Undesirable Evolution Can be Reversed in Fish, Stony Brook University
Scientists Show In an intriguing 21st century example of Darwinism, researchers demonstrate that fish will again grow to larger sizes and produce more young when size-selective fishing is eased
March 03, 2009
A long-term field and DNA study by the Institute for Ocean Conservation Science at Stony Brook University, University of Miami, Field Museum of Chicago and others has shown that young lemon sharks born at the Bimini islands, Bahamas, tend to stay near their coastal birthplace for many years. While shark research and conservation typically focuses on baby sharks confined to shallow habitats, or ocean-roaming adults, less is known about these intermediate-aged animals, which are the breeders of tomorrow and are roughly similar in development to human ‘tweens’ and teenagers. Tropical island-nations that sacrifice their nursery habitats to coastal development are therefore likely to lose not only babies but also much older sharks from their local areas, with potentially dire effects on the surrounding ecosystem. The study, conducted over a 14-year period at the Bimini Biological Field Station, is the cover article in the August issue of Molecular Ecology, a leading international scientific journal.
“It takes some sharks more than a decade to reach reproductive age, so we set out to better understand the phase of their development from when they are a couple of years old until they are on the verge of sexual maturity,” said lead author Dr. Demian Chapman, shark scientist with the Institute for Ocean Conservation Science at Stony Brook University (SBU) in New York, and an assistant professor at SBU. “We were very surprised to see that many lemon sharks lingered for years around the island where they were born -- often more than half of their development to adulthood.”
Fear of deep water and the bigger predators that live there combined with abundant prey in the mangroves around Bimini probably keeps these island-born sharks in safer waters near home for several years after their birth. “This means that using marine reserves and other local conservation measures may help protect sharks born around tropical islands for much longer than we thought,” Dr. Chapman explained. He suspects that future research could show that these stay-at-home behavior patterns are common among many shark species that live and breed around tropical islands. “If island communities develop all of their shark nursery habitats, like mangroves, or overfish baby sharks in local waters, then they will subsequently lose a big chunk of the older sharks as well,” he said.
Love them or not, sharks are essential to healthy oceans. Removing these top-level ocean predators will disrupt the local food web and cause negative consequences for other species and the ecosystem at large. Moreover, many tropical islands generate substantial revenue from shark-dive tourism, which this new research suggests will be heavily reliant on sharks born in local nursery areas.
During the course of the Bimini study, from 1995 to 2007, over 1,700 immature lemon sharks were caught, tagged and released. The implanted tags, plus subsequent recaptures and DNA analysis, showed that more than half of the 3- to 7-year-old sharks caught off Bimini were born locally and had lingered near their birthplace for years. Full results are described in the study, entitled, Long-term natal site-fidelity by immature lemon sharks (Negaprion brevirostris) at a subtropical island.
“In general, the survival of these intermediate-aged sharks is critical for sustaining shark populations,” said study co-author Dr. Samuel Gruber, Professor at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and Director of the Bimini Biological Field Station, who has been leading the overall lemon shark research program at Bimini since 1978. “Our study suggests that local conservation efforts can help many lemon sharks born at islands like Bimini survive through roughly half of their development to adulthood. Broader scale, sometimes international, management is needed to protect them after they’ve left their birthplace as adolescents and adults.”
Detailed information on how sharks disperse from their birthplace could be very useful for conservation efforts throughout the tropics, given that many tropical shark species are threatened by overexploitation to supply the trade for shark fin soup, for which demand is especially high in Asia. Between 22 and 73 million sharks are killed each year to supply the fin trade, and international management agencies are scrambling for solutions to stem severe shark population declines.
“Our study suggests that many tropical island nations may not have to wait for complex international shark regulations to be established in order to act,” said Dr. Chapman. “Their local management efforts could give immature sharks a chance to grow up in relative safety until they are big and ‘bad’ enough to roam deeper habitats far from home, where broader scale protection becomes more important.”
The research team is now extending its study to answer one of the great mysteries of shark biology: do sharks home back to their birthplace as adults? Co-author Dr. Kevin Feldheim of the Field Museum in Chicago, who led the genetics part of the study, said: “This research showed that most of the young sharks left the island by the time they were mature. Now we want to find out if they end up coming back to the place where they were born to breed, much like salmon and sea turtles do.”
The Institute for Ocean Conservation Science (IOCS) conducts scientific research about critical threats to oceans and their inhabitants, providing the foundation for smarter conservation policy. The Institute is a major research program of Stony Brook University’s School of Marine and Atmospheric Sciences and was founded as the Pew Institute for Ocean Science in 2003. For more information on IOCS, go to www.oceanconservationscience.org and www.somas.stonybrook.edu.
STONY BROOK, NY -- Undesirable evolution in fish -- which makes their bodies grow smaller and fishery catches dwindle -- can actually be reversed in a few decades' time by changing our take-the-biggest-fish approach to commercial fishing, according to groundbreaking new research published today by Stony Brook University scientists in Proceedings of the Royal Society B. The 10-year study was largely supported by a generous grant from the Institute for Ocean Conservation Science at Stony Brook University.
Intensive harvesting of the largest fish over many decades, while leaving the small fish behind, may have unintentionally genetically reprogrammed many species to grow smaller, said lead author Dr. David O. Conover, Professor and Dean of the Stony Brook University School of Marine and Atmospheric Sciences in Long Island, New York. Although Charles Darwin showed 150 years ago that evolution equips life forms to be better adapted to prosper in their environment, unnatural evolution caused by man's size-selective fishing is causing fish to be smaller, less fertile, and competitively disadvantaged. This has also been a loss for commercial fishers who seek big fish for their livelihoods, recreational anglers in pursuit of trophy fish, and seafood consumers who desire large portions on their plates.
This study demonstrates for the first time ever that detrimental evolution in fish can be reversed, and pokes a gaping hole in theoretical models suggesting that genetic changes are impossible to undo.
"This is good news for fisheries, but it also shows that reversal is a slow process," Dr. Conover said. "Over time, fish can return back to their normal size but the reversal process occurs much more slowly than the changes caused by fishing. So the best strategy is still to avoid harmful evolutionary changes in the first place".
Current fishery management plans are generally based upon assessment methods which do not incorporate long-term evolutionary dynamics. It could take years before evolutionary change is incorporated into such plans, since the concept remains quite controversial among scientists.
"It took scientists a long time to reach a consensus on climate change, and acceptance of this phenomenon might require a long time, too," Dr. Conover said.
Dr. Ellen Pikitch, Executive Director of the Institute for Ocean Conservation Science, said, "We now have proof that the negative ramifications of common fishing practices can linger for decades, leaving future generations to grapple with a legacy of diminished ocean productivity. It is essential that fishing be transformed to minimize its evolutionary and broader ecological consequences." Evolutionary dynamics are a fundamental principle of ecosystem-based fishery management (EBFM), a holistic approach that considers the connectedness of different species and the links between species and environmental influences, rather than managing each species in isolation. Dr. Pikitch was among the earliest proponents of EBFM, and lead author of a seminal 2004 article in Science on the concept.
"We have interfered extensively with the natural course of things, and while it is very encouraging that the harmful effects of size-selective fishing may be reversible, the length of the recovery period is sobering," said Dr. Pikitch, who is also a Professor of Marine Science at Stony Brook University. "Restoration of ocean fisheries requires prompt and widespread adoption of an ecosystem-based approach to fisheries management."
Fishery management is today largely based upon minimum size limits, which ban capture of fish below a given size, species by species. With few exceptions, maximum size limits do not exist, so the large fish can easily be fished out. "This is really bad news for fish because unlike humans, the bigger and older a fish is, the more offspring it produces," Dr. Conover said. "We're talking about a big fish producing ten times more eggs than the same species in a smaller size, but unfortunately, current fishery regulations make it dangerous to be big." The solution is to design regulations to protect large fish, he said. "If we stopped fishing out the largest fish, there would again be a benefit to being big, and genetic changes will occur that gradually trigger a population rebound."
Dr. Conover and colleagues worked at the Flax Pond Marine Laboratory on the north shore of Long Island, with six wild populations of the Atlantic silverside (Menidia menidia) captured from Great South Bay, NY. This marine harvested species is common to the east coast of North America. Researchers selectively removed the largest fish over the course of five generations (for the silverside, each generation amounts to about one year). They then allowed five generations of recovery, in which no size-selective fishing occurred. Mean body size of the fish declined rapidly during the size-selective fishing period. During the recovery years, body size gradually rebounded.
In this experiment, Dr. Conover's team estimated that full recovery would take about 12 generations, but recovery time in the wild could be shorter or longer depending upon the species and its environment. If their estimate is roughly correct, however, it means that recovery could take many decades for commercially harvested fish species, since they typically have a generation time of three to seven years. Cod have a five-year generation time, for example, which equates to a 60-year recovery period. Evolutionary change may explain why several cod stocks have still not returned to historical body size and abundance even though the fishery was closed years ago, Dr. Conover said.
In addition to Dr. Conover, authors on this paper are Dr. Stephan B. Munch of the Stony Brook University School of Marine and Atmospheric Sciences, and Dr. Stephen A. Arnott of the South Carolina Department of Natural Resources Marine Resources Research Institute.
The Institute for Ocean Conservation Science conducts scientific research about critical threats to oceans and their inhabitants, providing the foundation for smarter conservation policy. The Institute is a major research program of Stony Brook University's School of Marine and Atmospheric Sciences and was founded as the Pew Institute for Ocean Science in 2003. Visit us online at www.oceanconservationscience.org and www.somas.stonybrook.edu.
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