Assumptions on Overfishing Challenged

  • by Lowana Veal (reykjavik)
  • Wednesday, December 29, 2010
  • Inter Press Service

One example of this re-evaluation can be seen in relation to conventionally accepted practices whereby young fish are protected and older fish are exploited. Younger fish are generally smaller and weigh less than their older counterparts, so policies rely on assessing fish population by biomass.

For instance, in Iceland, certain areas will be closed to fishing when there is a large proportion of small fish in the area. Spawning and nursery areas will likewise be protected.

Defying convention, Prof. George Sugihara and his team at the Scripps Institution of Oceanography have begun to criticise the current scenario whereby small fish are protected and large fish are caught. They say that this system can alter the age pyramid within a given fishery, such as pollock, by removing the few, older fish that make up the top of the pyramid while leaving a broad base of faster-growing, small young fish.

Why does this pose a problem? 'It's not the young ones that should be thrown back, but the larger, older fish that should be spared,' said Sugihara. 'Not only do the older fish provide stability and capacitance to the population, they provide more and better quality offspring.'

The danger, according to Sugihara, is that current policies that manage according to current biomass targets while ignoring fish size pose risks that can destabilise the entire population. This instability can, in principle, affect the whole ecosystem, magnifying risk for all ecologically related fisheries.

This danger is especially prevalent when trying to rebuild fish stocks, Sugihara said. '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,' Sugihara warned.

In addition to the practice of exploiting fish based on biomass calculations, other conventionally accepted practices in the fishing industry are beginning to be re-evaluated by experts. One such practice has to do with using trends in trophic levels of fish to measure the health of world fisheries.

In 1998, using global catch data, stock assessments, scientific trawl surveys, small-scale fishery data, and modelling results, scientists developed a comprehensive test that used trends in the trophic levels of fish over four decades to measure the health of world fisheries. As the first of its kind, this system for analysing the health of fisheries was considered groundbreaking.

The trophic level of an organism shows where it fits in a food web, with microscopic algae at a trophic level of one and large predators such as sharks, halibut, and tuna at a trophic level of about four.

Twelve years later, confidence in this method of assessment is waning. A new study, carried out by Trevor Branch from the University of Washington, together with scientists in the U.S., UK, Canada, and Australia, reveals weaknesses in assessing ecosystem health from changes in the trophic levels of what is being caught.

Branch and his team based their findings on a large number of trawl surveys that consider the numbers and types of fish that actually live in these ecosystems, as well as catch data, to dispute the conventional wisdom.

Branch explained that there are two major weaknesses in the accepted metric: 'One, in just under half of all ecosystems, this catch metric goes up when the ecosystem gets worse, or goes down when the ecosystem gets better. Two, this metric can remain constant, even when increasing numbers of species are collapsing in the ecosystem,' he said.

The significance of these findings is crucial, Branch said, because 'the measure is the most widely adopted indicator by which the overall health of marine ecosystems is determined.'

For example, the UN’s Convention on Biodiversity uses the average trophic level of fish caught as the main measure of global marine diversity.

'The 1998 paper was tremendously influential in gathering together global data on catches and trophic levels, and it warned about fishing impacts on ecosystems,' Branch said.

'Our new data from trawl surveys and fisheries assessments now tell us that catches weren't enough. In the future, we will need to target limited resources in the best way, focusing on species that are especially vulnerable to fishing and developing indicators that reflect fish abundance, biodiversity and marine ecosystem health. Only through such efforts can we reliably assess human impacts on marine ecosystems.'

© Inter Press Service (2010) — All Rights ReservedOriginal source: Inter Press Service

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