By analyzing the impact of ocean warming and size-selective fishing on six generations of zebrafish, Australian researchers have found that fisheries can help buffer the impacts of climate change on their stock by fishing randomly rather than indiscriminately capturing large fish.

While the short-term impacts of individual stressors including ocean warming and fishing on marine life are well-known, the way these stressors interact over many generations is not as clearly understood. The study, published April 26 in PNAS, addresses this gap.

First author Henry Wootton, a Ph.D. candidate studying aquatic ecology at University of Melbourne, spoke with The Academic Times about the project.

"We took a large population of fish and split them into 18 populations, then applied these multiple stressors in different combinations over several generations, including a couple of common garden generations where you relax those stressors," he said, referring to populations with different genetics that are placed in the same environment to compare their response. "And then [we looked] at how those populations recover through time."

Multigenerational studies of this type are particularly important now, as scientists are only beginning to see the full scope of the impacts of climate change on marine species. Ocean warming has begun to cause mass migrations of marine species in search of more suitable climate. Climate change has also acidified the ocean, which interferes with fishes' sensory biology. 

"We're starting to get into that timeframe where we're starting to see impacts," Wootton said. "So, this is like a long-term problem that we're trying to address early on, in that sense."

Wootton explained that working in Australia allowed the research team to take advantage of the "world-class" management of Australian saltwater fisheries to establish a solid baseline against which to compare the effects of stressors such as warming and overfishing.

"Australia actually [is] quite lucky in the sense that we haven't industrially fished a lot of our populations here for all that long," Wootton said. "Our rates of declining fish population [are] actually quite low."

But while saltwater fisheries in Australia are doing relatively well, this is less the case for others around the world. According to a 2019 report in Science, global productivity of fisheries went down by an average of 4.1% from 1930 to 2010 because of ocean warming. Vulnerable regions such as East Asia experienced declines of up to 34%.

"In essence, we're talking about a global problem," Wootton said.

To analyze this problem, the team used a population of zebrafish, a freshwater species in the minnow family. The motivation for choosing this species was twofold. First, the fish are widely used as a model in biological experiments across disciplines, so it is well-known how to rear and maintain populations of them. Second, they reproduce fast, with a single generation taking only two to four months. This allowed the team to complete its six-generation experiment in just a few years. 

And while zebrafish may reproduce quickly, they are not expected to evolve fast enough to adapt to warming waters in the future, making them an apt subject for research involving ocean warming.

When the researchers manipulated the degree of warming and harvesting among the subpopulations of zebrafish, they found that warming the water by just 4 degrees Celsius — up to 30 degrees Celsius from the standard 26 degrees Celsius used by fisheries for zebrafish — significantly impacted their recruitment, or ability to survive to sexual maturity.

However, these effects weren't instant. Even in the warmer-water population, recruitment rates were steady and high for the first three generations. Only in the fourth generation did rates among the warm-water fish decline rapidly, by 30%. This figure increased to 50% in the fifth generation, and one population even experienced complete reproductive failure. The team, expecting "a much smaller decline," was shocked by the severity of the impact.

The researchers also tested how fishing for size could impact this decline. Fishing regulations around size are commonplace. For recreational fishing, a minimum size is often enforced to limit the number of fish removed from the ocean. Wootton explained that in commercial fishing, regulations take the opposite approach, because fish are being harvested rather than plucked from the wild.

"Some fishing regulations require that you put the big fish above a certain size threshold back, and that preserves those larger sizes, which are really important to fish population, in the sense that large females, we think, have huge reproductive output compared to small females," he said.

The team found that when fishing randomly, which preserves diversity in size, the fish populations in warmer water experienced less reduction in recruitment, though they still experienced declines. The team also found that recruitment quickly rebounded when all stressors were eliminated, suggesting that these effects, while drastic, are somewhat elastic. 

In light of these findings, the study proposes that "resilience of wild populations may be higher if fishing preserves sufficient body-size diversity, and windows of suitable temperature periodically occur."

The researchers are next interested in increasing the resolution of their findings by examining the impacts on different life history phases of the fish in more detail. Wootton has also begun working with the Australian government on management of freshwater fisheries, which are in "much more dire straits" than their saltwater counterparts, according to Wootton.

In the meanwhile, Wootton appreciates that the study has tangible takeaways that can be immediately applied to solving real industrial problems.

"Definitely one of the things that's sort of great about [fisheries science] is that you can do some some really interesting science, and then actually, it really matters in terms of direct application to [a] really important industry," he said. "It's one of the reasons why I love it so much."

The study, "Multigenerational exposure to warming and fishing causes recruitment collapse, but size diversity and periodic cooling can aid recovery," published April 26 in PNAS, was authored by Henry F. Wootton and John Morrongiello, University of Melbourne; and Asta Audzijonyte, University of Tasmania.