Sardines and anchovies cycle with climate

Pacific fish stocks may swing every 25 years.

Sardines flourish when waters are warmer of California and Peru. Credit: © alamy.com

Man's greedy take from the sea may not have caused the demise in the 1950s of Cannery Row - California's sardine-canning business and subject of John Steinbeck's 1945 novel. A natural, five-decade Pacific Ocean cycle may have been to blame.

Evidence compiled by Francisco Chavez of the Monterey Bay Aquarium Research Institute in Moss Landing, California, and colleagues suggests that the Pacific undergoes a physical and biological shift about every 25 years¹. The oscillation, similar in some ways to El Niño, seems to coincide with boom-or-bust cycles of the sardine (Sardinops sagax) population.

When Chavez saw all the data lined up he recalls thinking: "This is something big. It's happening everywhere in the Pacific and it's going to have a lot of influence."

The researchers plotted data from the past 100 years on global air temperature, Pacific sea surface temperature, naturally occurring carbon dioxide in the atmosphere, and abundance of seabirds, anchovies and sardines. When they smoothed out each graph using three-year averages, a closely matched flip-flop pattern emerged.

Sardines flourish in the half of the cycle when waters are warmer than average; Chavez dubs this period the 'sardine regime'. The half with cooler waters, when anchovies (Engraulis ringens) become the dominant small fish in the Pacific, he names the 'anchovy regime'.

During the sardine regime, waters are warmer off the coast of California and Peru and along the Equator, but cooler near Hawaii and off the coast of Japan. The air is also warmer and carbon dioxide levels are higher.

The pattern is exactly the opposite during the anchovy regime. Then the cooler waters off California and Peru also support more salmon, rockfish, seabirds and plankton.

These big changes every couple of decades affect all life in the ocean, working their way up the food chain to birds, marine mammals and ultimately humans.

Net changes

Steven Hare, a fisheries scientist at the International Pacific Halibut Commission in Seattle, admits that his field hasn't taken decades-long alternating patterns into consideration in the past. "We'll have to explore how these different regimes respond to fishing pressures. One may be able to withstand fishing and the other may be very fragile," says Hare.

But some are not convinced, because the group's data go back only 100 years - just two full cycles. John Hunter of the Southwest Fisheries Science Center in La Jolla, California, says that the best record for sardines and anchovies are sediments containing their scales that go back for thousands of years.

“This record for California seems to indicate that both stocks can be highly abundant at the same time John Hunter , Southwest Fisheries Science Center”

"This record for California seems to indicate that both stocks can be highly abundant at the same time," he says.

Currently, fluctuations on a ten-year scale are used to manage the Pacific sardine fishery. Sardine harvests are lowered when temperatures drop over a three-year period.

It is not known what might drive such an oscillation. Some researchers speculate that, like the El Niño cycle, which occurs over 3-7 years, it may originate from tropical atmosphere and ocean interactions that speed or slow circular surface currents, called gyres.

Climate puzzle

The oscillation also reveals a new kink in the global warming debate. The equatorial Pacific is the largest natural source of atmospheric carbon dioxide. Deep ocean water containing large amounts of the greenhouse gas is churned up to the surface.

One would expect more carbon dioxide to be released during the anchovy regime when more churning occurs. But Chavez and colleagues found the opposite.

This suggests that, on longer timescales, natural carbon dioxide levels "are controlled by more than just what's coming out of the tropical Pacific", says Michael McPhaden, who studies the ocean's role in climate at the National Oceanic and Atmospheric Administration in Seattle.

Chavez speculates that the higher levels of phytoplankton - microscopic marine plants - during the anchovy regime, which act as a carbon sink, may be offsetting the release of carbon dioxide at the Equator. But he and McPhaden agree that other carbon sinks in the ocean and on land must be investigated to complete the natural carbon-cycle picture.

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Related external links

Monterey Bay Aquarium Research Institute