What could make 17 scientists from eight European countries, most of them associated with their national fisheries research institutes, team up to author a rather short article describing a situation that is well-known and has been written about before?
The article, entitled 'Managing Evolving Fish Stocks' was recently published in Science 318, 1247 (2007). The list of the authors includes Dr Christian Jorgensen, the author for correspondence (christian.jorgensen@bio.uib.no), and M. Heino, whose article 'Does fishing cause genetic evolution in fish stocks?' was reviewed on this page in March 2004. In that article, apart from criticising single-species management, Heino wrote about the effect that trawling may have on the 'genetic make-up' of the target species. Heino was not alone. The effect of selective fishing had already been observed and described by several scientists, both before and later on.
By and large, when large number of scientists team up to sign an article, their intention is to make a strong statement aimed at affecting policies or actions. 'Evolving' fish stocks, however, can be interpreted in more than one way. The 17 write that while fishing pressure may substantially exceed natural mortality, its selective nature has induced evolutionary changes occurring much faster than previously thought, if not ignored.
Doubtless, sustained creaming off the faster growing, larger and more prolific (at least in terms of egg quality) individuals brings about changes in the stock in terms of age:length and age-at-spawning parameters. The composition of the population thus changes not only quantitatively but also qualitatively, the average fish becoming smaller and spawning at a younger age. Furthermore, the removal of the larger piscivores affects all species within the fished ecosystem and ecosystem processes, by reducing predation on small fish and other changes in the nutrient cycle and habitat components.
Among the fishery scientists who have criticised size selective fishing, some believe that it causes genetic change, while others think that with reduction of the selective pressure and improved feeding and other environmental conditions, the changes are reversible.
The 17, who are members of the ICES Study Group on Fisheries-Induced Adaptive Change, go beyond just size selection. They write that "increased mortality generally favours evolution towards earlier sexual maturation at smaller size and elevated reproductive effort and that selective fishing causes further evolutionary pressures”.
The 17 remain somewhat vague as to whether actual genetic changes have occurred. They write that fisheries-induced evolution may be slow to reverse or even irreversible, while predator-prey dynamics, competitive interactions, relative species abundances, and other ecological relationships will change over time.
Critical reaction
Soon, however, other scientists critically reacted to the article’s main thesis, insisting that none of the studies quoted in their article "have provided genetic evidence for the observed [phenotypic] changes. Because evolution is by definition a change in the genetic constitution of a population, an evolutionary change cannot be postulated without demonstrating a genetic basis”… for the observed changes, which are fully consistent with and could be… “due to simple environmental inductions”, - wrote Anna Kuparinen and Juha Merilä of the University of Helsinki (Science 320: 47-48 (2008)).
With all due respect to scientific disputes, I’m afraid that, instead of targeting 'how to avoid detrimental effects of size-selective fishing', the whole thing is going to evolve into a purely academic dispute as to what degree and how, if at all, exploited fish stocks are subject to evolutionary (i.e. genetic) changes.
Evolutionary impact assessment
On the more practical part, the article proposes that the ecological and evolutionary consequences of fishing should be evaluated and mitigated. Their tool - evolutionary impact assessment (EvoIA) - involves two main steps. The first relies on biological information and describes how human actions, (including fishing), lead to [genetic] trait changes. The second step is to assess how trait changes affect the stock’s utility to society, with 'utility' defined jointly by management and stakeholders, while the change in utility of a stock as a result of fisheries induced evolution is the 'evolutionary impact'.
Dr Jorgensen told me that the group’s approach is to “identify and quantify” societal, economic and other values other than biomass, “and then politically weigh costs and benefits partitioned to different groups in society...”, and that it would be a complex process that could lead to environmental and ecological considerations being given a more central place in management. Such a process would require a broad stakeholder involvement, including industry, society and NGOs.
However, the outcome of “politically weighing costs and benefits partitioned to different groups in society” would most certainly depend upon who’s making the weighing, and who speaks for the stakeholders and for the industry. This implies involvement of economists. But there are economists and then there are economists. There are no 'objective' economics, so that, eventually, the question remains: whose side the management is on?
Notwithstanding, 'Managing Evolving Fish Stocks' raises two important points: one – it implies the inadequacy of the present understanding of the dynamics of fished stocks; and two – it indicates that changes in the size/age/maturity/fecundity relations should be considered for management purposes. A pity that the authors refrain from pointing more explicitly to the main vector of those changes: the sustained for generations selection of the larger individuals out of the exploited fish populations by regulated mesh size in fishing nets.
But the world keeps rolling as usual. While the European legislation (EU Council Regulation 1967/2006) stipulates that by 2010 all Mediterranean trawlers are obliged to implement the square-mesh codend, WWF wants it now. The shift to square mesh "will benefit fish and fishermen in the Mediterranean”, it says, “and reduce the detrimental impact of trawl fishing in the region”. It "will make trawling more selective – meaning the capture of less immature juveniles and non-target species, and reducing discards – and thus allowing the Mediterranean's fragile marine life, damaged by years of indiscriminate trawling, to begin recovery”. In short, more of the same old selective fishing. And not a word on evolutionary changes…
