Changing environment may be to blame for reduced fish populations

Rutgers study weighs influence of biological, environmental factors

Skaggerk cod, a variety of Atlantic cod that spawns off the coast of Norway. Photo courtesy of Rutgers University

A new study aimed at determining how best to boost recovery of Atlantic cod has researchers pondering whether ongoing fishing and environmental changes, rather than evolution, are behind failed recovery of many stressed fish populations.

Research published Monday, April 5 by Rutgers University, including genetic sequencing of this iconic species, offers major implications for ocean conservation, says Malin Pinsky, an associate professor in the School of Environmental and Biological Sciences at Rutgers University-New Brunswick.

“Fish populations around the world have collapsed and many have not recovered,” said Pinsky.

“Evolution has often been used as an excuse for why recovery of Atlantic cod and other species has been so difficult. Cod now mature at a much earlier age, for example.

“Our findings show that reducing fishing pressure and addressing other environmental changes, including climate change, will be important for letting these populations recover. Ongoing fishing and environmental changes, rather than evolution, are the more likely explanations for failed recovery,” he said.

“Recovered fish populations would be able to feed more people, provide more jobs and help sustain the environment,” he noted.

Plans are to share results of the study led by Rutgers and the University of Oslo with the New England Fishery Management Council and other fishery managers. Meanwhile the immediate next steps are focused on other species of fishes to understand whether these results apply broadly or are unique to Atlantic cod, he said.

The study was published in the journal Proceedings of the National Academy of Sciences, and also online at EurekAlert, the online science news service of the American Association for the Advancement of Science.

To date no similar studies have been done on Pacific cod, according to researchers at the Alaska Fisheries Science Center in Seattle.

The Atlantic cod species may be about eight million years old. Pacific cod, a major commercial groundfish fishery in the Bering Sea and Aleutian Islands area and the Gulf of Alaska, split off from Atlantic cod evolutionarily some three to four million years ago, when some stray fish swam through the then-open Bering Strait from the Atlantic Ocean.

Bottom swelling fish such as Atlantic cod are often found near structures such as shipwrecks. Photo courtesy of NOAA/Rutgers University

Atlantic cod can grow to 51 inches and 70 pounds and live for over 20 years. Pacific cod, by comparison, live less than 20 years, may grow over 6 feet and weigh up to 33 pounds.

There has been considerable discussion over the last few decades centered on whether cod have evolved in response to fisheries, a phenomenon known as fisheries-induced evolution.

Cod that were the focus of the study are now maturing at a much earlier age, for example, and the concern has been that if the fish have evolved, they may not be able to recover even if fishing is reduced, Pinsky said. Cod populations with late-maturing individuals can produce more offspring and more effectively avoid predators, he said. They are also better protected against climate variability, more stable and less likely to collapse.

Both theory and experiments have suggested that fishing can lead to an earlier maturation age, but prior to the Rutgers study nobody had tried sequencing whole genomes of the cod from before intensive fishing to determine whether evolution had occurred.

In this study for the first time, researchers sequenced the genomes from Atlantic cod that last lived over 100 years ago. Using DNA found in fish scales and ear bones from 1907 in Norway, 1940 in Canada and modern cod from the same populations, researchers determined that decades of intensive fishing are unlikely to have caused Atlantic cod to evolve. The northern Canadian population of cod collapsed from overfishing in the early 1990s, while the northeast Arctic population near Norway faced high fishing rates, but smaller declines, the study said.

A genome is an organism’s complete set of chromosomes or genetic instructions.

DNA, or deoxyribonucleic acid, is the hereditary material of organisms in the genome. Genome sequencing is the process of determining the entirety, or near entirety, of the organism.

Studying the differences in sequencing of cod from 1907, 1940 and modern cod, researchers determined that cod likely did not evolve in response to fisheries.

“There were no major losses in genetic diversity and no major changes that suggested intensive fishing induced evolution,” Pinsky said. “We cannot entirely rule out that evolution happened, but it’s more likely that the fish are developing earlier as a response to their environment and would be able to develop and mature later if the environment changes, benefitting the species.”

The study concludes that their research complements conclusions from literature reviews and evolutionary modeling that the direct impacts of fisheries on populations and ecosystems are a more pressing concern than effects of fisheries-induced evolution.

“A big question, said Pinsky,” is whether other species, especially those with shorter life spans, may show signs of evolution in contrast to the long-lived cod.” The research group is now investigating this question by DNA sequencing 100-year-old specimens from the Smithsonian National Museum of Natural History.