NOAA scientists show off catches of tiny cod at Kodiak in 2018

 

Warmer temperatures on the ocean bottom were key in causing the  cod crash in the Gulf of Alaska.

That’s the conclusion of a NOAA Fisheries study that connected low numbers of cod larvae, juveniles and adults to loss of spawning grounds in the 2013–2016 heatwave called “the Blob” – the largest warm anomaly ever recorded in the North Pacific.

Pacific cod are unique among all cod species because they only spawn once in a season and have eggs that adhere to the ocean floor. The researchers said females can actually place their eggs in habitats with temperatures that optimize hatch success.

But Pacific cod eggs have a very narrow bottom temperature range for hatching success from about 37 to 43 degrees, much narrower than Alaska pollock or Atlantic cod. During the heatwave, Gulf of Alaska waters reached nearly 61 degrees, compared to a norm closer to 50 degrees.  Right after, biologists saw no first year cod.

Lead researcher Ben Laurel collected first year cod from nearshore waters around Kodiak in 2018 to bring back to the Alaska Fisheries Science Center in Newport, Oregon. Their data on young cod cover every year from 2005.  They’ve followed the tiny fish to see how they are recovering from blob effects.

 “A lot can happen in that first year of life that we would like to learn more about to predict whether or not these year classes that come through are actually going to survive. But there is always this variability and uncertainty that we have to be braced for.”

Laurel said in a previous interview that things might be taking a turn for the better.

“So in 2017 the ocean temps started to get back to normal and we did see signs of some fish, which is good because we hadn’t seen fish earlier than that. But again, we’re just looking at one year – in 2018 also seeing some young fish around.”

The research is providing a window into how Alaska cod will fare in a changing climate.

“It’s sort of a dress rehearsal for things to come. And it’s encouraging we had really responsive actions to this really drastic reduction in the population. I’m encouraged by that but also tentatively nervous about what’s in line for the future. Everybody should be braced for that uncertainty.”

The report titled Loss of spawning habitat and prerecruits of Pacific cod during a Gulf of Alaska heatwave appears in the Canadian Journal of Fisheries and Aquatic Sciences.

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