A research team found that fishing results in directional selection, leading to fish with smaller body sizes. As natural size variations disappear, the population becomes more vulnerable to environmental changes. The findings are in a paper that was just published in the journal Biology Letters.
A large international team of researchers studied populations of zebrafish (Danio rerio), a species commonly used as a model for studying fish dynamics. The team consistently removed the largest fish over five generations, replicating the effects of fishing. After five generations of harvesting, the experimental population was left alone for another six generations. A separate population of zebrafish was harvested sustainably; the researchers protected the largest fish in the group and then left the population untouched for six generations.
Populations that had experienced size-selective harvesting showed reduced variability in sizes. The fish were also much smaller on average when compared to other populations. Populations that had experienced harvesting with protection for larger individuals showed normal body size variations.
Removing the largest fish in a population leads to directional selection, a type of natural selection in which an extreme of a trait is favored above others. In this case, small fish were favored. When the largest fish were consistently removed, evolutionary pressure resulted in smaller fish on average, even when the group was left alone for six generations. Variation within a population is necessary to deal with environmental changes, such as reduced food availability, so this type of fishing can lead to population crashes.
The study emphasizes the importance of preserving variability in fish populations. Fishing policies that focus on only protecting the smallest fish may result in less variable populations over time. Populations with little variation are much more vulnerable to environmental changes and will also take longer to recover from overfishing. If the fish become progressively smaller, it also creates problems for fisheries managers. Instead, the research team recommends that larger fish are preserved to increase the sustainability of fish stocks. The authors hope that their findings will encourage more sustainable fishing practices.
Silva Uusi-Heikkilä et al. Altered trait variability in response to size-selective mortality. Biology Letters (2016).