Fishing Can Reduce Size Variation in Fish
Fishing typically removes large individuals from the population. Over time, this leads to evolutionary shrinking of fish and also erodes the natural genetic diversity of fish stocks. New study from the University of Turku shows that removal of the largest individuals from populations can decrease variability in fish size which in turn can hinder their adaptation to environmental changes.
Fishing can cause heritable changes in average fish size, as minimum size regulations direct fishing towards large individuals. For example, pikeperch in the Finnish Archipelago Sea have shrunk and reach their sexual maturity earlier as a result of intensive and size-selective fishing.
– In addition to shrinking the average body size, fishing can also reduce growth and size variation among individuals in a fish stock. It is very important to preserve this kind of natural diversity as it helps the fish to adapt to the constantly changing environment, states Postdoctoral Researcher Silva Uusi-Heikkilä from the Department of Biology.
When food is scarcely available, individuals that grow slowly will survive while fast growing fish will starve. Therefore, populations that consist of individuals growing at different rates will be better off as at least some of them will survive in fluctuating environments.
The joint research project of the University of Turku and Åbo Akademi University studied how fishing affects the size and growth variation in fish.
– We have studied the evolutionary effects of fishing experimentally using zebrafish as a model organism, says Uusi-Heikkilä.
The zebrafish were harvested for several generations, after which the fishing was halted for six generations. The researchers found consistent differences in body size variation among differentially harvested zebrafish populations.
– Removing large fish led to lower size variation compared to a harvesting strategy where the largest fish were protected, says Uusi-Heikkilä.
Decreased size variation caused by fishing can have serious consequences. Fish stocks' recovery from overfishing can be extremely slow or even impossible. In addition, fish populations' ability to adapt to changes in their environment deteriorates as their diversity decreases.
– Also from this point of view, protecting large individuals is very useful, concludes Uusi-Heikkilä.
JV
Photo: markus53