Genetically speaking, captivity is fishy business.

Feb 03, 2012

Christie, MR, ML Marine, RA French and MS Blouin. 2012. Genetic adaptation to captivity can occur in a single generation. Proceedings of the National Academy of the Sciences

Synopsis by Tamara Tal

Researchers follow the pedigree of almost 13,000 steelhead during 15 years and find the descendents of the wild fish that reproduced well in the hatchery were less likely to produce progeny in the wild due to unintended genetic changes.

It takes just a single generation in captivity to genetically change a wild fish's offspring so they are less able to reproduce in the wild, report researchers who studied three generations of the endangered steelhead fish from Oregon's Hood River.

In this evolutionary conundrum, the more successful a wild caught fish is in captivity – by producing high numbers of offspring – the worse off the offspring are at reproducing successfully in the wild. This study is the first to explain the cause: unintentional genetic adaptation to captivity.

It turns out that the wild parental fish bred in captivity produced offspring that quickly adapted to their new hatchery conditions. However, they lost the ability to flourish in the wild. These captive-born fish – and their wild-born offspring – produced fewer young when allowed to spawn naturally in their home streams after returning from their multi-year ocean sojourn.

This so-called genetic domestication of wild born fish and their offspring render them less fit in the wild. In the context of this study "less fit" means that fewer of the captive born offspring (called the F1 generation) gave rise successful fish of their own (the F2 generation).

This research has important implications for current conservation approaches aimed at replenishing dwindling stocks of wild fish like salmon and steelheads. It also opens the door for scientists to track down which traits are being selected – that is, those traits that help the fish better survive – during captivity. If identified, breeding programs could be tweaked to produce captive-bred fish that will be better equipped to survive and successfully reproduce in the wild.

The results of the study are consistent with previous studies reporting that captive-born individuals released into the wild have much lower fitness than their wild-born counterparts. Proposed ideas to explain the phenomenon include relaxed natural selection (less healthy animals survive in the hatchery), inbreeding among close relatives and unintentional genetic adaptation to captivity.

Researchers at Oregon State University and the Oregon Department of Fish and Wildlife teamed up to perform a 15-year study that included almost 13,000 fish. The adult wild-caught steelhead were bred in captivity to produce the F1 generation. The F1 fish were raised in captivity, released into the wild, migrated out to sea and returned 2-3 years later to their original hatching river to spawn. After hatching, the wild born F2 generation then migrated out to sea and returned 3-4 years later to the same river to spawn.

Because the fish had to maneuver past a dam to get to the spawning grounds, researchers counted every fish moving upstream and collected fin tissue samples for genetic analysis. By sequencing eight highly variable locations in the fish genome in each of the 12,700 fish in the study, researchers were able to create intricate fish lineages to track each fish back to its captive born parents and wild born grandparents.

The study yielded several important conclusions. The wild fish used as the original brooders – the grandparents – were unintentionally domesticated in a single generation. In other words, fish that had a genetic advantage in captivity gave rise to young with genetic traits suited for captivity. Intriguingly, domestication by natural selection has a trade off. The offspring – and eventually grand-offspring – of fish that were successful in hatcheries were less able to compete and successfully reproduce in the wild.

The results allow researchers to discount the competing theories of relaxed natural selection and inbreeding as significant factors that contribute to this rapid reduction of fitness in the wild.

More research is needed to understand which genetic traits matter most in captivity and in the wild. If these are identified, hatcheries could produce fish better able to prosper under natural conditions in the efforts to conserve endangered species and restore declining wild fish populations.

While this study concentrated on reproductive success, it's well known that other stresses like chemicals in the environment or pathogenic infections can add up to slash wildlife survival - often through increased predation, weakened immune systems and abnormal sex development. Producing captive-born fish better able to adapt to conditions in the wild may benefit the animals that face these other stressors, too.

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