Warming water not good for fish

by Jeff Helsdon | September 14, 2021
Largemouth Bass Fry (Photo by Todd Pearsons/Engbretson Underwater Photography)

University of Regina Professor Peter Leavitt led an international study published in the journal Nature in June. His study looked at data from lakes with longstanding records across the northern hemisphere, including two sites in Ontario — the IISD Experimental Lakes Area near Kenora and Dorset Environmental Science Centre.

Warmer water means less oxygen

Looking at data recorded between 1980 and 2017, researchers found the average water oxygen level decrease at the surface was 5%. In deeper water, it was a more drastic average of 19%. The main driver for the decreases is water temperature increases since warmer water can hold less oxygen.

“What that means is lakes are losing fish habitat space at the bottom,” Leavitt said.

With huge oxygen losses at the bottom, fish are forced to move up in the water column into warmer waters, which are less optimal for many species.

“With no oxygen at the bottom and warmer areas at the surface, there is an overlap and no home for fish,” Leavitt said, adding this could impact recreational and commercial fisheries.

Algae also a consideration

Leavitt said the effect is generally more pronounced near areas with human activity and less intense in the boreal forest. Human-populated areas have greater nutrient inputs that fuel more algae growth. As algae dies, it drops to the bottom, and uses more oxygen as it decomposes.

Although not part of the study, when asked about Lake Erie and its dead spot driven by algae, Leavitt said this was the “poster child” for the impacts he was talking about.

“We’re not at the breaking point for many lakes yet, but we are headed in the wrong direction,” he said.

To reverse or stabilize the trend, Leavitt said human nutrient input needs to be controlled. The larger issue is stopping the increases in the earth’s temperature.

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