PIT tag scanning after marked salmonids uncovers the prey preference of Great Cormorants in the River Dalälven estaury

The Great Cormorant is often considered a voracious fish eater and previous studies have shown that cormorants can consume a large proportion of the young salmonids that migrate from nursery habitats in streams to the sea. But how many young salmonids, so called smolts, are actually consumed in Swedish water courses?

Researchers at the Swedish University of Agricultural Sciences and DTU Aqua in Denmark have now analysed how many smolts are being eaten by Great Cormorants and Herons in the River Dalälven estaury.

“Our data clearly shows that sea trout smolts are more susceptible to bird predation than salmon smolts. Hatchery-reared fish are also more prone to predation than wild fish”, says Torbjörn Säterberg, researcher at the institute of Aquatic Resources at SLU, and the main author of the scientific article published in the scientific journal Ecosphere. 

That a larger proportion of sea trout than salmon smolts are being eaten likely depends on the different fish species´ migration behavior. Salmon smolts migrate fast through streams and their estuaries, while sea trout spend more time in estuaries. This increases the risk that a sea trout is consumed in comparison to a salmon. Hatchery-reared fish is less well adapted to a natural environment and hence tend to be consumed to a larger extend than fish that grow up under natural conditions.

River Dalälven is extensively stocked with hatchery-reared trout and salmon smolts, and since 2017 many of those have been PIT tagged before being released in the river. PIT tags are small electronic marks with unique IDs that are inserted into the body cavity of the smolts.

The study is based on data from twenty four thousand hatchery-reared smolts being released in 2017-2021. In addition, in 2019-2021 one and a half thousand smolts, reared under natural conditions, where caught, PIT tagged and released in the river. Bird colonies where scanned using a hand held PIT tag reader during three consecutive autumns, revealing how many smolts where consumed by breeding birds.  

“As we know how many smolts where marked and how many of those marks that where recaptured in bird colonies we can get a direct measure of bird predation, that is, how large proportion of the fishes there are that are being eaten”, says Torbjörn Säterberg.

The study shows that birds, and especially Great Cormorants, consume a significant proportion (approximately 31 % of the hatchery-reared trout and 13 % of the hatchery-reared salmon) of the smolts migrating from River Dalälven. As similar figures have been document in other systems this suggests that birds have the potential to regulate salmonid stocks.

“Although our results clearly show that Great Cormorants consume a significant amount of the smolts migrating from River Dalälven it is unclear to what extent this affects the salmonid life cycle in general, for example if bird predation affects the number of adults that return to the river for spawning. In order to answer this question we would like to, in addition to continue tagging fish and scanning bird colonies, also scan and analyze how many marked individuals there are that return to the river. “

Read the article: Species- and origin-specific susceptibility to bird predation among juvenile salmonids