Letting young fish escape creates instability and 'boom and bust' fisheries says top level scientific report.

- Fishing News 25. April 2008:- A TOP level scientific report says that normal fisheries management techniques of targeting bigger, mature fish and imposing minimum mesh and landing sizes to allow young fish to escape is "exactly wrong".

The report says this kills off the best breeding fish, allows too many young fish to survive, and puts pressure on available food sources, leading to instability in the stocks and a kind of `boom and bust' cycle. It also says that managing stocks by setting current biomass targets while ignoring fish size can exacerbate this instability.

The new study is published in the respected scientific journal Nature following a major research project at Scripps Institution of Oceanography at the University of California San Diego. The research involved a distinguished team of government and international experts including two UK chief scientific advisors to the government - Sir John Beddington of Imperial College London, current adviser, and Lord Robert May of Oxford, a former adviser. It is based on data from the California Co-operative Oceanic Fisheries Investigations (CaICOFI), a programme based at Scripps, which has monitored fish and oceanographic activities of the California current for more than 50 years.

Fishing can alter the age pyramid of stocks by removing the few large, older fish that make up the top of the pyramid, leavíng a broad base of faster growing small young fish, says the report. This rapidly growing base is unstable - "a finding having profound implications for the ecosystem and the fishing industries built upon it". Professor George Sugihara of Scripps said the data show that fished species seem to be significantly less stable than unfished species, and that this appears to be caused by alteration of the age structure of the stocks.

The report says: "Imagine a container of water with one 500 1b fish. With food, it grows a little bigger. Without food it gets a bit smaller. Imagine the same container with 500 one pound fish. They eat, reproduce, and the resulting thousands of fish boom, quickly outstripping the resources, and the population crashes. "These many smaller fish with the same initial 'biomass' as the larger fish can't average out the environmental fluctuations, and in fact amplify them through higher turnover rates that promote boom and burst cycles." Fishing typically extracts the older, larger members of a targeted species and fishing regulations often impose minimum size limits to protect the smaller, younger fishes. That type of regulation... is exactly wrong," said Professor Sugihara. "It's not the young ones that should be thrown back, but the larger, older fish that should be spared. Not only do the older fish provide stability and capacitance to the population, they also provide more and better quality offspring."

He said that the danger is that current policies that manage according to current biomass targets (without significant forecast skill) while ignoring fish size pose risks that can further destabilize the population. This instability can extend to the whole ecosystem, much like a stock market crash or a domino effect, and magnify risk for the fishing industry itself as well as those of ecologically related fisheries.

'This is especially true when trying to rebuild fish stocks, Prof Sugihara says. "This may be the most important implication of this work, as we attempt to rehabilitate fisheries," said Prof Sugihara. "Regulations based solely on biomass harvest targets are incomplete. They must also account for age-size structure in the populations. "Current policies and industry pressures that encourage lifting bans on fishing when biomass is rehabilitated - but where maximum age and size are not - contain risk." This is currently the case with Atlantic swordfish, for which industry pressures to resume fishing are based on the restoration of historic biomass levels, even though the swordfish are clearly undersized. "In the extreme case, the danger of such unstable dynamics for certain populations for management is that harvest targets may lag the population, potentially making things worse," said Prof Sugihara.

"A high harvest target may be set after an especially abundant period when the population may be poised to decline on its own. Likewise future abundant periods may represent missed opportunities, despite current low abundances. As senior officials of the Canadian Department of Fisheries and Oceans have said, we are often a year behind in our stock projections.'"


Here is the abstract from the NATURE magazine and reference to the cited article:

Why fishing magnifies fluctuations in fish abundance

Christian N K Anderson, Chih-hao Hsieh, Stuart A Sandin, Roger Hewitt, et al. Nature. London: Apr 17, 2008. Vol. 452, Iss. 7189; pg. 835, 5 pgs

Abstract (Summary)

It is now clear that fished populations can fluctuate more than unharvested stocks. However, it is not clear why. Here we distinguish among three major competing mechanisms for this phenomenon, by using the 50-year California Cooperative Oceanic Fisheries Investigations (CalCOFI) larval fish record. First, variable fishing pressure directly increases variability in exploited populations. Second, commercial fishing can decrease the average body size and age of a stock, causing the truncated population to track environmental fluctuations directly. Third, age-truncated or juvenescent populations have increasingly unstable population dynamics because of changing demographic parameters such as intrinsic growth rates. We find no evidence for the first hypothesis, limited evidence for the second and strong evidence for the third. Therefore, in California Current fisheries, increased temporal variability in the population does not arise from variable exploitation, nor does it reflect direct environmental tracking. More fundamentally, it arises from increased instability in dynamics. This finding has implications for resource management as an empirical example of how selective harvesting can alter the basic dynamics of exploited populations, and lead to unstable booms and busts that can precede systematic declines in stock levels.

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