How much fish do grey seals along the coast eat?

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The grey seal is known for its long snout and horse-like head.

Photo: Michael Poltermann / Institute of Marine Research

By analysing their stomach contents and faeces, and multiplying the results by the number of grey seals in Norway, researchers estimated that they consume 8,000 tonnes of fish per year. Saithe, cod and wolffish were their favourite foods.

Over a period of eleven years, marine scientists collected stomach and stool samples from grey seals in Finnmark, Nordland and Rogaland.

In total they analysed fish remains found in 381 samples.

"The otolith of a fish is like a data logger, and it provides us with information about the individual’s species, age and size. It passes through the seal’s digestive system", explains marine scientist Kjell Tormod Nilssen.

Seal faeces reveal their diet

By analysing the otoliths found in faeces on small islands and skerries, and in the stomachs of seals that had been shot, researchers were able to find out what each individual had eaten.

Cod and saithe were the most common fish in the seals’ diet in all regions, followed by wolffish, haddock and various types of flounder.

"But there are variations from place to place, season to season and year to year. For example, in Finnmark capelin was the only fish on the menu in 2009. In Rogaland, meanwhile, grey seals don’t eat much wolffish", says Nilssen.

Multiplied their results by the number of grey seals in Norway

"The seals’ diet is interesting in its own right. But it’s even more interesting to find out what impact the seals have on the fish stocks in question", says Nilssen.

That is because opinions are divided on the matter.

To work out the total amount of fish eaten by grey seals in Norway, the researchers multiplied the fish they had found by the needs of the whole Norwegian grey seal population, most recently estimated at 3,850 individuals.

After a complex calculation involving calorific values, they estimated that grey seals eat the following amounts of fish, split by management area:

  • From Lista to Stad: 517 tonnes of fish
  • From Stad to Lofoten: 3,923 tonnes
  • From Vesterålen to Finnmark: 3,644 tonnes

In total, that’s 8,084 tonnes of fish. That number includes 3,059 tonnes of saithe, 2,598 tonnes of cod and 1,364 tonnes of wolffish.

By comparison, marine scientists recently recommended that fishers should be able to harvest up to 171,982 tonnes of saithe in the Barents Sea in 2020 without any negative long-term impact on the population.

Fish otoliths. (Foto: Erlend A. Lorentzen / Institute of Marine Research)

For the northeast Arctic cod population, the most recent quota advice is 689,672 tonnes. The coastal cod, on the other hand, is struggling all along the Norwegian coast. Only DNA tests can distinguish northeast Arctic cod from coastal cod.

Unlikely to affect the coastal cod population

Marine scientist Kjell Tormod Nilssen believes that if the grey seals have any impact on the coastal cod population it is small.

"35,000-39,000 tonnes of coastal cod were fished commercially per year over the period 2003-2015. Anglers and tourists are estimated to have caught 13,000 tonnes in 2015. There is therefore little reason to believe that the grey seal’s consumption has any significant impact on the stock", he says.

Reference

Kjell Tormod Nilssen, Ulf Lindstrøm, Jon Ivar Westgaard, Lotta Lindblom, Taija-Riitta Blencke & Tore Haug (2019) Diet and prey consumption of grey seals (Halichoerus grypus) in Norway, Marine Biology Research, 15:2, 137-149, LINK: https://doi.org/10.1080/17451000.2019.1605182