The release of captive-bred animals, such as the Atlantic salmon, has been a common practice for nearly 150 years to restore or supplement wild populations. However, researchers have discovered that there are ecological and genetic consequences such as survivability and producing offspring.
A study by scientists from Ireland, Finland, and the United Kingdom was recently published in the journal Proceedings of the Royal Society B. The researchers discovered that salmon from the Burrishoole Catchment in Ireland can only produce one-third as many offspring gas wild salmon in the North Atlantic.
Evolutionary biologist Ronan James O'Sullivan from University College Cork shared that the "years where you have a greater input from the captive-bred Atlantic salmon-the ability of the population as a whole to produce more wild-bred fish is reduced in subsequent years." The study proves that the assumption of wild and captive-bred fish are ecologically equal is not true.
O'Sullivan shared that releasing captive-bred salmon into the North Atlantic is not a replacement for wild species. It is also alarming that there are increasing populations of captive-born salmon whose reproduction rates continue to decline.
He explained that when hatcheries produce a self-sustaining population of healthy salmon, there is no way to safely stock the fish. For years, salmon have been stocked in the wild in the Pacific and Atlantic oceans.
Researchers have tagged and recorded the genetic data of nearly all the salmon passing through the Burrishoole catchment where there are several lakes and waterways. The captive-bred salmon are released into the ocean as smolts (juvenile stage), unlike farmed salmon that just stay in aquaculture farms.
The wild salmon that used to swim through the catchment would lay eggs downstream as they head back to the ocean. Now, the captive-bred salmon cannot move upstream and are separated from the wild species unlike before when the two species would mate in the ocean. Tagging the fish allowed the researchers to count the offspring of each salmon.
Genetics, Ecology, and Climate Change
Several factors are affecting the salmon populations such as genetics. In the wild, the fish can choose who to mate with. At the hatchery, managers choose with captive-bred fish to mate together within a limited gene pool.
As a result, there is a loss of genetic diversity in the hatcheries, leading the mutated or silenced genes necessary for surviving in the wild. One genetic trait necessary in the wild is emerging from torpor or lower body temperature and reduced metabolic rate to survive the winter.
Another factor is ecological, meaning that captive-bred salmon are less fit to survive in the wild since hatcheries do not have predators. As a result, they are larger and females produce more eggs.
O'Sullivan is also worried about how the Atlantic salmon is affected by climate change. Warmer waters in Spain and France have resulted in the extinction of wild salmon in their regions. "If increasing temperatures make the rivers in Ireland less viable for fish to live in, coupled with the fact that rivers are producing less fish because of stocking policies, then it would be a perfect storm."
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