How Warm Can Cod Tolerate? Exploring the Thermal Limits of a Cold-Water Fish

Cod, a commercially important fish species, have traditionally been regarded as thriving in cold waters. However, recent studies have shed light on the surprising adaptability of cod to a wider range of temperatures than previously thought. In this article, we’ll explore the thermal limits of cod and how they are coping with the effects of climate change.
Cod’s Thermal Tolerance: Surprising Findings
A comprehensive study conducted in 2024 by researchers from several European universities, including the National Institute of Aquatic Research in Denmark (DTU Aqua), equipped more than 2000 cod from eight different North Atlantic stocks with advanced temperature gauges. The results, published in the “Marine Ecology Progress Series,” revealed that adult cod can handle much warmer water than originally thought.
The study found that some cod were able to survive in water as cold as -1.5°C, while others swam comfortably in water nearly 20°C above zero. This indicates that cod are relatively adaptable fish that can tolerate a wider range of temperatures than previously believed. However, the researchers noted that cod are more conservative in their choice of water temperature during spawning, consistently seeking out water between 1°C and 8°C.

Spawning: A Vulnerable Stage for Cod

The study suggests that the egg and larvae stages of a cod’s life may be particularly vulnerable to the effects of climate change. While adult cod can tolerate a wide range of temperatures, their spawning behavior is more sensitive to changes in water temperature.
Researchers found that when springtime temperatures in the Gulf of Alaska were higher than 6°C, fewer larvae and juveniles less than a year old were observed in the ecosystem. This indicates that cod’s narrow thermal window for hatch success may be a limiting factor in their ability to adapt to rising ocean temperatures.
Northward Shift: Challenges and Consequences
As waters off Alaska continue to warm and sea ice disappears, cod are already starting to move northward into the Bering Sea in search of cooler waters. However, this region may lack comparable habitats to those that cod rely on in the Gulf of Alaska, and the Bering Sea may be too cold for their eggs due to cod’s narrow thermal window for hatch success.
The northward movement of cod has economic and ecosystem implications. The reliability of the Pacific cod fishery in the Gulf of Alaska will be affected, and the shift may also impact other fisheries, such as the tanner crab fishery in the Bering Sea. Additionally, if cod replace Arctic cod as a food source for predators in the Northern Bering and Chukchi Sea, there could be a decrease in the quality of food available for species like seabirds, ringed seals, narwhals, belugas, and other fish.
Adapting to Warmer Waters: Potential Mechanisms
Researchers are exploring potential mechanisms by which cod may be able to adapt to warmer waters in future generations. For instance, they are investigating whether the eggs of fish that have acclimated to slightly warmer temperatures have higher hatch success in warmer water, or if there is evidence of warm-tolerant genotypes.
However, longer-term studies are needed to answer these questions conclusively. In the meantime, the effects of climate change on cod populations are already being observed. A study published in 2024 found that during recent marine heatwaves, some cod stocks may never fully recover, as the warmer the water, the more larval and 4-year-old cod died, possibly due to increased predation by zooplankton that feed on cod larvae.
In conclusion, while cod have traditionally been considered cold-water fish, recent studies have shown that they are more adaptable than previously thought. However, their spawning behavior and early life stages remain vulnerable to the effects of rising ocean temperatures. As cod shift northward in search of cooler waters, it is crucial to understand the potential ecological and economic consequences of these changes and explore ways to mitigate the impact of climate change on this important fish species.