For over seventy years, fishery managers have gathered data about the Northeast Arctic Atlantic cod in order to better manage this economically important resource. As previous research has shown, fishing induces evolutionary changes, whether selecting for a particular trait or by simply pulling too many fish from the sea. But no one had used that data in asking whether evolutionarily informed fisheries management strategies would improve harvest. Now, in an Early Edition PNAS paper, Eikeset et al. incorporate that data into a bioeconomic model and suggest that reducing the current mesh size of nets would also reduce the overall economic costs to Northeast Arctic Atlantic cod fisheries, especially given regular overfishing.
From an ideal sustainability standpoint, fishers would only catch the largest and most mature cod, and never overfish. However, feeding our growing population means improving harvest. Classical fisheries management strategy suggests reducing mesh size to catch intermediate (and larger) fish, and so still allow smaller fish to escape, grow, and reproduce before being caught. In contrast, evolutionarily informed fisheries management acknowledges that intermediate-sized mesh nets puts selective pressure on fish to mature earlier and at smaller sizes, and overfishing increases that selective pressure. But “Maturing earlier may also reduce fecundity,” the authors write, “because individuals are smaller when they reproduce.” In addition, there are several other positive and negative effects of fisheries-induced evolution, which the authors also discuss.
In sum, the authors found that “evolution has a positive effect on the fishery if fishing mortality is set optimally, but a negative effect if fishing mortality is high,” though reporting that “it is not immediately obvious why” given the various feedback loops of biomass, growth, and maturation, among others the team included in their bioeconomic model. Still, the authors found that simply reducing the mesh size of nets is “sufficient to avoid any evolutionary costs… because it leads to individual growth that is fast enough to negate any detrimental effects of early maturation.”
In contrast to much of the existing literature, “which tends to sketch a gloomy picture of the potential consequences of fisheries-induced evolution,” the authors “very surprising” and “comforting” finding is that that fishing “can cause evolution of faster growth, allowing the population to withstand higher harvest pressure and prevent stock collapse.” Still, the authors do not oversell their conclusions, admitting that their calculated evolutionary costs (and benefits) are negligible for managers who optimally manage their fish stocks under classical fishery management techniques, even if net mesh sizes remain the same.
Recognizing that few fish stocks are optimally managed, however, and that overfishing is a recurrent problem, the authors warn such evolutionary costs with current nets, while small, “may just be enough to push a fish stock from the state of overexploitation into collapse.” The last major collapse of an Atlantic cod fishery occurred in the Northwest Atlantic cod fisheries in the mid-1990s. The fishing moratorium put into place is still in effect: the Atlantic cod population off the coast of Maine has yet to recover.
Economic repercussions of fisheries-induced evolution. Anne Maria Eikeset, Andries Richter, Erin S. Dunlop, Ulf Dieckmann, and Nils Chr. Stenseth. PNAS. Published online before print July 8, 2013, doi: 10.1073/pnas.1212593110