating most of the offspring of these species. On the other hand, nutritional studies conducted during the breeding season (so far only in small areas) have shown that cormorants mainly feed on species with little commercial value. However, the diet varies greatly from area to area, and it may change over time if a cormorant colony inhabits the same area for a long time and potentially affects the local fish populations.

No research data is available on the diet of cormorants outside their breeding season. The research data available on the feeding behaviour of seals and their movement near fishing traps is also scarce. Owing to the lack of research data, public discussion on cormorants and grey seals’ impact on the populations of target species for fishing and fisheries occasionally becomes too heated, so research activities on the diet, behaviours, and impact on fish populations of these species should be enhanced. In addition to pressures relating to the restriction of populations, there are currently both national and international conservation pressures aimed at both the grey seal and the cormorant.

Under the ‘The grey seal, the cormorant and fisheries’ research theme:

• The diet (amounts, species, temporal variation) of cormorants in coastal areas is studied, and estimates on the cormorant’s impact on the populations of the most important coastal species, such as pikeperch and perch, are produced.
• Research data is produced for the other impacts of the cormorant on the scenery, vegetation on breeding islets, and other sea bird populations.
• The seasonal movement and dispersal of seals is studied, in particular, the behaviour and movement of individuals caught in trap nets.
• The diet of seals is studied using various research methods, and the impact of seal populations on fish populations in the Baltic Sea is assessed.
• An international health and welfare study into seal populations is carried out, and the number of parasites in seals is monitored, together with their breeding success. The extent of the by-catch mortality of seals is assessed and monitored in accordance with international recommendations.

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