Ocean Bottom Temperatures are Key to Pacific Cod Numbers

A federal fisheries study focused on the influence of water temperature on egg hatching success concludes that the dramatic loss of Pacific cod in the Gulf of Alaska in 2019 was due to lack of optimal spawning habitat on the ocean bottom.

The NOAA Fisheries study, led by biologist Benjamin Laurel, connected low numbers of Pacific cod larvae, juveniles and adults to loss of spawning habitat in the 2013 – 2016 marine heatwave, known as “the Blob” – the largest warm anomaly ever recorded in the North Pacific.

Loss of optimal habitat for the Pacific cod eggs occurred during and immediately following the heat wave, causing a significant ecological as well as a significant economic impact on the second most valuable commercial fishery in Alaska.

Also worth noting, researchers said, is there are a number of other ways in which temperature can affect fish survival. For example, warm water can affect cod prey and predators, which may subject cod at different ages to starvation or higher predation.
NOAA’s Alaska Fisheries Science Center, reporting study results on Feb. 20, said the fishery experienced a 58 percent cut in its annual catch limits in 2018 and a fishery closure in 2020.

Their report, “Loss of spawning habitat and prerecruits of Pacific cod during a Gulf of Alaska heatwave” was published in the Canadian Journal of Fisheries and Aquatic Sciences.

“We combined results of laboratory studies, stock assessment model output and survey data to help us better understand what happens to Pacific cod in warm and cold years” Laurel said.

“We found that the recent three-year heatwave 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).”

Laurel and colleague Lauren Rogers determined that Pacific cod eggs have a narrow optimal range for hatching success, only 3-60 C, much narrower than related species like walleye Pollock and Atlantic cod.

“Pacific cod are unique among cod species,” Rogers said. “They only spawn once in a season and have eggs that adhere to the sea floor. Pacific cod females can actually place their eggs in habitats with temperatures that optimize hatch success. However, during these warm years, it may have been more challenging to find suitable habitat because the warmer water temperatures extended into the ocean depths.”

Laurel and his team have successfully raised age-0 juveniles (fish in their first year of life) from the Gulf of Alaska to adulthood in their Newport, Oregon laboratory, and have also spawned fish successfully in the same laboratory setting. For this experiment, eggs were collected, fertilized and then tested under different temperatures to determine ideal hatching conditions.

The team created an index of spawning habitat suitability, using University of Alaska data going back to 1994, to understand what may be behind observed changes in abundance in larval, age-0 and adult fish surveys over the years.

In 2015, 2016 and 2019, due to a substantial lack of suitable habitat for Pacific cod eggs, the probability of successful hatching was less than half that observed in 2012, when ocean temperatures were considered ideal. NOAA Fisheries also recorded during the three warm years the lowest abundances of Pacific cod larvae and age-0 Pacific cod.

Notably the abundance of age-3 fish from 2015, fish born during the height of the heatwave, was estimated at a record low based on subsequent surveys, researchers said.

It is too early still to say how many fish hatched in 2016 and 2019 and will make it to age 3, so Laurel and Rogers plan to continue their research to see if these initial findings will hold true for those year classes too.