Fishing pressure can affect evolutionary pathways of foraging activity in the remaining fish population

In commercial fishing operations, fishing gear types are designed to target individuals based on characteristics such as swimming behavior and depth. Removing large numbers of individuals from a population can radically shape the evolution of the remaining, unfished population. One theory is that through the removal of foraging fish, less active fish become more prevalent. On the other hand, another theory predicts fishing is selective to fish that mature faster. Fish maturing faster means they do not reach as large of a size because once a fish matures, more resources are devoted to reproduction instead of growth. Under this scenario, fish reach smaller sizes and therefore remain vulnerable to predators for more of their lives. Under this theory, faster maturing fish are more vulnerable to predation, therefore leading to reduced foraging activity and risk-taking. Like the second theory, the third theory favors fish with shortened life spans, but instead predicts increased foraging.

Marion Claireaux, Christian Jorgensen, and Katja Enberg from the Institute of Marine Research in collaboration with the University of Bergen in Norway employed a computer model to investigate how different commercial fishing types may influence the foraging activity of fish populations. To catch fish, various types of fishing gear are employed that are classified by the activity the fish are typically engaged in when they are most likely to be caught. The categories of fishing gear are as follows: gears targeting hiding/resting fish (Figure 1), gears targeting foraging fish (Figure 2) and unselective gears.

Figure 1. Midwater trawls are used to catch fish that are hiding or resting in a school. Source: www.afma.gov.au

Fish activity stages ranging from resting/hiding to feeding and everywhere in between were modeled along with fishing gear types to test the effect of the three gear selection patterns on fish foraging activity.

Figure 2. Pelagic longlining targets species like tuna using baited hooks to capture actively feeding fish. Source: www.afma.gov.au

The researchers analyzed multiple variables such as mortality and growth to determine the different categories of fishing gear effects on the evolution of fish foraging activities. The model determined that fish targeted while foraging will tend to forage less and fish targeted while resting/hiding will forage more. The researchers concluded that fishing may affect the way natural mortality and behavior evolves in fish populations.

Because of the effects fishing gear can have on the development of fish populations through changes in foraging activity, managers should consider not only population estimates but also the prevalent types of gear being used on different fisheries. This study supports the hypothesis that mortality will increase when fishing gear is selective. Moreover, targeting of hiding/resting fish was found to lead to even greater mortality than those targeted while foraging. Managers that ignore these fishing-caused effects could underestimate stock size and potentially jeopardize the sustainability of economically important populations. Including the effects of fishing on fish populations would be beneficial to avoid population collapses and help to prevent conflict between managers and fishers.

If you want to learn more, check out Marion Claireaux’s blog on their project’s website: http://conevolher.imr.no/en/projects/conevolher/news/evolutionary-effects-of-fishing-gear-on-foraging-behaviour-and-life-history-traits

Reference

Claireaux, M., Jorgensen, C., Enberg, K. 2018. Evolutionary effects of fishing gear on foraging behavior and life-history traits. Ecology and Evolution [online serial] 1-11. DOI: 10.1002/ece3.4482

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