THIS CONTENT IS BROUGHT TO YOU BY the Institute of Marine Research - read more

Do whales compete with fishermen?

The debate has raged for years. New research provides a possible answer.

This humpback whale was chasing capelin, east of Jan Mayen in 2018.
Bente Kjøllesdal Communications Adviser
Presented by: Presented by: Presented by: Institute of Marine Research
Published

Minke whales gulp down herring, harbour porpoises devour capelin, and grey seals hunt cod along the seabed.

Every year, marine mammals consume 11 million tonnes of fish in the North Atlantic.

Do they eat fish that fishermen could have caught?

The debate has raged for many years, in many countries.

New research shows how the abundance of marine mammals and fish influence one another. The findings were unexpected.

“The relationship is highly asymmetric. We see that variations in abundance of marine mammals can have major consequences for fish stocks, but not necessarily the other way around. Changes in fish abundance does not significantly affect marine mammals,” says marine scientist Benjamin Planque.

Who eats how much of whom?

The researchers have looked at how marine mammals – such as whales, porpoises, and seals – interact with the most commercially important fish stocks in the Barents Sea and the Norwegian Sea.

Using data from research cruises, fisheries, and diet studies, researchers reconstructed the feeding habits of predators over a period of more than 30 years, from 1988 to 2021.

By modelling the food web, the researchers were able to calculate how much of each prey species could be consumed by each predator.

This, combined with causal analyses, makes it possible for researchers to examine concrete relationships, such as how changes in one species affect another.

For example: If the biomass of marine mammals increases by 1,000 tonnes, what would be the consequence for fish stocks – and vice versa?

One more tonne of whales = one less tonne of fish

More whales generally mean fewer fish.

The researchers tested over 1,000 possible configurations of the food web. On average, they found a negative effect that was close to one-to-one.

“This means that if there are 1,000 tonnes more marine mammals in one year, we can expect about 1,000 tonnes less fish the next year,” says Planque.

More fish in the sea, on the other hand, can have a positive but modest effect on marine mammals. If there are 1,000 tonnes more fish one year, then we can expect 13 tonnes more marine mammals the next year.

“This is probably because marine mammals are dietary generalists. If fish availability is poor, they’ll eat something else, like krill or squid,” he says.

Tightly packed herring lying in layers on a fishing deck.
“A fish that's not eaten by a marine mammal is not necessarily available to the fishery – it may have been eaten by another fish, migrated elsewhere, or died from other causes,” says Planque.

Removing marine mammals does not lead to more fish

The study suggests that large increases in marine mammal populations could make things more challenging for fisheries:

“With increasing marine mammal poplations, fish stocks may decline, especially if those stocks are already under pressure from other factors such as overfishing or climate change,” says Planque.

However, reducing marine mammal numbers is not an effective solution for increasing fish stocks.

“To have a positive effect – meaning more fish available for fisheries – one would have to hunt enormous numbers of marine mammals. Far more than what is current practice today,” says Planque.

Over a longer period of about 30 years, these effects become smaller, as ecosystems adjust.

The marine scientist explains that this is because ecosystems have built-in balancing mechanisms. Species can switch to different food sources when their preferred prey becomes scarce, and reduced competition can allow the remaining prey to grow and survive better.

Need for better monitoring

The researcher points out that there are large uncertainties in the estimates.

The negative impact of marine mammals on fish stocks could be large, but the exact size is unclear. The effect might be smaller than estimated, or it could be up to three times greater. 

In contrast, the impact of fish on marine mammals is smaller but more robust.

“These uncertainties stem both from data limitations and from the fact that interactions within a food web are highly complex and only partially understood,” says Planque.

To reduce this uncertainty, one of the key measures is improved monitoring and more precise long-term estimates of both how many marine mammals live in the North Atlantic and what they eat.

Reference:

Planque et al. From trophic flows to causal effects: marine mammal–fish interactions in the Norwegian Sea and Barents Sea, Proc Biol Sci, 2026. DOI: 10.1098/rspb.2025.2363

More content from the Institute of Marine Research:

biological diversity natural sciences partner institute-marine-research biology marine research
Powered by Labrador CMS