Seafish response to ‘Nature' letter on fisheries measurement

November 17, 2010 15:29
In response to the letter published today in ‘Nature' by Branch et al, ‘The trophic fingerprint of fisheries', Philip MacMullen, Head of Environmental Responsibility at Seafish, said:

"This new work reinforces the view that marine ecosystems are remarkably robust - in contrast with some previous research that has predicted a catastrophic future of oceans populated only by plankton and jellyfish. Those studies used overly simple datasets to produce an index known as mean trophic level (MTL). Detecting and tracking changes in marine biodiversity requires a more sophisticated approach to measurement and we welcome this new analysis. It may well lay the basis for better and more sustainable management of fisheries and their ecosystems, reports with reference to Seafish. 

"MTL indices derived from fish catch data are often used as a proxy for the health of marine ecosystems. Branch et al have queried their uncritical use and explored possible causes for discrepancies between them and with other, related indicators.

"Branch et al note that data from catch, stock assessment and trawl surveys are all inherently selective to some extent, such that the raw data will contain bias and the datasets may not be mutually compatible. Furthermore, the inclusion or exclusion of several species - and the trophic level index ascribed to each - can be critically significant in deriving a mean trophic level. In this respect, the authors express some concern over the inclusion of certain ephemeral pelagic species, such as anchoveta, that are subject to extreme fluctuations in abundance, independent of fishing mortality.

"Correcting for these, and other variables, the authors then re-analysed data to produce a range of modified MTL indices.  They conclude that, in many of the large marine ecosystems studied, the situation was quite different from the simplistic picture more commonly described. Whilst human impacts had reduced biomass significantly, MTL had fluctuated and then recovered. In their own words: ‘Indicators such as catch MTL use readily available data and are quick and easy to calculate, but without improvement are ineffective measures of trends in biodiversity.'

"The research also shows how critically important it is to manage fishing effort and other human impacts.  Overfishing is shown to be a real threat but understanding ecosystem impacts and changes in biodiversity requires a more sophisticated approach than many workers have used in the past. We believe it's clear that better datasets are required and that the fishing fleet should have a key role to play in co-operative research.  Comprehensive catch sampling by well-trained crew should add substantially to our understanding of ecosystem dynamics and our ability to manage our impacts more effectively.

"It seems that a continuing and diverse supply of seafood is a more likely future diet than the jellyfish burgers and plankton soup which we've been promised by some."

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