STANFORD (US) — Overfishing has a detrimental effect on population numbers, whether it’s happening to big fish in a small pond or small fish in a big one.
An analysis of more than 200 assessments of global fisheries finds that populations of small fish such as sardines and anchovies were at least as likely to have collapsed at some point in the last 50 years as stocks of large fish.
The finding runs counter to a longheld assumption that dramatic population declines suffered by large predatory fish, including tuna, sharks, and marlins, indicate they are at the greatest risk of extinction.
“We were expecting to see a strong pattern with large, top predators showing the highest probability of collapse,” says Malin Pinsky, a graduate student in biology at Stanford University. “We were really surprised to find that just isn’t the case.” Instead, small species were up to twice as likely to have suffered a major decline.
The analysis is reported online in the journal Proceedings of the National Academy of Sciences.
Small fish are a vital link in the oceanic food chain and when a species suffers a plunge in population, it hits the mammals, birds, and other fish that depend on that species for food.
“There are relatively few species at that level in the food chain, so if one of them collapses, it can have a big impact,” Pinsky says. “It is a big deal.”
Smaller fish tend to be short-lived and therefore reproduce and mature faster than large species, so a drop in their population tends to last about five years. But not every small fish population bounces back that quickly. A prime example is the collapse of the sardine fishery in Monterey Bay, California that took decades to recover.
Over 25 percent of the world’s fisheries consist of small fish, primarily for use in animal feed, fertilizer, and nutritional supplements.
“The important lesson is that all species of fishes can collapse once humans decide to eat or use them, from sardines to swordfish,” Pinsky says. “This really contrasts with what scientists, managers, and the conservation community have often assumed up until now.”
Collaborating with researchers at Rutgers University and Dalhousie University, in Halifax, Canada, Pinsky and Stephen Palumbi, professor of marine science, used two data sets, one compiled by the Food and Agriculture Organization of the United Nations and another housed at Dalhousie, to which researchers around the world contributed.
When team members began examining the data, they were seeking to figure out a way to predict fisheries collapses, in hopes of then developing ways to head them off.
“We looked at everything from small species to really large species and asked how frequently fisheries collapses occur for the whole range,” Pinsky says. “We were not focused on small fishes when we started out, but that was what popped out.”
“Local managers and fishermen have known about individual fishery collapses for years,” says Palumbi. “It took looking at 50 years of data and hundreds of fisheries to realize that these collapses among small species actually add up to a whole lot. Bringing a halt to overfishing is the best way to reduce collapses in the future.”
Most of the available data came from fisheries in the developed world, so some of the findings of the study may not apply to the developing world where management practices differ.
The National Science Foundation, the Cedar Tree Foundation, and the Natural Sciences and Engineering Research Council of Canada funded the research.
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