Overfishing, climate variability appear to be culprits, says Rutgers marine biologist

A Rutgers marine biologist studying the rise and fall of fish populations worldwide recently made a counterintuitive discovery: ocean species that grow quickly and reproduce frequently, such as sardines, anchovies and flounder, are more likely to experience dramatic plunges in population than larger, slower growing fish such as sharks or tuna.

Why is this counterintuitive? Because for life on land, the situation is in stark contrast.

“Rabbits are doing pretty well compared to rhinos,” said Malin Pinsky, assistant professor of ecology and evolution in the School of Environmental and Biological Sciences. “Mice thrive while lions, tigers and elephants are endangered.”

Sardines are one of the small, fast-growing fish species likely to experience dramatic plunges in population due to overfishing. Photo: Rich Carey, Shutterstock

After studying population changes in 154 species of fish worldwide over 60 years, Pinsky was surprised to see marine equivalents of rabbits and mice collapsing to low levels – still shy of extinction but serious enough to disrupt ocean food chains or fishing-based societies.

In his research, published this week in the journal Proceedings of the Royal Society B, Pinsky sought an answer to this riddle. In nearly all of the cases, overfishing was the culprit.

"Climate variations or natural boom-and-bust cycles contribute to population fluctuation in small fast-growing fish,” he noted, “but when they are not overfished, our data showed that their populations didn’t have any more tendency to collapse than other fish.”

For example, this effect is apparent in sardines off the coast of southern California, whose populations have fluctuated naturally for thousands of years. But these fluctuations are not enough to explain why so many fast-growing fish species have collapsed in recent decades – meaning a drop to less than 10 percent of historical levels. With the advent of efficient fishing vessels and techniques after World War II, population collapses started to occur much more frequently in sardines and anchovies, which are valued for pet food and fish oil.

Malin Pinsky, assistant professor of ecology and evolution in the School of Environmental and Biological Sciences. Photo: Shonda Foster

“Overfishing is a problem throughout the world and across all species, including slow-growing fish like sharks, many of which are in serious trouble,” said Pinsky. “But it turns out that fishery collapses are three times more likely in the opposite kinds of species – those that grow quickly.”

Combining climate variability with high levels of fishing greatly increases the risk of collapse, Pinsky says.

“If environmental factors are driving the population down, previously sustainable levels of fishing might suddenly drive a collapse,” he said. “The proper response would be to quickly change fishing practices, but every political or bureaucratic process has some lag.”

Pinsky will examine this further with summer flounder, a popular east coast sport fish, as populations drop off the coast of the Carolinas in response to warmer waters and increase off the Mid-Atlantic States.

Pinsky’s analysis relied on data from fisheries management agencies worldwide, and used mathematical analyses developed by collaborator David Byler in operations research and financial engineering at Princeton University. The work is important, he notes, because species declines can affect the viability of sea life higher in the food chain and human societies that rely on fishing to supply food and economic support.

The National Science Foundation provided major funding for the research.