I bred parrots and other birds for zoos and for the pet trade for years before I moved to NYC. I often thought about applying my avicultural and ornithological skills and talents to propagate birds for release into the wild, to supplement wild populations that are decreasing. But in addition to the practical challenges of captive breeding birds, I often wondered about the evolutionary implications of doing this: because every individual is precious in a captive breeding program, they are subjected to relaxed natural selection criteria, so would I be raising birds that were less fit? If so, could captive breeding potentially damage the very wild population that it is intended to help?
Last week, a paper was published in Science that addresses some of my concerns. This paper, which focused on salmonids instead of birds, was published by a group led by Hitoshi Araki, a zoologist at Oregon State University (OSU) in Corvallis, studied local hatchery programs that are intended to enhance threatened populations of Pacific salmon and steelhead trout on the west coast of the United States.
Every year, hatcheries release in excess of five billion juvenile salmonids into the North Pacific Ocean, both for anglers to catch and also to replenish diminishing wild populations. But these numbers also provide a huge dataset for researchers to study. Previously, researchers found that when hatcheries rely on captive-reared fish for their broodstock over many generations, their fish populations develop a distinct gene pool due to the effects of reproductive isolation and genetic drift. But curiously, one study noted that even hatcheries that rely on local wild fish for their broodstock end up producing captive-reared fish that are genetically distinct from wild populations. Is there an evolutionary significance to this genetic difference? Does this genetic difference negatively affect reproductive success in captive-reared fish?
In spite of some early research by Araki (in conjunction with another group) which suggested that the reproductive success of first-generation captive-reared fish might be indistinguishable from that of wild fish, Araki wanted to better understand the genetic effects of captive breeding on the reproductive success of captive-reared fish in the wild. To do this, Araki’s OSU team designed an experiment where they compared the percentage of offspring produced by each of two pairings for captive-reared fish (C) that were born in the same year, raised in the same hatchery and released at the same time, as follows;
- captive-reared fish produced by two wild-born parents (C[WxW]) or
- captive-reared fish produced by a wild-born parent and a first-generation captive-reared parent (C[CxW])
Araki’s estimated the reproductive success for 547 C[CxW] and 193 C[WxW] fish collected from three salmonid “run years”; when adult fish return to their natal rivers from the ocean to spawn. Using DNA fingerprinting, the team established the parentage of 355 wild-born returning salmon as having either one C[CxW] or C[WxW] parent. Based on these data, Araki’s team found that the reproductive success for C[CxW] salmon was only 55% of that for C[WxW] salmon.
Amazingly, this study found that reproductive success declined sharply after a short time in captivity (figure 2a, below);
When a meta-analysis was performed comparing this study’s data to previously published data, they all described a remarkably fast decline in reproductive fitness associated with captive-breeding: there was a 37.5% decrease in productivity after just one generation in captivity (figure 2b, above).
The evolutionary mechanism underlying this reproductive decline is unknown, but I’d be willing to bet that relaxed natural selection criteria are playing a strong role, which also would alter the resulting gene pool without increasing the overall mutational load.
I find these data to be alarming. Do these data reflect what is happening for other vertebrates, such as birds and mammals? Based on my own experience and knowledge, I would not be surprised to learn that there are sharp declines in reproductive success for captive-bred and -reared birds that have been released into the wild, just as there are for these fish. Further, because birds have a long learning/imprinting period (unlike fish), I suspect that the effects of captive breeding on wild avian populations would be more profoundly damaging, and those damages would extend beyond genetics to include behavioral and cultural effects. I think that Araki’s team’s study has important and potentially far-reaching implications for captive-breeding programs.
“Genetic Effects of Captive Breeding Cause a Rapid, Cumulative Fitness Decline in the Wild” by Hitoshi Araki, Becky Cooper, Michael S. Blouin. Science 318:100-103 (5 October 2007 | DOI: 10.1126/science.1145621) [PDF]. (images, quotes)