Fisheye view

Complex interplay of several factors decides fisheries’ fate

 
Last Updated: Saturday 04 July 2015

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IT MAKES sense to fish when their numbers are high. Fishing targets are based on abundance of fish stocks and are the mainstay of most management plans in the US and a growing number of other countries. It is thought that if a stock reaches a certain abundance the potential harvest would be maximised.

But fish stocks fluctuate both in abundance (indicating high numbers) and productivity (the stock’s ability to increase in population). While productivity can increase or decrease due to changes in abundance caused by fishing, there are also examples where changes in productivity are entirely unrelated to abundance. Researchers from the US and Argentina used a database of trends in harvest and abundance of 230 fish stocks to find the proportion of fish stocks in which productivity is primarily related to abundance.

The results show that potential harvest of fish is closely linked to abundance only in 18 per cent of the stocks assessed. For the other 82 per cent stocks, potential harvest was primarily controlled by irregular shifts in environmental conditions or was random and not controlled by either abundance or environmental factors.

“We can think of fisheries like natural savings accounts, where we’re trying to harvest the interest—what fisheries scientists call the ‘surplus production’—without causing a long-term decline in the principal or abundance of mature adult fish. Scientists have generally operated under the assumption that the ‘interest’ is determined by the abundance of mature adults,” says the study co-author Olaf Jensen of Rutgers University. “Our research shows that this is rarely the case. Instead of operating like a simple savings accounts, fisheries are more like volatile stocks where the rate of return is determined by a variety of complex factors outside the control of managers,” Jensen said.

In many cases natural causes are the reason stocks are in low abundance, rather than overfishing, although fishing will cause even lower abundance in such cases. “Improved methods for early detection of such shifts may permit fishery managers to reduce harvest in time to avoid collapse,” write the authors in the study published online on January 14 in Proceedings of the National Academy of Sciences.

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