Captive bred fish are less successful in the wild


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Two recently published studies have cast fresh doubt over the value of captive breeding projects aiming to reintroduce threatened species into the wild.

Captive bred animals released into the wild have been well documented to have lower survival and reproduction rates than their wild counterparts, but research published in the journal Biology Letters asked whether this phenomenon was heritable, ie. was this poor performance passed down through successive generations once fish were in the wild?

The study focused on a wild U.S. population of Rainbow trout (Oncorhynchus mykiss), which had been supplemented with hatchery raised fish, released to boost numbers for fisheries or conservation.

The researchers assessed the genetic parentage of several generations, and so calculated whether parents were wild fish, hatchery fish, or both.

What they found, was a change in population fitness and reproductive success according to where the parents were born, and that the genetic effect of captive breeding was not removed by a full generation in the wild.

Fish with two captive bred parents were found to be 63% less reproductively successful than fish with two wild parents, while fish with one captive bred and one wild parent, were found to be 13% less reproductively successful. On a population level, the introduction of hatchery fish was found to make that population 8% less fit than a purely wild population.

A similar study presented in the journal Proceedings of the Royal Society B, found corroborative results, this time analysing 37 years of Irish Atlantic salmon (Salmo salar) data.

Like the rainbow trout, millions of farmed hatchery salmon are released and escape annually.

In this research, climate change was factored into the calculations, with the scientists looked at egg-to-smolt survival rates.

The researchers reported that increased winter water temperatures had a negative effect on survival of future generations, which may make the eggs hatch early, putting fry at risk of starvation and depletion of energy reserves. This situation was found to be much worse for offspring of hatchery fish, indicating that populations "enhanced" with captive bred fish will be less able to quickly evolve and adapt to the rising winter water temperatures caused by climate change.

The effects of natural selection, vital to ensure individuals are best adapted to their environment, are quite different in farmed fish. It is believed that faster metabolic and growth rates, higher aggression, and reduced predator avoidance are part of the reason captive bred fishes are less successful in the wild.


Araki, H., Cooper, B., Blouin, M.S. (2009). Carry-over effect of captive breeding reduces reproductive fitness of wild-born descendants in the wild. Biology Letters 5, 621-624.

McGinnity, P., Jennings, E., deEyto, E., Allott, N., Samuelsson, P., Rogan, G., Whelan, K., Cross, T. (2009). Impact of naturally spawning captive-bred Atlantic salmon on wild populations: depressed recruitment and increased risk of climate-mediated extinction. Proceedings of the Royal Society B 276, 3601-3610.