If we are to sustain fish as a global food source, then fisheries and conservation managers need to take account of new evidence showing how overfishing of the larger fish in a population actually changes the gene pool in favour of smaller less fertile fish.
The fishing community has been aware of the shrinking size of individual fish for over a century, and as the century has progressed, scientists have noted that these fish grow more slowly and smaller females produce fewer young than their predecessors. The scientific community has been divided as to the reason for this. One theory, Fisheries Induced Evolution proposed as a hypothesis since the early part of the twenty first century is that our activities in removing the larger fish from populations is driving evolutionary change. The other school of thought has been that the changes are induced by responses to environmental factors, such as changes in food supply.
A paper in Frontiers in Ecology and the Environment led by fish geneticists at Bangor University, with contributions from the University of East Anglia, the University of the West Indies and the Max-Planck-Institute for Developmental Biology, has proved for the first time that the change towards smaller fish does take place at the DNA level, and within a relatively short time period of a few generations.
This lab-based study using a model tropical fish, the Trinidadian guppy, replicated fishing conditions in the wild, and while other studies have replicated and observed a reduction in fish size over successive generations before, this study also included a rigorous review of fish populations' DNA, and in particular, genes associated with body size, throughout the generations. The resultant change in DNA linked to growth (while there were no other significant DNA changes- ruling out inbreeding or random changes) proves that changes in body size and maturation is an evolutionary response to overfishing. What was seen under laboratory conditions has probably already taken place in any number of commercial fishing grounds.
Statistical modelling has already suggested that recovery from DNA change will take between five and ten times longer than simple recovery from overfishing- if the population is able to revert to the full range of DNA diversity at all. This means that current estimates for how quickly commercial fisheries will recover from declines and overfishing are probably far too optimistic. In terms of fish as a food source, not only do such genetically-based shifts lead to the need to harvest more smaller individual fish for the same tonnage, but this also has repercussions for the wider marine community and environment.
Prof Gary Carvalho, of Bangor University's School of Biological Sciences explains: "Our findings have major implications for the sustainability of harvested populations. The consequences of a shift in the genetic make-up of harvested fish to smaller less fertile individuals has serious global consequences for the environment and for global fishing industry.
"We would urge the scientific community, policy makers and managers to consider the capacity of harvested stocks to adapt to, and recover from, harvesting and predation."
"I'd advocate that response strategies should be devised to mitigate the intensity of fishing by protecting over-fished stocks, by routine revaluation of the size range of fish caught and by regional adjustment of fishing pressures. A combined strategy is needed to reduce the overall impact of fishing and to encourage natural replenishment of stocks."
"In order to successfully manage harvested resources, it is vital that we apply population genomic approaches to detect and mitigate detrimental genetic effects of harvesting selection. Collectively, our findings highlight the importance of developing high-density genetic maps and genomic tools to assess the evolutionary impact of selective harvesting in the wild," he adds.
Much of the work was conducted by Serinde van Wijk studying under a Bangor University funded Doctorate. She explains:
"Our attempts to conserve fish communities by regulating the size of fish that can be fished for, and by removing specifically the larger fish, may have had opposite effects to those intended. As well as losing the capacity to produce large sized and productive fish, specific fish populations may also be at risk of losing other specific adaptions by selective fishing, such as adaptions to particular location characteristics, like colder water or migration routes. The loss of these genetic 'types' may mean that populations may not be able to recover completely or at all."
Isabella Lövin, MEP, (Greens, Sweden), member of the Fisheries Committee in the European Parliament commented:
"This study comes at a crucial moment just as the European Union is reforming the Common Fisheries Policy. It shows that fish stocks may be even more vulnerable than we had thought and underlines how important it is to apply the precautionary principle in fisheries management and to rebuild fish stocks to levels much closer to their natural state."
Explore further: Under threat: Kenya's iconic Nairobi national park
More information: Paper: Experimental harvesting of fish populations drives genetically based shifts in body size and maturation. Frontiers in Ecology and the Environment doi: 10.1890/120229