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New Model Describes Why Collapsed Overfished Fish Stocks Don’t Recover

November 16, 2006

Contact: Christopher Dudley
Date: November 16, 2006

A new conceptual model developed by scientists at the Pew Institute for Ocean Science describes how lasting collapses of important fish stocks may be controlled by self-amplifying “feedback loop” processes operating within marine ecosystems. These can prevent recovery from the effects of overfishing, according to an article published in the December issue of Fish and Fisheries.

“Imagine the difficulty of maintaining the stability in an African veldt ecosystem if adult antelopes and zebras were to voraciously hunt and consume young lions and cheetahs,” says Andrew Bakun, Ph.D., a scientist with the Pew Institute and a professor of marine biology and fisheries at the University of Miami Rosenstiel School of Marine and Atmospheric Science.

In the case of the ocean, it is extremely common for prey fishes to catch and consume the eggs, larvae, or juvenile stages of predatory fish that prey upon them as adults. Additional uniquely marine sources of instability, such as outbreaks of jellyfish and noxious algal blooms, may trigger one another to form a cascading sequence of destructive effects that transforms the ecosystem to a non-productive wasteland.

“This is very different from the way we are used to understanding ecosystems on land, where such radical instability among populations is far less evident,” adds Bakun.

In the study, Bakun and co-author Scarla Weeks of the University of Queensland in Australia reconstruct the history of the marine ecosystem off the southwestern African coast of Namibia, which several decades ago was severely overfished by foreign fishing fleets. As the local sardine resource was decimated, a malignant series of feedback processes was set into motion that transformed the formerly productive regional ecosystem which had once yielded more than a million tons per year of desirable fish to one that is now dominated by jellyfish and “trash” species of fish with little commercial worth. Even though fishing pressure has been relaxed, no progress has been made toward restoring the fishery to its former productive state.

Bakun and Weeks blame that decline on the following proposed “feedback” sequence.

  • First, the sardines were overfished in the core of their habitat range, leading to a change in the distribution of sardines.
  • Lack of consumption by the formerly enormous sardine population allowed an overabundance of zooplankton to build up in a key area, which in turn stimulated an outbreak of medusas and other zooplankton-consuming species.
  • These expanded to dominate the entire regional ecosystem, eating the larvae of sardines and other important fish, preventing the crucial sardine population from recovering, and trapping the entire region in a state of low fishery productivity that has lasted to the present time.

“Given that each individual female fish is capable of spawning tens or hundreds of thousands of eggs each year, it is a puzzle when their populations do not rebuild as expected after fishing pressure is removed,” says Ellen Pikitch, Ph.D., director of the Pew Institute for Ocean Science. “We urgently need to solve that puzzle in order to successfully develop ecosystem-based management systems that will be effective in preserving the integrity of our precious marine ecosystems.

“Studies such as this one that probe directly into the intrinsic mechanics of ecosystem operation and population-scale biological interaction are essential efforts in that direction,” Pikitch adds.

The mission of the Pew Institute for Ocean Science is to advance ocean conservation through science. Established in 2003 by a generous multi-year grant from the Pew Charitable Trusts; the Pew Institute for Ocean Science is a major program of the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

Click FISH and FISHERIES for abstract

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