Anthropogenic evolution threatens fish stocks

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For millennia, humans have used selective breeding on domesticated animals to alter their size, shape, behavior and other characteristics. But now we’re actually doing it for wild animals on a global scale through another form of ‘artificial selection‘ because animals are forced to adapt to the way we change and damage the environment.

In research published in Proceedings of the National Academy of Sciences, University of Melbourne ecologist Dr John Morrongiello and his colleagues explored the combined effect of fishing and climate change on fish populations and their evolution.

Overfishing removes large fish from populations, leaving smaller fish that reproduce less well. Photo: Getty Images

The team, which included PhD student Henry Wootton from the University of Melbourne and Asta Audzijonyte from the University of Tasmania, found that over several generations, these human pressures result in smaller fish that reproduce less well.

Over time, this can be expected to disrupt social structures by distorting population demographics and displacing fish from established habitats or else perishing, putting existing fisheries at risk of collapse.

Dr Morrongiello says it’s important to look at fishing pressure and climate change in combination.

“While pressures from fishing activity differ across the world, fish everywhere are facing pressure from warming temperatures,” he says.

Dr. Morrongiello’s broader research at School of Biosciences explores the ability of local fish species populations to respond to these environmental stressors over multiple generations.

“Fishing pressure and climate change both act as selective agents of evolution,” he says.

“The value of these long-term, intergenerational experiments is that we can first look at the short-term impacts of stressors, and then we can allow populations to be exposed to them for multiple generations.

Rising water temperatures appear to increase fish metabolism, which may make reproduction more difficult. Photo: Getty Images

“If individuals are in low physical condition or don’t like hot weather, they won’t reproduce and they will die.

“Those who survive will move on and reproduce in the next generation.”

Over time, populations might be expected to acclimate and resist warming better, but in lab experiments with zebrafish, researchers found the opposite.

Initially, they found that warming temperatures reduced egg size, prolonged the time for early development in young fish, and increased metabolism and energy expenditure in mature individuals, which reduced the energy available for reproduction. . Despite this, populations as a whole could indeed still cope but only temporarily.

“After three or four generations in the populations studied, the key indicators of population collapse really started to show up,” says Dr. Morrongiello.

These indicators include all changes in the ecosystem that affect population abundance and social structure – such as overfishing. As overfishing decreases population sizes, there may be additional consequences associated in the form of increased larval mortality and reduced maturation rates.

Warmer environments are likely to increase overall metabolism in fish, and to meet this need, more food must be consumed and growth slowed.

This can impact reproduction and survival over generations.

Experiments have shown that zebrafish can only temporarily adapt to rising temperatures. Photo: Oregon State University/Wikimedia Commons

“They’re trying to offset some of those extra energy costs from the warmer conditions by doing things like making bigger eggs, which is good for big fish with a great ability to produce eggs, but when we’re also fishing the big animals, after for a while you are left with just small fish that cannot produce many eggs,” says Dr. Morrongiello.

“In zebrafish, females are larger than males. If you fish preferentially for large fish, you target females and the population ends up with only males.

“This phenomenon can occur in other species with sexual dimorphism or when fishing activity targets spawning aggregations and larger females with greater spawning capacity are caught.

“After several generations, there were not enough females left to produce enough eggs, and the eggs they produced were not large enough to offset the costs of development at high temperatures, and the population sagged. collapsed,” says Dr Morrongiello.

“If you think about trophy hunting on land, we see the opposite effect there.

“We see this targeting of males because the males have ornamental antlers or have large horns. In fish, we know that large females produce far more eggs than small females and therefore have a disproportionate impact on the number of offspring produced.

In previous research from 2019 published in the British Ecological Society’s Journal of Animal Ecology with Philip Sweetman and Ronald Thresher from Australia Commonwealth Scientific and Industrial Research Organizationn (CSIRO), Dr. Morongiello found that removing larger dominant individuals disrupts established social hierarchies within fish populations.

The loss of larger fish from populations can disrupt social structures. Photo: Getty Images

This left smaller fish to compete for newly unclaimed resources. These previously less dominant fish become larger and more aggressive in the absence of larger ones, resulting in the restructuring or collapse of their social hierarchies.

Dr Morrongiello also warns of the potential for range shifting – a phenomenon where a species moves in response to environmental pressures – such as warming temperatures.

“When the waters warm, species have to move and they follow the favorable warmer conditions.”

“Off the coast of Australia is the East Australian Current. It’s a ton of warm water coming down the east coast of Australia, and that current is getting stronger, it’s getting hotter, and it’s pushing further south.

“So we see species that used to live off Victoria and New South Wales are now found off Tasmania where the water is cooler.

“We’re also seeing tropical species that used to be in the barrier reef now moving into Sydney Harbour.”

Warming temperatures are changing the range of different types of fish, with tropical fish now found in Sydney Harbour. Photo: iStock

“Coastal cold-adapted species living in the waters off Tasmania will have nowhere to go as the water temperature continues to rise as they will run out of ribs and these populations will fall off the edge of their world, therefore, to say.”

This warming process, known as tropicalization, also poses new challenges to stable fisheries that are centered on specific populations of fish but now must adapt their practices to accommodate other species of fish moving through their waters.

While acting on climate change is crucial, consumers can also help by only eating fish from sustainable fisheries certified by groups like the Marine Stewardship Council.

And he says policymakers need to find regional solutions, because fish don’t recognize international borders.

Potential solutions include establishing fishing exclusion zones or regulations mandating the use of standardized fishing gear to avoid targeting the largest fish and to maintain diverse and stable populations.

“Countries like Australia have a real responsibility to help. It is in Australia’s interests to have good and sustainable fisheries in our neighbour’s waters. Just as it is good for them that we have the same.

The authors of this article, Ameer Lambrias and Chenyang Yuan, are both MSc students at the University of Melbourne whose studies include Scientific communication.

Banner: Getty Images

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