Can artificial cooling, or thermal refuges, offset the effects of ocean warming on fish?

Ocean warming is harming fish, leading scientists to explore artificial options to cool things down

Ocean warming is putting fish in warm water. Average global ocean surface temperatures have increased by 0.13 degrees-C per decade for the last century, linked to loss of marine habitat and sudden declines in fish species.

In some regions, a rapid increase in water temperature has occurred Too fast for the fish to adjust, leading to changes in their geographical distribution. While some fish swim toward the poles in search of new habitats with deeper, cooler water, others rest in thermal refuges.

Thermal refuges, also called cold water refuges or cooling zones, are discrete areas where water temperatures are at least 2 degrees-C cooler than ambient conditions found elsewhere throughout the watershed, explains Chris Sullivan, a Ph.D. candidate in the Department of Natural Resources and Environment at the University of Connecticut.

“Thermal refuges are really critical for the short-term survival of the [fish species],” Sullivan said in Protector. “We will likely see their roles emphasized as weather progresses and climate changes.”

Understanding the need to retreat

The effect of warming water on fish populations is clear: Increasing water temperature affects spawning, affects foraging habits and increases the risk of disease; one study predicts up to a 34-fold increase in fish kill events by 2100 based on current climate projections.

Thermal refuges provide valuable relief from the heat. Sullivan said colder water is often found in side channels, lateral springs or other places where rising groundwater lowers temperatures; Riparian vegetation covering streams and rivers also has a cooling effect. Cold water species such as trout, salmon and cod migrate to thermal refuges for hours to weeks at a time to cope with extreme heat.

Currently, it is unclear how fish find thermal refuges. Tommi Linnansaari, chair of Atlantic salmon research and professor at the University of New Brunswick, says that “fish have amazing spatial cognition,” and points to some cues that may lead them to colder water. .

It is possible for fish to sense cold water coming downstream and swim in that direction; fish can also smell schools of fish gathered in a thermal refuge and move toward their scent.

Despite their ability to navigate to cooler waters, the number of natural thermal refuges there are rejectedwhich makes it harder for fish to beat the heat and more susceptible to the effects of warming water temperatures.

“Even the loss of a single patch [of cooler water] across the riverscape…could damage that part of the river,” Sullivan said. “We may not see the mass disappearance of thermal refuges, but even if we lose one or two, it could harm the cold water fish depend on.”

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The hot debate in the cold water

Rising water temperatures and longer, more frequent marine heatwaves along with the shrinking number of thermal refuges have led to debates about using artificial methods to cool the water.

Researchers at the Coastal Hydrology Lab at Dalhousie University in Canada received funding to explore the possibilities. Their first experiment was to create underground trenches that redirected water into rivers and streams before it flowed back into the main bodies of water. Early findings showed that the approach lowered the water temperature by several degrees, which was usually enough to help the fish cool off.

The second experiment involved building wells and pumping cold groundwater into rivers and streams. Groundwater is only 9 degrees-C (48 degrees-F), which is up to 20 degrees cooler than the surrounding water.

In a separate project, the Cheticamp River Salmon Association in Nova Scotia, Canada, placed rock piles known as deflectors to make river channels narrower and deeper – and thus cooler – to help protect Atlantic salmon.

Experiments have shown that it is possible to create areas with lower water temperatures for fish to beat the heat, but there is disagreement as to whether creating artificial thermal refuges is a good idea. Linnansaari called for “careful and multi-faceted consideration of the consequences,” in his research.

Opponents have expressed concerns that groundwater is a limited resource and that pumping it into rivers and streams depletes supplies and can create ripple effects in the ecosystem. There is also great potential for abuse.

Linnansaari points to fishing camps that pump groundwater to cool the surrounding water to attract large salmon populations. He called it “a kind of arms race where whoever has the biggest bomb wins.” Regulation, he believes, will be essential for the widespread creation of artificial thermal refuges.

These methods may have some use in smaller bodies of water but are impractical in the ocean where rising temperatures also cause damage to fish species.

“Think of the vastness of our coastlines and the geographic range in which we see some of these different fisheries and organisms; it’s not just scale,” said Ryan Walter, associate professor at the Center for Coastal Marine Sciences at CAL Poly. “You need a massive system. Water is really heavy; it takes a ton of energy to pump that [and] where does that energy come from? It doesn’t make sense.”

Researchers may not agree on the correct approach to providing temperature relief to stressed fish, but there is consensus that there is a need for continued efforts to address the issue.

“There are a lot of smart people working in a lot of different regions trying to figure this out,” Sullivan said. “I’m excited to see where science takes us.”

@GSA_Advocate