MARINE HEATWAVES MAKE THINGS WORSE

Marine heatwaves make things worse: ‘Lurv’ takes over kelp and fish shrink. Marine heatwaves amplify the effects of historical overfishing along the coast: fish become fewer as kelp replaces kelp forests.

 Author: Bente Kjøllesdal

The sugar cane forests in the Skagerrak are threatened by increasing numbers and more severe marine heatwaves. 

“When the kelp forest disappears, it is replaced by filamentous algae. These are what we call ‘lurv’,” explains marine scientist Kjell Magnus Norderhaug. 

Marine scientists have studied the consequences of clearing the kelp forest, and the “litter” spreads like a blanket on the seabed. 

The findings show that marine heatwaves amplify historical effects of overfishing: 

“The fish in the coastal ecosystem are getting smaller,” says Norderhaug.  

“That two such completely different impacts as climate change and fishing can have similar consequences in the ecosystem is an important and worrying finding.”

The Blue Forest

Kelp forests are key ecosystems along the Norwegian coast. 

They house a variety of animals. 

Some thrive hidden on the bottom and some spread out in the “open spaces” between the treetops in the blue forest. Others live their lives along the stems, on the leaves or swimming between them in search of cover and food. 

This is where fish hatch and grow up. At night, predatory fish come in from the depths to eat.

More and smaller small fish

Marine scientists have taken a closer look at the fishing community around the Bolærne archipelago, in the outer Oslofjord.

The area has varying cover of kelp and moss, but otherwise has similar environmental conditions and topography.

The researchers have analysed the differences in the abundance, size, and composition of benthic fish – fish that live close to the seabed.

“We found more fish in these carpet-forming algae than in the kelp forest, but the fish were smaller,” says Norderhaug. 

Good hunting grounds

The study suggests that changes in housing and habitat have different effects on different fish. The murky habitat selects for small fish. 

Along with codfish and wrasses, gobies are among the most common and numerous families in coastal ecosystems. 

They live well hidden on the seabed, where they are invertebrates and themselves food for larger fish. 

“On the one hand, the gophers were more numerous in the furrow area, but they were also smaller in size. This may indicate that the gophers are more exposed to predation pressure in open habitats such as dens,” says Norderhaug. 

For cod fish, the kelp forest is particularly important while they are young. 

The kelp forest is their breeding ground, rich in hiding places. When the cod fish grow up, they become roaming predators, and their function changes – the kelp forest is then the hunting ground, where they find their next meal. 

“Our findings indicate that kelp is also a good hunting area for predatory fish that swim around. There were just as many of these that hunted in the water column in kelp as in kelp forests. Small fish, on the other hand, seemed to avoid the water column in open kelp landscapes,” says the marine scientist. 

Disappeared fish

The areas that the marine scientists have investigated have a century of heavy fishing behind them, which has left its mark. 

Until the 20th century, the coastal ecosystem was dominated by kelp forests and large predatory fish, but by the 21st century, these large reefs had disappeared. 

“What we see is that marine heat waves amplify these effects,” says Kjell Magnus Norderhaug. 

The frequency and intensity of marine heatwaves increased in the 2000s. The marine scientist points out that this could have devastating consequences for the food web along the coast.

“It can lead to changes that are difficult to reverse. The thick blankets of kelp that cover the seabed make it difficult for kelp spores to settle and grow. 

“It can be a vicious cycle for efforts to restore kelp forests. Marine heatwaves are becoming more frequent with climate change, causing kelp forests to disappear, causing the kelp to take root and then displace the kelp forests from growing back. 

“Marine heatwaves can thus help lock a habitat into a degraded state, dominated by small species. This undermines the management efforts being made to restore larger predatory fish, which today depend on and benefit from the kelp forest for their growth,” concludes Norderhaug.

Image: The results showed that fish from common families – such as gobies and codfish – were smaller in the kelp than in the kelp forest. The illustration shows kelp gobies. Photographer: Erling Svensen / Institute of Marine research/HI

Reference: Norderhaug, KM, Kleiven, PJN, Wernberg, T., Synnes, A.-E., Filbee-Dexter, K., Espeland, S., Thormar, J., Kartveit, KH, Christensen, L. and Moland, E. (2025),  More but Smaller: Marine Heatwaves Exacerbate Size Truncation in Overfished Fish Communities in the Skagerrak . Ecol Evol, 15: e71404.
https://doi.org/10.1002/ece3.71404

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