Research News

Study Explores Combined Impacts of Ocean Warming and Acidification on Pacific Cod

As the climate changes, the ocean is simultaneously warming and acidifying. Both have been shown to adversely affect the vulnerable early life stages of Pacific cod. But until now, the interplay of these two environmental stressors was unknown.

A new NOAA Fisheries study takes a look into the future by replicating predicted Alaska ocean conditions in the laboratory. The study is the first to look at the interactive effects of ocean warming and acidification on Pacific cod. Researchers evaluated the growth and survival of eggs and larvae at combinations of temperature and acidity representing current and future ocean environments. The findings can help improve predictions to ensure climate-ready Alaska fisheries and communities in the future.

Alaska Fish Face Multiple Climate-Driven Stressors

Ocean warming and acidification are escalating faster in Alaska and other high latitude regions than anywhere else on the planet. Alaska produces more than half of the nation’s wild-caught seafood. Local communities depend on the ecosystem for their livelihoods. Understanding how climate-driven stressors will affect these valuable resources is critical to ensuring resilient Alaska communities and a strong blue economy in the future.

Because fish are cold-blooded, temperature profoundly affects their growth and behavior. The effects of acidification are more nuanced. Fish can buffer acidity in the bloodstream but at an energetic cost.

Very young fish—eggs and larvae—tend to be the most sensitive stages of the fish life cycle to both warming and acidification. Unlike adult fish that can move to cooler waters, these tiny creatures are incapable of migrating to more favorable conditions. Subtle differences in their growth and condition can make a big difference to the number of young fish that make it to the adult population.

image2 750x500 Ocean Acidification Warming Laboratory

Laboratory set up for ocean acidification and ocean warming experiments with early stages of Pacific cod. Large tanks in the background provide chilled and ambient seawater that are mixed to temperature. Small round tanks house eggs and larvae at different temperatures and acidification levels. Acidification is controlled by bubbling carbon dioxide into the seawater: more carbon dioxide causes higher acidity (lower pH). (Image credit: NOAA Fisheries/Emily Slesinger)

Pacific cod is an important subsistence food, a key species in the food web, and the second-largest commercial groundfish catch in Alaska. Understanding how warming and acidification together may affect this ecologically, culturally, and commercially valuable species is key to predicting future climate impacts.

“Knowing how individual species will respond to multiple climate stressors helps us put together this puzzle to figure out what species are more vulnerable than others,” Emily Slesinger said. “And that will help guide ecosystem-based fisheries management to ensure their sustainability.”

Partnerships and Unique Laboratory Capabilities Expand Possibilities for Studying Alaska Species

Finding answers posed some hurdles. “It’s really challenging to study these fish that are found in remote and cold environments,” Slesinger said.

They met those challenges through partnerships and a specially equipped laboratory facility.

Fishermen played a key role. To obtain eggs and larvae for experiments, they first needed live adults, ready to spawn. Commercial fishermen used baited fishing pots to catch adults off Kodiak Island during the spawning season.

Fertilized eggs were shipped to the NOAA Fisheries Alaska Fisheries Science Center Laboratory in Newport, Oregon. The eggs and larvae were reared at six combinations of temperature (3°C, 6°C, 10°C) and acidification (ambient and high acidity). Scientists measured their hatch success, growth, nutritional condition, and mortality throughout the nine-week experiment.

“The Newport lab is very special,” Slesinger said. “What is most unique is our seawater system that can chill water to Arctic temperatures. The lab has the capacity to house many fish at all life stages and the expertise to keep them thriving. And it has the facilities to run these complicated experiments on the younger life stages.”

Experiments Reveal a Dominant Stressor

The study found that overall, Pacific cod's early life stages were more sensitive to warming than to acidification:

  • Temperature affected all measured traits: growth, condition, and mortality
  • Mortality of both eggs and larvae was highest at the warmest temperature
  • 6°C was the optimal temperature for hatching, condition, and survival
  • At the warmest temperature (10°C), Pacific cod grew the fastest but were in poor condition with high mortality rates
  • At the coldest temperature (3°C), mortality rates were lower, but both growth and condition were also low
  • Acidification effects were more modest and inconsistent
  • The single instance of a negative effect of acidification was at the optimal 6°C temperature

“We’re seeing that either temperature or acidification drove the response—they were not additive. At optimal temperatures, Pacific cod may be impaired by acidification. At the warm temperature, they were overwhelmingly negatively impacted, but there was pretty much no effect of acidification,” Slesinger said. “Our findings suggest that temperature is the dominant stressor and ocean warming is likely the greater threat.”

The research highlights the complexity of species- and stage-specific responses to multiple climatic stressors and the importance of understanding them.

“This work is exciting because we are able to provide information about these fish that can be useful for future management. It is really important to me that my research can be directly helpful to societies, economies, and communities,” Slesinger said. “This research contributes to our common goal of ensuring productive, sustainable fishing into the future.”


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