A study coordinated by researchers from the University of Helsinki and University College Cork in Burrishoole, Ireland, demonstrates that the number of offspring produced by a hatchery-reared salmon during its lifetime and survived to adulthood, is no more than roughly one-third of the corresponding number for wild salmon spawning in the same river. Furthermore, a mixed population composed of both wild and hatchery-reared salmon produced much fewer offspring in years where the share of captive-bred spawning fish is higher.
Previous research has already shown that hatchery-reared salmon or their offspring are less successful in nature compared to the wild members of their species. Before this, however, no related evidence encompassing the entire lifetime of salmon has been collected.
On the basis of DNA samples, the researchers in Burrishoole calculated the number of offspring that survived until reproductive age for each adult salmon returning from the ocean to the river.
“Salmon migrating to spawn in the river and returning to the ocean had to use a route where we collected DNA sample from each individual fish. Subsequently, we established a family tree for the salmon, linking it with a monitoring dataset covering 40 years,” says Academy Research Fellow Tutku Aykanat from the Faculty of Biological and Environmental Sciences, University of Helsinki.
Knowledge potentially relevant for the Finnish setting
In Ireland’s Burrishoole, hatchery-reared salmon are transplanted into a river where salmon also spawn efficiently in the wild. In Finland, such mixing of salmon populations is not being practised. Finland has rivers relying solely on natural spawning such as Teno and Tornio, as well as rivers whose salmon stock is entirely dependent on transplantation.
“In spite of these differences, we can apply the knowledge gained in Ireland also to the Finnish setting. The results indicate that supplementing salmon into a wild salmon population, for example, in the Tornio and Teno rivers, would not be advisable. But if we can get to the point that natural salmon spawning is successfully revived in waterways such as the Oulujoki river and Lake Saimaa, it would be sensible to discontinue salmon transplantation altogether and instead invest in boosting reproduction in the wild,” says Craig Primmer, Academy professor from the Evolution, Conservation and Genomics research group.
The researchers point out that further research is needed to determine what happens when wild and hatchery grown salmon breed with each other. They speculate that the genes of the offspring produced through such interbreeding are less suited to surviving the dangers of life in the rivers and oceans compared to wild salmon. If this turns out to be true, the large-scale introduction of hatchery-reared individuals into nature could in many cases cause more harm than good.
The research group was composed of international researchers from the University of Helsinki in Finland, University College Cork and the Marine Institute in Ireland, Queen’s University Belfast in Northern Ireland and the University of Edinburgh in Scotland.
The project has been funded, among others, by the Academy of Finland.
Ronan James O'Sullivan, Tutku Aykanat, Susan E. Johnston, Ger Rogan, Russell Poole, Paulo A. Prodöhl, Elvira de Eyto, Craig R. Primmer, Philip McGinnity, and Thomas Eric Reed. Captive-bred Atlantic salmon released into the wild have fewer offspring than wild-bred fish and decrease population productivity. Proceedings of the Royal Society B, Published:21 October 2020. https://doi.org/10.1098/rspb.2020.1671