Bringing the Big Fish Back

By Eric P. Palkovacs, Assistant Professor of Ecology and Evolutionary Biology at UCSC

Fish are smaller today than they were several decades ago. In my previous column, I described how the selective removal of large fish can cause evolutionary downsizing in harvested populations. Here, I discuss how evolutionarily informed conservation strategies can be designed to restore big fish. Biologists working to formulate such strategies are looking to a surprising source for inspiration – agricultural pests.

An important challenge in fisheries is to sustainably harvest fish populations without driving evolutionary reductions in body size. In an analogous situation, a major challenge in agriculture is to kill crop pests without causing the evolution of pesticide resistance. Although the context is different, the underlying management challenge is similar. In both situations, the goal is to devise management strategies to remove part of the population without causing unintended evolutionary side effects for the remaining wild population.

Insect pests inflict major crop losses worldwide, leading to the widespread use of pesticides. A pesticide may kill most of the harmful bugs in the population, but a small fraction of the population often contains the genetic variation needed to survive the chemical onslaught. This resistant fraction can soon come to dominate the population, rendering the pesticide ineffective. In order to maintain the effectiveness of pesticides, farmers have adopted a number of strategies to forestall unwanted evolution. In a surprising twist of fate, the same set of underlying evolutionary principles that allow farmers to combat pesticide resistance can be applied to restore large fish.

One strategy to prevent undesirable evolution is to devise strategies that mimic natural mortality schedules in the wild. Humans often kill a very large proportion of the target population. Simply reducing the body count can be an effective strategy to slow unwanted evolution. In agriculture, this can mean using less potent pesticides or employing natural enemies such as predators or parasites. In fisheries, such an approach means reducing the intensity and size-selectivity of fishing.

In most fish populations, juveniles suffer naturally high death rates due to predators. However, once fish grow large, they become more resistant to predators and may expect to live a long time. The 60 lb. (27 kg) Atlantic cod that cruised the waters off Newfoundland in the early 1900s had few natural predators.

Selectively harvesting large individuals therefore represents a fundamental shift from the natural pattern. Employing fishing strategies that protect large fish can bring fishing mortality closer into line with natural mortality, thereby helping to prevent evolutionary downsizing. Slot limits (Keeping fish caught within a specified size range) can be an effective management tool to achieve this goal. Slot limits focus fishing pressure on mature but moderately sized individuals, thereby reducing impacts on the largest fish.

A complimentary method is to set aside protected areas where the natural selection regime still prevails. Setting aside reserves provides a storehouse of genes that can supply variation to the population outside the reserve. In agriculture, this strategy is referred to as planting a refuge crop. Refuge crops serve as storehouses for genes that confer susceptibility to the pesticide. The flow of genes out of the refuge can thereby prevent the widespread evolution of pesticide resistance. In fisheries, the establishment of marine protected areas (MPAs) can play a similar role in preventing pervasive evolutionary downsizing. The population of large fish within MPAs can serve as a source of genetic variation that can naturally flow into
harvested populations outside the reserve. Maintaining genetic variation is important because it may be difficult or impossible to restore valuable genes if they become completely lost from the population.

The types of management strategies described can prevent evolutionary downsizing in harvested fish populations, but implementing such strategies requires trade-offs. Just as farmers planting refuge crops must sacrifice the immediate productivity of some of their acreage for the long-term benefit of reduced pesticide costs, so too must fisheries sacrifice some current catch for the long-term benefit of retaining large fish in the future population. The benefits of large fish include economic value in commercial fisheries and ecotourism and ecological value for the unique role they play in ecosystems. The long-term benefits of restoring large fish are well worth the short-term costs.