When North Atlantic fish stocks fell to record lows in the 1990s, conservationists called for limits on commercial fishing. But even with stricter fishing regulations, Atlantic cod continue to dwindle. Now research shows that ocean warming could be worsening the situation, by causing wild swings in the fish's food supply.
Growing cod depend on swimming fish larvae and other kinds of plankton. When fry hatch in early spring, they feed mainly on the eggs of tiny aquatic crustaceans called copepods. Once the cod reach a certain size they begin eating euphausiids, a plankton high in Vitamin A, and other fish larvae.
To investigate the interplay between plankton and cod populations, Gregory Beaugrand, a research scientist at the CNRS in Wimereux, France, and colleagues surveyed data from 46,777 samples collected during the last 4 decades by merchant ships in the North Atlantic and North Sea. Between 1963 to 1983, the team discovered skyrocketing levels of the plankton that cod fry like to eat. That bumper crop matched a huge increase in the number of 1-year-old cod.
The team also observed a more recent decline in copepods. Not only has the overall supply shrunk, but the copepods are maturing later so that the cod larvae don't have access to their preferred prey size at the right time. This change in food abundance happened at the same time as the fall in adult cod populations. The food depletion has exacerbated the impact of overfishing, the authors speculate in the 10 December issue of Nature.
The unfavorable changes in the plankton ecosystem parallel a warming of the sea surface, Beaugrand says. "If this warming goes on it is unlikely that the cod will remain in the North Sea even if there is a zero cod catch in the coming years," he says. "This kind of study adds to the evidence of the way that climate moves its influence up the food chain," says Arthur Miller, an oceanographer at the Scripps Institution of Oceanography in San Diego.