Ocean bottom heat proved deadly to P-cod eggs

Water temperatures are particularly critical in first year of fish’s life

Pacific cod eggs highly sensitive to the temperature on the bottom of the ocean were doomed in large numbers in 2015, 2016 and 2019, and it’s still too early to say how many fished hatched in 2016 and 2019 in the Gulf of Alaska will make it to age three.

That’s the conclusion reached by biologists with NOAA’s Alaska Fisheries Science Center, whose study was announced on Feb. 20, in a research report published in the Canadian Journal of Fisheries and Aquatic Sciences.

“We found that the recent three-year heat wave and return to similar conditions in 2019 potentially had the greatest effect on spawning habitat for the years we had available data (1994 to 2019),” said Benjamin Laurel, the NOAA Fisheries biologist who led the study. To reach their conclusions Laurel and colleague Lauren Rogers combined results of laboratory studies, stock assessment model output and survey data, comparing what happens to Pacific cod in warm and cold years.

Water temperatures are an important component of fish habitat in every stage of the life of a fish, and during the first year of life in particular, fish eggs are very sensitive to changes in environmental conditions. Laurel and Rogers determined that the Pacific cod eggs have just a narrow optimal range of ocean temperatures for hatching success, one much narrower than other related species like walleye Pollock and Atlantic cod.

They also noted that there are a number of other ways in which temperatures can affect fish survival, and that warm water can affect cod prey and predators, subjecting cod at different ages to starvation or higher predation.

Laurel and his team were successful in raising age-zero juvenile cod, fish in their first year of life, to adulthood, using juveniles collected from the Gulf of Alaska and brought to their lab in Newport, Oregon. They have also had success spawning fish in the lab setting and testing them under different temperatures to determine ideal hatching conditions.

“We found that habitat suitability (optimal water temperatures) can be a reliable early indicator of pre-recruit abundance and adult abundance,” Laurel said. “It can provide an early warning to help resource managers and the industry anticipate how future warming events in the Gulf of Alaska may affect this lucrative fishery so that steps can be taken to ensure its continued sustainability.”

The team said it is still too early to say how many fish hatched in the warm years 2016 and 2019 will make it to age three. They said they plan to continue their research to see if their initial findings hold true for these year classes.