The fittest fish escape trawling

Humans have used trawling nets to catch fish for hundreds of years, making us one of the most effective predators for certain fish species. It is well known that we can shape characteristics of fish populations through ‘fishery-induced evolution’, where we catch the most desirable and largest fish from the population while trawling. Much less is known about an individual fish's vulnerability to trawling. For example, are some fish consistently more likely to be captured? And, might this capture vulnerability be due to aspects of their physiology already known to be important for evading natural predators in the wild, like metabolic rate and swimming performance?Shaun Killen, Julie Nati and Cori Suski, of the University of Glasgow, UK, and the University of Illinois at Urbana-Champaign, USA, set out to answer these questions using a population of minnows in a swim tunnel outfitted with a trawling net. While you might think it is less optimal to test such questions using a surrogate species in the laboratory, Killen and colleagues point out that these questions would have been impossible to answer with large-scale trawling in the wild.Knowing that quick, anaerobically powered movements such as darting are often used by fish to avoid predators and could be used to avoid oncoming trawls, the team measured various aspects of metabolism after exhaustively exercising fish. They measured how much extra oxygen each fish consumed after exercise before returning to their baseline metabolic rate (metabolism for everyday functions). Then, the researchers grouped fish in shoals and assessed their ability to resist capture in the trawl when the water flow was set to a constant speed of 38 cm s−1. Finally, the authors assessed swimming performance when they increased the water flow rate to simulate the fish trying to escape and measured the speed at which the fish switched from aerobic swimming (smooth and sustained movement) to anaerobic swimming (burst and glide movement). This switch was a measure of aerobic or endurance performance, and could be important for fish out-swimming a trawl net. They also measured the speed at which the fish failed to sustain anaerobic burst swimming and was unable to continue swimming in order to evaluate the animal's anaerobic power.The researchers first found that a fish's vulnerability to trawl capture was highly repeatable as the same fish were captured repeatedly across multiple trawling sessions. The fish that avoided capture also had greater anaerobic capacity and anaerobic swimming performance, they consumed more oxygen after exercise and were able to sustain burst and glide swimming at greater speeds: so anaerobic movements could be important for escaping an approaching trawl. In addition to increased anaerobic performance, fish that avoided capture had greater aerobic swimming performance and they maintained smooth locomotion at higher water speeds. The authors suggest that aerobic endurance could be important for out-swimming trawls after their initial approach.Killen's team is the first to identify that physiological traits can determine a fish's vulnerability to capture by a trawling net and that this vulnerability is very consistent. While more research is needed to fully understand the relationship between various aspects of exercise physiology and capture, they have provided an intriguing first investigation. Their results suggest that fisheries have the potential to shape physiological traits in heavily trawled fish populations. Indeed, by capturing all the ‘slow-pokes’ we might be causing our most consumed fish to be better at escaping our trawl sweeps.

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