Sudden decline in salmon growth may signal ecological change

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FEwer salmon from the southern region of Norway, a country that provides 20% of the world’s supply, are returning home to the rivers where they spawned after going to sea.

A new study published in Scientists progress today (March 4) suggests that the marine ecosystem reached a tipping point around 2005, when salmon caught in Norwegian rivers also experienced a sudden and persistent change in body size.

The study looked at the body size of 52,000 Atlantic salmon found in 180 Norwegian rivers between 1989 and 2016. Researchers from the Norwegian Institute for Nature Research, the Institute for Marine Research and Rådgivende Biologer combined the data from several years of effort to investigate the phenomenon. , likely caused by climate change, which researchers, as well as fishermen, have been reporting for years. They found that the growth rate and number of salmon returning from the sea suddenly declined in 2005 and numbers have not recovered since – a change that not only affects salmon and their prey, but possibly the fauna at all levels of the Norwegian marine ecosystem. .

Preserving and Predicting Salmon’s Past

Study co-author Knut Wiik Vollset, a marine biologist at the Norwegian Research Center (NORCE), says The scientist that his collaborators at Rådgivende Biologer reported a decrease in the growth of salmon on the west coast and rivers of Norway several years ago. So Vollset and his colleagues collected and compared salmon size data from 1989 to 2016 from multiple datasets across Norway to investigate what was causing salmon stunting.

Atlantic salmon (Salar de Salmo)

HELGE SKOGLUND, NORCE LFI

The researchers calculated the age and growth rate of the salmon by measuring the size of their scales. For 30 years, it has been common practice in Norway for commercial fishermen to provide scale samples of their catch for fisheries research. Like the rings on a tree, salmon scales grow throughout a salmon’s life, increasing linearly in diameter as a salmon does. Each year, the young salmon move down the river to the sea, where they go through a period of rapid growth. Once they reach a specific size, usually after a year, they return to the same river in which they spawned, where they are either captured or spawned (most die shortly after spawning). Since salmon venture into the sea around the same time each year, researchers can also estimate how long they have been at sea.

Vollset says he and his team expected growth changes to “happen gradually over time.” Instead, the researchers saw a “dramatic change suddenly occurring in 2005.” The researchers also found that the salmon began to spend more time at sea after 2005. Larger and heavier salmon returned to the rivers, indicating that the number of years the salmon had been at sea had increased. increase. The researchers say this is important because the longer salmon stay at sea, the more vulnerable they are to predation.

In order to find out why this dramatic change in the size and behavior of the salmon occurred, Vollset and his colleagues enlisted the help of marine biologists from the Norwegian Institute of Marine Research, asking if they had seen anything which could help explain the decline in the growth rate.

Marine researchers have examined the growth and abundance patterns of other marine species. ” We saw that [the decrease in growth] was clearly co-emerging with some of the patterns they were observing,” says Vollset. “There was a 50% reduction in zooplankton. . . in the North Atlantic Sea. Then, about two years later, the growth of mackerel collapsed in the same way as that of salmon. These results suggest that after 2005, mackerel and salmon had less to eat.

Changing tides

Researchers believe that rising temperatures in the Norwegian Sea, stemming from a potentially climate change-related decrease in the amount of frigid Arctic water flowing south each year, could be at the origin of these changes. Over the past three decades, the current of Arctic water has slowed, Vollset says. “In 2005, it completely disappeared,” he explains. It has since been replaced by a rush of warm, nutrient-rich water, which researchers say has led to widespread changes throughout the ecosystem, including making the salmon’s food less plentiful and disrupting their growth. .

“You might expect things to get warmer, populations to grow faster,” said Peter Westley, a marine biologist at the University of Alaska, Fairbanks, who was not involved in the work. The scientist“But what this indicates is that as things get warmer, the food web . . . has changed in such a way that salmon are becoming food limited.

This study shows that even small changes in temperature “can have a disproportionate effect on things people care about, like the size and growth of Atlantic salmon, in this case,” Westley says.

To see “A “climate disaster”: the salmon of the western United States on the edge of the abyss

Lisa Cozier, a research ecologist with the National Oceanic and Atmospheric Association who studies Pacific salmon, says The scientist that she was “not surprised” by the results, as similar decreases in body size with increasing temperature have been documented in some species of Pacific salmon. “We have known for a long time that catches are correlated with climatic factors. . The big question is how climate change is going to drive a long-term trend here.

Some researchers have long suspected that climate change can cause ecological systems to rapidly alter, potentially making them more fragile. Vollset says there is no indication that the Norwegian marine ecosystem as a whole is in danger. However, last year’s UN climate report mentioned tipping points 97 times, identifying 12 tipping points that could lead to irreversible damage to specific ecosystems and further spur climate change, although the impacts of these ecological changes are poorly understood.

Although it is clear that salmon eat zooplankton, data regarding the availability of the majority of salmon food sources during this period was not available. Although the correlations between salmon growth, zooplankton availability and temperature are strong, “we still don’t know a lot about the mechanisms,” Vollset says. For example, human activity and predation, as well as parasites from escaped salmon, can also affect growth.

Westley suggests that genetic changes could also impact salmon growth. “The question is: are these changes only environmental?

For example, an article published in Science last month discovered a genetic component of early salmon maturation that is affected, both directly and indirectly, by commercial fishing. The researchers identified an allele, vgll3, which resulted in late maturation and larger salmon size. Fishing in rivers selected for salmon that remained smaller in their early growth cycles, making them less attractive to anglers, and decreased the prevalence of this allele in some salmon populations. However, fishing for capelin, one of salmon’s main food sources, for salmon farms has reduced the amount of food available to salmon at sea and selected for early growth, indirectly increasing the prevalence of vgll3.

In a particular river located on the North Atlantic coast of Norway where the Scientists progress study did not see a decline in salmon growth, the researchers behind the Science report that salmon grow faster on average and spend less time at sea. Nevertheless, evolutionary biologist Tuktu Aykanat, who co-authored the recent Science paper, tells The scientist that Vollset’s study is a “very important paper” and separately suggested that genetic analysis is the “next step” in finding the mechanism behind shrinking salmon.

To see “2,000-year-old salmon’s DNA reveals secret to sustainable fishing

Vollset says his the study has implications for salmon management. “If you want to keep the salmon population alive, you have to manage it in such a way that they are resilient to these dramatic changes if they occur,” he says. He says people may not want to “take too much from these populations, because next year you don’t know if these changes are happening or not.” Larger fish, which are generally more attractive to anglers, may have been in the ocean longer to mature and may already be more vulnerable to predation.

Westley points out that researchers have trouble predicting when these changes will occur, making the future of many marine populations uncertain. “As we move forward, when will the next surprising regime change occur?” said Westley, “and when will we know that we are in a new regime?”

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