Impacts of Thermal Stress on Aquatic Species

Biologists release mussels into the Powell River, (Credit: Gary Peeples / USFWS via Flickr CC BY 2.0)

Under a rapidly changing climate, aquatic ecosystems are exposed to varying temperature extremes—both cold and hot—leading to thermal stress on aquatic species. As defined by Scient Direct, thermal stress refers to the impact on normal physiological functioning as a result of extreme temperatures.

Depending on the severity of temperatures and the species, reactions can vary and shape ecosystems. Water level changes, hypoxia, nutrient cycling, vegetation growth, and other physical/chemical characteristics of lakes are shaped by climate conditions.

As a result, a better understanding of the influence of thermal stress on key species and overall ecosystem health plays a key component in fisheries management. From an aquaculture perspective, understanding the influence of thermal stress and the limits of farmed species can improve yields.

How Fish Respond to Warming

Like other aquatic organisms, fish have preferred habitats in terms of temperature, water depth, salinity, dissolved oxygen levels, and other water quality conditions. Temperatures that exceed or fall below ideal habitat standards for each species place stress on native species and impact the ecosystem as a whole.

This thermal stress can have devastating impacts on fish populations, including fish kills, disease, distribution changes, shortened lifespans, increased fry mortality, stunted development, and other negatives that lead to population declines.

The loss of one species can have cascading effects on other organisms in the fishery. For example, the loss or decline of predator species can lead to booms in prey populations and declines in vegetation or micro-organisms as prey species can feed freely, according to Scripps Scripps Institution of Oceanography.

Mitigating thermal stress in a natural environment can be difficult due to being unable to suddenly cool or warm lakes and rivers. For this reason, NOAA advises that managers take an ecosystem-based approach to managing fisheries, monitoring water quality conditions to inform stocking and evaluate the vulnerability of the fishery.

A similar issue exists in aquaculture settings, so managers must ensure that pens/ponds are monitored frequently in order to prevent fish kills, adjust practices, and prevent further warming or cooling. A 2022 study found that feed restriction could alleviate chronic thermal stress impacts, though this topic is still being researched.

A NexSens real-time data buoy deployed in Norway Beach equipped with sensors that monitor temperature, pH, conductivity, dissolved oxygen, turbidity and chlorophyll levels. (Credit: Cautus Geo)

Mussels and Clams

Like fish, shellfish like mussels and clams are negatively impacted by thermal extremes. Different from fish though, mussels and clams are not able to escape poor conditions easily, with limited mobility as adults. Meaning that mussels and clams are trapped and often die as a result of thermal stress.

Beyond increased mortality among adult specimens. Thermal stress can impact the development of larvae, who start life soft-bodied and then develop their shells over time under the right conditions.

Thermal stress is one variable that can prevent the body from calcifying and forming the hard shell associated with mussels and clams. A 2024 study also highlights that thermal stress paired with ocean acidification can impact feeding and digestive physiology.

Considering the large number of freshwater mussels endangered or threatened in the U.S. and their important role in the ecosystem, managers should consider how best to protect and preserve these organisms.

For example, extreme heating or cooling events may require direct intervention wherein the mussels are translocated to another location to prevent die-offs.

In aquaculture facilities, operators must take steps to minimize thermal stressors. Some possible steps for bottom-dwelling species like mussels and clams could be the use of shade structures (inorganic floating barriers or natural vegetation) to block solar radiation.

Conclusion

Knowing how weather extremes will impact certain species helps fisheries managers plan out strategies based on existing knowledge of how species respond to specific temperatures. At the same time, aquaculture facilities can face great product losses and declines in yield—both in quality and quantity—as a result of thermal stress.

Under a rapidly changing climate, where instances of thermal stress are expected to grow in frequency, monitoring water temperature is critical for fisheries and aquaculture management moving forward.