Coastal safe havens may no longer protect Pacific cod

“Climate change” is no longer a distant concept; it’s an ongoing reality with tangible consequences. A manifestation of this can be seen in marine heatwaves that are altering the traditional coastal safe havens, making survival difficult for young Pacific cod in the Gulf of Alaska.

Cod population hit hard by heat waves  

With a focus on the marine heat waves in 2014-16 and 2019, researchers have discovered that young Pacific cod, who have traditionally found safety around Kodiak Island, no longer enjoy the defenses these near-shore nurseries used to offer. 

Instead, they’re undergoing significant alterations in their abundance, growth rates, and diet. It’s estimated that only the top 15 to 25 percent of Kodiak Island’s cod population with the largest size endured the summer heat waves. 

Even as temperatures recede, these cod are still struggling to return to their pre-heat wave size and diet.

Unreliable safe havens 

Study lead author Hillary Thalmann is a graduate student in the Department of Fisheries, Wildlife and Conservation Sciences at Oregon State University. 

“These coastal habitats aren’t supporting fish in the same way that they used to as a result of marine heat waves,” said Thalmann.

“That’s a novel finding, because we don’t always look at the nurseries as a place where size-selective mortality could be occurring rapidly.”

Ripple effects of climate change 

The evidence gathered from these observations could potentially have widespread implications for marine fish populations across the globe, as climate change causes marine heatwaves to become lengthier and more frequent. 

The Pacific cod, a favorite choice for fish and chips, is the second-largest commercial groundfish fishery off the coast of Alaska.

Struggles of the Pacific cod

The coastal nurseries traditionally act as safe havens for young Pacific cod, who begin feeding and growing in these areas from around three months of age. 

However, the heatwaves recorded water temperatures of 58 degrees Fahrenheit, nearly 6 degrees above normal, making survival a challenge. These two heatwaves are considered the most extreme warming events in the Northeast Pacific Ocean’s records. 

The impact on the cod population was so severe that the fishery had to close in 2020, leading to a federal disaster declaration in 2022.

Focus of the research 

In this study, the focus was on the physiological disruptions that young cod faced while inhabiting the coastal nurseries. 

To investigate, the researchers analyzed juvenile Pacific cod collected from 16 sites around Kodiak Island during the summers of 2006-2019.

Size-selective mortality of Pacific cod

The analysis of the samples revealed that the August fish were 30% larger than the projected size based on the July growth rate, with almost no small fish present in the August sample. 

The only viable explanation for this was to consider only the top 15-25% of the July sample and project their growth to August. It’s probable that the size-selective mortality was a significant factor in what the researchers witnessed.

“If we removed the little guys and grew the big guys – the top 15-25% – through to August based on the growth rates we saw earlier in the summer, then we got the size range that we see in those heat wave years,” noted Thalmann. 

“It’s important to show that with heat events like this, size-selective mortality can continue in the cod population beyond just their early life in the open water.”

“We saw these differences in size in the nursery, and we tried to explain them with growth rates and tried to explain them with diet, but we couldn’t explain it all.”

“There was something out there, probably size-selective mortality, that was the major driver for what we were seeing.”

Cautionary tale for climate change 

Changing ocean conditions may result in Pacific cod moving further north to find suitable growth environments, or there may be a shift towards larger cod being the sole survivors who pass on their genetic information. 

“If the marine heat waves continue, there will probably be some changes in both the distribution and the quality of these populations,” said Thalmann. “I don’t think it’s the end of fish and chips, but I do think it’s a cautionary tale for climate change and the shifting dynamics of fisheries in warm temperatures.”

The study was a collaboration with co-authors Zoe Almeida, Kaitlyn Osborne, Kaylee Marshall, and Jessica Miller at the Oregon State University, and Benjamin Laurel at the NOAA Alaska Fisheries Science Centre in Newport, Oregon.

The research is published in the journal Scientific Reports.

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