Forage fish like these sardines travel in large schools, making it easy for fishermen to catch a large fraction of the population all at once.

Forage fish like these sardines travel in large schools, making it easy for fishermen to catch a large fraction of the population all at once.

Hiroya Minakuchi/Minden Pictures

Sardines hit hard by overfishing

It may seem like sardines are, well, packed like sardines in the ocean—the tiny fish travel in schools of millions. But they and other small forage fish like herring and anchovies aren’t immune to the effects of commercial fishing. A new study looks at seven species over 25 years and finds that overfishing exacerbates natural boom-and-bust cycles of the fish, which are not only a food source for humans but for countless larger marine animals.

The study “is a thought-provoking piece of work” says Jason Link, a fisheries scientist at the National Oceanic and Atmospheric Administration in Woods Hole, Massachusetts. And it strengthens the case that forage fish deserve more protection, he adds.

There’s no mystery about the status of tuna and other large predators; many of these stocks are clearly overfished. But it’s harder to know the impact of fishing on forage fish populations. That’s because populations of these small fish naturally alternate between abundant and scarce. Whether commercial fishing makes the collapses more frequent, prolonged, or severe has been hard to pin down. “There was this notion that … fishing doesn’t really matter,” says ecologist Timothy Essington of the University of Washington, Seattle.

Essington and his colleagues turned to stock assessments—regular surveys of fish populations that are collected by both commercial fisheries and research vessels. They analyzed 55 stocks from around the world of seven forage fish species, which had been tracked for at least 25 years. Then they used the stock assessments to not only calculate the overall fish populations, but also local fishing rates—how much of a fish population was being caught at a given time. When fishing rates were high, the scientists found, populations collapsed to levels six times lower than they would have been otherwise. The collapses weren’t more frequent, however, the team reports online today in the Proceedings of the National Academy of Sciences.

This highlights a problem in fishery management: “We usually increase fishing at exactly the same time the population is struggling,” Essington says. Because the forage fish tend to congregate in large schools, it remains easy for fishermen to catch many fish at once; but as populations decline, one school can be a larger fraction of the total population. “But the problem is that once you get to these lower abundances, you end up in this really risky zone.”

Using the new data set, Essington and his colleagues also developed a simulation to explore the risk. If fishing is halted when populations start to decline, forage fish remain plentiful, the models show. “If you just turn off all fishing and let the populations do what they would have done naturally, most of them will completely recover within a few years,” Essington says, suggesting a time frame that fishing bans might be enforced for.

That makes sense. “In general terms, limited harvest when populations are low is wise,” Link says. But before fishing restrictions could be put in place, more research is needed on exactly what the cutoff for each fish species would be, and how long a restriction will let the fish recover, he notes.

Climate change makes the problem even tougher. “It’s an excellent study, but the large omission here is that of climate, and we know that climate drives the dynamics of many of the stocks that they studied,” says David Checkley of the Scripps Institution of Oceanography in San Diego, California, who was not involved in the new work. To keep forage fish abundant, the role of both climate change and commercial fishing will need to be teased out so the populations can be protected from both, he says.

*Correction, 9 April, 5:25 p.m.: The original image that accompanied this story was not one of sardines, as indicated. We have replaced the image.