Scientists at University College Cork (UCC) are investigating differences in the breeding of salmon.
This iconic fish is native to the world’s two biggest oceans and the rivers draining into them. The Atlantic Ocean has only one species, the Atlantic salmon, while the Pacific has several species.
Irish salmon are Atlantic salmon and spend their juvenile phase in rivers, before migrating to sea to grow and mature. To complete their life cycle, they must return to their river of origin to spawn.
They are regarded as a keystone species. Commercial drift-netting for salmon has been banned in Ireland since 2007. A government advisory group had warned that the salmon population would fall to catastrophic levels if drift-net fishing was not banned. However, the controversial measure, welcomed by recreational angling interests, but opposed by coastal communities, which lost a traditional fishery, has not solved the problem of disappearing stocks.
The number of adult salmon returning to Irish rivers continued to fall, with fears of its extinction. Various factors have been blamed, and climatic change in particular for influencing the distribution of fish stocks in the North Atlantic.
The Burrishoole Fishery is one of the facilities of the State agency, the Marine Institute, an impressive location, to which I have been a number of times. It is about 5km from Newport, in County Mayo, a premier salmon fishery and research area. It has been using genetic fingerprinting techniques developed by the UCC researchers, examining whether the introduction of ‘captive-bred’ salmon into the wild could be a response to declining numbers.
Ronan O’Sullivan, of the Environmental Research Institute (ERI), co-lead author of the research report, says that “the introduction of captive-bred salmon into the wild is a common management response to natural or human-driven declines in salmon numbers and is sometimes done to increase the numbers of fish available for angling”.
The ERI was established in 2000 by UCC as a flagship institute to integrate over 400 researchers from 20 university schools and departments and six research centres to work together. The UCC researchers used genetic fingerprinting and say they have shown that ‘captive-born’ salmon have as little as one third of the lifetime reproductive success of wild salmon.
Their research team included colleagues at the Marine Institute, Queen’s University Belfast, the University of Helsinki, and the University of Edinburgh.
Wild salmon have low survival rates as juveniles in rivers, probably because of natural limits of ecosystems, such as competition, predators, and food scarcity, whereas salmon bred in a protected hatchery survive at much higher rates. There has been hope that this would allow large numbers to be moved to the wild at a suitable stage.
Earlier studies of Atlantic salmon did indicate that captive-bred fish or their offspring might perform less well in the wild, though evidence across the full life cycle was lacking.
“We have demonstrated that ‘captive-born’ salmon have as little as one-third of the lifetime reproductive success of wild salmon spawning in the same river,” Mr O’Sullivan said.
“Moreover, the overall productivity of the mixed population was much lower in years where ‘captive-bred’ fish comprised a greater fraction of potential spawners. We looked at the lifetime reproductive success of salmon spawning naturally in the wild. For each adult fish that returned to the river from the sea, we counted up the total number of offspring they produced across their lives that themselves survived to spawning age. We used a genetic pedigree, coupled with four decades of salmon data from the Marine Institute’s research facility on the Burrishoole Catchment.
“The results show that ‘captive-bred’ fish that are deliberately, or inadvertently, introduced into the wild contribute fewer offspring to the next generation than wild fish and are not a substitute for natural wild spawners. They do not enhance the conservation status of naturally self-sustaining salmon populations,” Mr O’Sullivan said
Further research is needed to work out what is happening when the wild and captive salmon mix, according to the UCC research team, which suspects that hybrid offspring produced by matings between captive and wild parents are genetically less well-equipped to deal with life in the river.
“If true, this means that the widespread release of captive fish into the wild might actually do more harm than good in many cases,” the UCC researchers say.