The ones that get away—does intensive fishing make fish harder to catch?

Are humans changing the course of evolution in wild fish populations? A new study making media waves today provides tantalizing evidence of a mechanism by which this may happen. In our new Naturally Speaking Reports, one of our editors caught up with Institute Senior Research Fellow Shaun Killen, the author of the study, to find out more about his work and a phenomenon called ‘fisheries-induced evolution’.

The ones that get away—does intensive fishing make fish harder to catch?

It’s startling to consider that within a commercial fish population an individual fish may have a greater  chance of being caught by humans than dying of natural causes. As a result, our intensive fisheries may do more than deplete fish stocks, they may be one of the strongest drivers of evolutionary change in wild populations—a phenomenon called ‘fisheries-induced evolution’.

In this relationship, fishing seems to change fish growth rates and their capacity to reproduce. But, could selective fishing  actually drive fish to become less catchable? And if so, what are the actual processes that influence whether a fish is more or less vulnerable to being captured?

Most of the current evidence on fisheries-induced evolution comes from work on body size. Commercial fishing gears tend to select on size, and in heavily fished areas marine biologists have found that fish tend to be smaller and reproduce earlier in life. This may reflect how a population survives when fishing targets the largest individuals. But, body size is influenced by a wide range of environmental and physiological processes, which are difficult to disentangle. Also, while body size may be a desirable property for commercial fisheries, there are a range of other traits, besides size, that could affect which individuals actually end up in the net.

Within fish populations, just as with humans, physiological traits vary between individuals—some fish are better athletes than others, or have different energy requirements. Yet until recently, nothing was known about the role of these traits in determining which individuals are most likely to be caught, and how the ones that get away affect the rest of the population.

The question of whether commercial trawling could be driving evolutionary change is the subject of a study published today in Proceedings of the Royal Society B, led by Senior Research Fellow Shaun Killen. The study, which looked at the contributions of physiological traits such as swimming ability, metabolic rate, and indicators of aerobic and anaerobic physical fitness, shows for the first time that physiological traits do play a role in determining which individuals are captured.

For the study, Shaun teamed up with an old colleague from the USA, Dr Cory Suski, an Associate Professor in the Department of Natural Resources and Environmental Sciences at the University of Illinois at Urbana-Champaign. Cory joined the Institute last year while on sabbatical, bringing with him a wealth of experience studying the vulnerability of fish to being captured by anglers.

How to simulate a trawl

The first challenge for the team was to create an appropriate lab-based simulation of a trawl, using wild-caught common minnow as a model shoaling fish. It may sound strange to use minnows in a trawling simulation, but while it is true that there is no commercial fishery for small minnows, the use of smaller stand-in species allows researchers to examine questions and scenarios that would impossible with larger, commercially important species such as cod or haddock. Interestingly, minnows behave very similarly to these species when in the mouth of a small trawl.

It also turns out that fishnet stockings also provide a crude yet effective means to outfit a swim tunnel to simulate events occurring at the trawl mouth!

In a series of experiments, the team filmed schools of seven individual minnows in swim tunnels as they were exposed to a gradually increasing flow speed, which was then maintained at a constant speed—equivalent to a human sprint. They then placed a trawl net behind them, and over the course of 10 minutes recorded how much time each fish spent in the net. After resting up for several days, the same fish were tested for their aerobic and anaerobic swimming performance. Here they were exposed to a constantly increasing flow rate, not unlike climbing on a treadmill and having it increase in speed —this would first challenge your aerobic running, then your anaerobic sprinting as it sped up, until you fall off the end. This provides the speed of maximum anaerobic performance, which varies between individuals.

In the study, those individuals with greater anaerobic swim ability were those ultimately able to propel themselves ahead of the mouth of the net without tiring, or able to use rapid bursts to sprint away either around or over the net—much as they would to evade other predators.

The work suggests that if physiological traits do play a role in susceptibility to capture, then it is certainly possible that commercial trawl fisheries may select for these traits, and so affect the physiological traits of the surviving population. Though whether these traits can become hardwired into the population is another question, one that would need to establish whether traits such as metabolic rate and swimming ability (aerobic and anaerobic capacity) are heritable between generations.

The team are now developing much more sophisticated sea loch-based replicas of trawls that allow for simulations of many different aspects of real trawl design.

Are we selecting for faster fish?

But if the fish that survive are the better swimmers, are we selecting for faster fish? Is this a bad thing? We put the question to Shaun who told us,

‘There are always trade-offs…there are reasons why all fish aren’t amazing athletes. Investing in athleticism requires greater resources, and humans change the balance of this. Selecting for faster fish might have the trade-off that these fish require more food—an activity which itself might expose them to more predators as they forage. They may also reproduce more slowly and be more sensitive to environmental change.’

Shaun points out that physiological traits such as metabolic rate are linked with a fish’s ability to tolerate changes in temperature, oxygen levels and food availability. This has repercussions for how well these populations are able to adapt to changes in their environment. Shaun adds,

‘It’s plausible that there could be an interaction between two of our biggest threats right now, overfishing and climate change, which hinges on the physiology of these animals.’

Listen to Shaun discussing the work with Jackie Leonard (@JackieLeonard01) for BBC World Service


Further reading

Shaun S. Killen, Julie J. H. Nati, & Cory D. Suski (2015). Vulnerability of individual fish to capture by trawling is influenced by capacity for anaerobic metabolism Proceedings of the Royal Society B, 282 : This paper is #openaccess

Jim Caryl for NaturallySpeaking reports

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