Fish Growing Faster in Warming Waters

John Roach
for National Geographic News
April 23, 2007
Fish are fattening up faster near the Pacific's surface, which is warming, while species in the deep sea are growing more slowly as that water cools, a new study says.

The shallow-living fish are growing 20 to 30 percent faster today than they were 50 years ago, according to the researchers' analysis of fish ear bones.

The faster growth rates closely match a warming trend in the ocean's surface waters.

"There's no question that the shallow-water fish are tracking our local version of global climate warming," said Tasmania, Australia-based Ronald Thresher, a fisheries biologist with the Commonwealth Scientific and Industrial Research Organization.

The faster growth, he added, could make the near-surface fish more resilient to overfishing. (Related: "Warming Oceans Put Kink in Food Chain, Study Says" [January 30, 2007].)

By contrast, deepwater fish are growing 20 to 30 percent slower than they were 50 years ago. Their slowing growth rates correlate with a long-term cooling of the deep waters.

The cause of the cooling trend is unclear. But analysis of deepwater corals suggests it has been going on for centuries and may be independent of global warming, Thresher said.

The slowing growth of deepwater fish raises concern about commercially valuable species such as orange roughy, Thresher and his colleagues note this week in the journal Proceedings of the National Academy of Sciences.

"It looks like there's more than one factor going on which might reduce their productivity—not just the fishing but also environmental changes," Thresher said.

For example, fish that grow slowly—and therefore stay juveniles longer—are weaker and more susceptible to predation and starvation.

(See a map of overfishing's effects around the world.)

Growth Rings

The researchers examined bony structures in fish's earlike organs to determine the growth rates of more than 500 commercially caught fish. The fish ranged in age from 4 to 128 years.

"If you take one of these [bony structures] and whack them in half, you can see these little growth increments," Thresher said.

The increments are similar to tree rings. Wider spaces between rings represent faster growth.

The team saw "a pretty substantial slowdown in the growth rates for the deepwater ones and a very significant increase for the shallow water ones," Thresher said.

Temperature Affect

When the researchers compared the fish growth rates with surface and deepwater temperature records, the team found a tight fit.

Thresher said several scientific studies indicate that temperature has an effect on the growth rates of cold-blooded animals, including fish.

"The temperature thing is a highly significant correlation and makes a lot of good sense from a biological perspective," Thresher said.

Frank Schwing is an oceanographer with the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in Pacific Grove, California.

He commented in an email that the discovery that water temperature affects growth rates is not surprising. But, he said, it "is just one of many, often competing, impacts of climate change" on fish.

For example, warming oceans could also disrupt the food chain, shift migratory patterns, and disturb reproductive cycles. Fisheries managers need to consider all these factors, he said.

Boris Worm is a marine biologist at Dalhousie University in Halifax, Canada, who studies changes in marine biodiversity.

Impressed by the new study, he said the climate effect on long-lived and easily overfished deepwater species like orange roughy is of particular concern.

"If you have fishing pressure and at the same time this slowing down of growth rates due to climate change, you have a double-whammy effect," he said.

The finding, Worm added, should make managers of these deepwater fisheries more cautious.

"And we're not really managing them very carefully to start with."

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