Warmer Bering Sea may benefit an Alaskan flatfish

 Northern rock sole - NOAA
Northern rock sole - NOAA (KTUU)
Published: Nov. 25, 2019 at 7:22 PM AKST
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While the repercussions of climate change are complex and many impacts are unknown, newly published research suggests that one winner in a shifting environment is Alaska's Northern rock sole.

The Northern rock sole is a flatfish that is commercially harvested, although it is fished significantly less than Pollock and Pacific cod.

Females grow up to 27 inches, while makes grow up to around 19 inches. The North Pacific Fishery Management Council set the acceptable biological catch for the fish in the Bering Sea and Aleutian Islands at 143,000 metric tons for 2020, yet in 2018 only 60% of the total allowable catch was harvested.

Research by NOAA Fisheries biologists suggest that the fish have higher reproductive success in warmer years, meaning that a higher percentage of eggs laid will grow to become part of the catch-able population.

The investigation started after surveys of juvenile showed dramatically different results in the same location.

"One year we went in this area between Nunivak Island and Cape Newenham offshore and we found very high densities of the animals. We estimated that there were billions and that was in 2003 - a warm year," said Dan Cooper, a research fisheries biologist with NOAA's Alaska Fisheries Science Center.

He said it's the opposite in cold years.

"We went back in 2010 to try and study these animals, and we couldn't find any age 0 Northern rock in 2010, and we noted that was a very cold year. The cold pool was actually sitting on much of the survey of the Northern rock sole nursery area, so we can up with this idea that possible the cold pool was affecting the nursery area and survival in the first year of life."

The cold pool is a section of very cold bottom temperature waters, usually defined as less than two degrees Celsius in the Bering Sea. The cold pool and its reduction in recent years has shown to have significant impacts on the marine ecosystem, affecting animal movement and the food web. Previous research indicates that juvenile fish exposed to the cold pool develop slower and could increase mortality.

Northern rock sole spawn offshore and depend on winds and currents to drift larvae to nursery areas closer to shore where they continue developing.

"Our group looks at fish's first year of life because recruitment, or adding fish to the catchable population, is one of the biggest factors that determines the size of the fish population, and most of the variation in reproductive success happens during the first year of life," Cooper said.

Cooper and fellow researchers used data from 1982 through 2014 and wind and temperature data to try to understand why some years there would be billions of inch-long North rock sole in an area while another year there would be very few.

"We looked at winds during the larval drift period and we looked at temperature and we looked at the temperature on the northern nursery area," Cooper said. "We really only have two factors, so the best models we've created only explain about half of the variation we see in recruitment."

Cooper says that from 1982 through around 2003, the cold and warm years in the nursery area happened seemingly at random. The early 2000s brought a warm period, followed by a cold period from 2007 to about 2013. Cooper says there has been another warm period since then.

"In general, our findings would suggest that if there's more warm years, there will be more rock sole that survive the first year of life," Cooper said. "But things are complex, and other things could change."

Adult rock sole feed on worms and other organisms on the bottom of the ocean. Sharks, marine mammals and larger fish including Pacific cod are their primary predators.

Although understanding how water temperature and the reduction of the cold pool impact populations including the rock sole can help managers make better decisions, researchers say they need to continue collecting data over a longer period to determine if the trend is temporary or indication of more permanent changes.

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