Sockeye Salmon Can't Handle the Heat

As rivers along the Pacific coast get warmer, the survival rate of migrating sockeye salmon will plummet. Experimental exposure of adult sockeye from the lower Fraser River run to warm water resulted in high fish mortality.

Salmon kept four weeks in 18 °Celsius water had nearly twice the mortality rate of salmon living in 10 °C water.

Later when the survivors were released to continue their upriver migration to Weaver Creek near Harrison Lake in southwest British Columbia, mortality of hot water fish was again double that of the fish from cooler water. While migrating, the death rate for fish kept in cool water was similar to that of a control group which never had their journey interrupted by scientists.

The final outcome at the spawning beds for the 50 fish that started in each group was dramatically different. Altogether 21 salmon from the ten-degree tanks made it to spawn, whereas only 1 female and 5 male sockeye that had lived in warmer water reached their spawning grounds. This study is the first to show that adult sockeye salmon exposed to high temperature are less successful at migrating and thus less likely to reproduce.

A kidney parasite, Parvicapsual minibicornis, was the key difference between the two groups of salmon. It severely infected 44 percent of the fish that died in warm water, yet didn't appear in any of the fish held in cool water. This is consistent with other research showing that temperature plays a role in the prevalence of kidney infection among sockeye. Salmon from warm water were also more covered in fungus than were their cool-water counterparts.

River water temperatures of 18 °C aren't unusual these days. Peak summer temperatures on the Fraser River's main stem have risen 1.5 °C since 1940. The fish in this study had entered the Fraser in early September 2004 when water was an exceptionally warm 21 °C.

There's more, though, than just climate change that's putting salmon into hot water. Many of the late-run Fraser sockeye stocks now enter the river several weeks earlier than the run did a decade ago. The salmon start upriver in mid-August when water measures at least 18 °C, rather than in September or October when temperatures cool to 10 to 14 °C. The sudden onset of this behaviour in 1995 puzzles fisheries biologists who don't yet understand why the shift in timing occurred.

But the consequence for the sockeye is remarkably high levels of mortality, up to 90 percent, during river migration. This research suggests that late-run sockeye who get an early start on upstream travel aren't coping well with higher river temperatures.