Research shows Pacific cod can withstand warming better than Arctic cod
Arctic cod are proving highly susceptible to warming ocean temperatures, but the good news is that Pacific cod, the kind caught commercially in pots and trawls around Alaska, not so much.
A NOAA lab in Newport, Ore., is reportedly the first to successfully spawn Arctic cod in captivity, and the resulting science has shown that the species has a very narrow window of temperature viability.
In waters between 32 and 36.5 F., and even colder with the lower freezing point of salt water, Arctic cod do very well. However, water above 41 degrees is fatal to them.
The same lab has done research on Pacific cod and walleye pollock, and found a much higher range of temperature tolerance.
NOAA’s Dr. Ben Laurel said that Arctic cod offer an interesting comparative species to Pacific cod, which has a range from the central Bering Sea to Puget Sound.
His lab did experiments with Arctic cod as the focal species, specimens around 4 to 5 inches, and also brought back Pacific cod and pollock of the same size.
They reared them for six weeks with a consistent diet and temperature range, taking them out every couple of weeks to weigh and measure them, getting a good sense of their growth.
The growth rates were similar in the Pacific cod and pollock, he said.
“They are very adapted to take advantage of a couple ranges of temperatures. From 9 to 12 degrees C. (48-54 F.), these 4-5 inch fish do very well, but they also can survive at higher temperatures with a little bit of decreased growth performance. They also survive very well at cooler temperatures.”
He noted that the magical tipping point seems to be, in terms of their growth performance, around 2.5 to 4 degrees (36.3 to 39.2 F.), and there, he said, Pacific cod start to really out-perform Arctic cod, growing at as much as three times as fast. Pollock seem to be even a bit more cold-adaptive.
“That sort of fits with what our understanding is of their distribution,” Laurel said.
Pacific cod are voracious eaters, they can consume a lot of prey, and put on a lot of growth at those optimal temperatures.
“When you get under 3 degrees C. (37.4 F.), even given all the food they want, the growth rates of Arctic cod are actually higher. So (Pacific cod) have this little glitch,” Laurel said.
Arctic cod have a much higher fat content than Pacific cod and pollock, but researchers do not think that is an issue in their temperature sensitivity.
Laurel noted that warm-blooded creatures use fat as insulation, but cold-blooded creatures are not any warmer than their surrounding environment.
“Why we’re interested in fat is because of its energetic content just for those animals to survive when there’s no food around,” he said. “Overwintering is a real big deal for these guys. There’s not much going on in the system, and they have to sort of feed off fat supplies.”
He said the other big concern is what feeds off them.
“If we don’t have Arctic cod around or if these Arctic cod are sort of burning through their fat just to operate and keep them going in a warmer temperature, they’re less nutritious. There are big consequences.”
Laurel said that ocean acidification does not seem to have a big impact on Pacific cod.
His colleague at the lab, Tom Hurst, is doing that research, and has found that, at least at egg and larval stages, ocean acidification does not seem to have a strong effect on growth and development.
“There are some subtle things there, and whether or not that affects their survival later on is uncertain, but they seem to be pretty robust,” Laurel said.
He added that now what Hurst is doing is looking at some sort of behavioral impacts of ocean acidification on Pacific cod, and those experiments are ongoing.
Kodiak herring has shut down before it really got started due to immature roe, missing year classes and cautious management.
Alaska Department of Fish and Game fisheries biologist Brad Fuerst said a lower harvest was expected this year after looking at the year-classes from last year during the harvest, which dropped the Guideline Harvest Level area-wide.
So far this year, fish were only harvested out of Kiuluda Bay, which had a GHL of 75 tons, and Ugak Bay, which had a GHL of 350 tons.
The GHL in Kiuluda Bay was reached, but not in Ugak Bay.
Fuerst said there were a lot of immature fish and not very many older fish, and area management biologist James Jackson erred on the side of caution and shut things down, at least for now.
“Looking at last year’s scale samples, we’ve got a lot of three- and four-year-old fish coming up, but not much between four- and nine-year-olds.”
He added that there are not a lot of recruitment fish in the mix.
He said there is still quota on the table, and the department is working with fishermen and spotter pilots to watch for fish and let them know if herring show up, but at this point most of the fleet is looking to get switched over to salmon.
He said Jackson and other ADF&G staff were out doing surveys on the west side of Kodiak, but the fish they were finding were either small schools or juvenile fish.
He said there were no gillnetters registered for the fishery, but around 20 seiners had signed up.
He added that dock talk was for prices around $300 per ton, far above the reported price of $50 per ton for the Togiak herring fishery which also is under way.
The spring Kodiak fishery is a sac-roe fishery. The food/bait fishery takes place in the fall.
The Kodiak spring fishery is different from most herring fisheries around the state, where there is basically one large spawning biomass that fishermen target in openings that last from minutes to hours. Kodiak’s fishery opens on set days and times.
Cristy Fry can be reached at email@example.com.
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