Salmon suffer heat stress too: growth stops at 18 C, some die at 26 C

Freshwater salmon at 26 C show elevated liver-damage markers and heat-shock protein gene expression up to 16 times higher

Water temperature found to affect salmon more than salinity in both fresh and saltwater — findings expected to guide fish farm temperature management

Research findings image [Korea Institute of Ocean Science and Technology]

Salmon, a cold-water species, have been found to suffer heat stress during periods of high summer temperatures. Growth stops when water temperature reaches 18 degrees Celsius, and some individuals die at 26 degrees. Salmon raised in 26-degree freshwater also showed internal warning signs, with heat-response genes expressing up to 16 times more than normal.

The Korea Institute of Ocean Science and Technology (KIOST) announced Thursday that a research team led by Dr. Kang Hye-min had published findings from an analysis of growth, liver function and genetic responses in juvenile Atlantic salmon exposed to high water temperatures.

Atlantic salmon at the smolt stage used in an experiment analyzing the effects of high water temperatures on the species' growth and physiological functions. [Korea Institute of Ocean Science and Technology]

Salmon are born in rivers and migrate to the ocean to grow. Before making that transition, their bodies undergo changes to adapt to saltwater environments — juvenile salmon at this stage are called smolts. During the smolt phase, the fish are particularly vulnerable to external environmental factors such as changes in water temperature.

The research team divided 192 Atlantic salmon smolts among four freshwater tanks and four saltwater tanks. Water temperatures were set at 14, 18, 22 and 26 degrees Celsius. Using 14 degrees — the temperature at which salmon grow best — as the baseline, the team raised temperatures in the remaining tanks by 1 degree per hour. Once the target temperatures were reached, the fish were observed for five days.

Only the salmon in the 14-degree tanks gained weight. Those at 18 degrees and above showed no weight change, and at 26 degrees some individuals died in both the freshwater and saltwater tanks.

Liver function also showed abnormalities. Salmon raised in 26-degree freshwater had significantly elevated ALT levels, a marker of liver damage. Blood protein levels fell in both the freshwater and saltwater tanks. The research team concluded that high water temperatures stressed the liver and disrupted energy metabolism.

Researchers at the Korea Institute of Ocean Science and Technology dissect Atlantic salmon to analyze physiological changes caused by high water temperatures. [Korea Institute of Ocean Science and Technology]

Protective genes also responded strongly. Expression of HSP90, a heat-shock protein gene that repairs proteins deformed by heat, increased approximately 16 times in freshwater and about 8.5 times in saltwater at 26 degrees compared with 14 degrees. Expression of GPX, an antioxidant enzyme gene that neutralizes reactive oxygen species — a cause of cellular damage — also rose.

Researchers at the Korea Institute of Ocean Science and Technology collect liver tissue from Atlantic salmon exposed to high water temperatures to analyze changes in liver function. [Korea Institute of Ocean Science and Technology]

Freshwater and saltwater differ in salinity, which affects how salmon regulate salt levels in their bodies. Yet in this experiment, both environments showed increased HSP90 and GPX expression and changes in liver function indicators as water temperature rose. The research team concluded that water temperature had a greater effect on the salmon's physiological responses than salinity did.

However, the study was a short-term experiment conducted on juvenile Atlantic salmon over a maximum of five days. The deaths and physiological changes observed at 26 degrees cannot be directly applied to all salmon, and further verification under a range of conditions is needed.

"As high water temperatures recur every summer, managing temperature at fish farms has become increasingly important," Kang said. "The three indicators identified in this study all changed together when water temperature rose in both fresh and saltwater environments." She added that the findings could serve as indicators for monitoring heat stress throughout the entire salmon farming cycle — from freshwater rearing to the transition to the sea.

The findings were published in the Journal of Thermal Biology.

adastra@heraldcorp.com