Once in their lifetime, coho salmon undergo an arduous upstream migration to reach the same spawning ground they left as larvae years earlier. Transitioning from sea to freshwater, swimming against the current for hundreds of kilometres, navigating rapids and avoiding predators, salmon migrations are perilous journeys that require elite athletic performances to complete. Recent increases in river temperatures are further challenging the stamina of salmon, meaning that fewer fish may survive the migration and reproduce, which worries conservationists, fishers and indigenous peoples. The mortality is often higher in female salmon, which is perhaps unsurprising, as they must complete the migration while hauling a large mass of eggs up the river – the fish equivalent to running an up-hill marathon, in the summer heat, while pregnant. Just like you don't see many pregnant Olympians, female salmon may also be at a disadvantage when it comes to exercise, but whether pregnancy is to blame entirely is still unclear.
Krista Kraskura at the University of California Santa Barbara, USA, and her international team of collaborators set out to test whether mature female salmon struggle with exercise and whether higher water temperatures affect them more than the males. In a tremendous sampling effort, the team intercepted migrating salmon in the rivers of British Columbia, Canada, and transported them to the lab. There, they measured the athletic performance of females and males in a swim tunnel, a fish-treadmill of sorts, at three different temperatures: present low (9°C) and high (14°C) river temperatures, and a climate change scenario (18°C) that the fish do not yet see in the wild. The results were a surprise.
Across all of the temperatures, female salmon, despite carrying their heavy egg loads, matched the performance of the leaner males in all tests: swimming to exhaustion, escaping a simulated predator and recovering from bursts of activity. During exercise, female and male salmon needed similar amounts of oxygen, indicating that the larger female gonads did not impair performance as had been predicted. On the contrary, at rest and during recovery from exercise, female salmon actually required less oxygen than males, indicating that females may be more conservative with their energy stores, which is critical as salmon stop feeding during their journey. But once the temperatures were raised, both female and male salmon struggled in some way.
In warm waters that mimic predicted river temperatures due to climate change, salmon of both sexes were able to maintain their swimming performance surprisingly well. However, at the higher temperatures, all salmon needed longer to recover from the effort. This is a critical finding, as migrating salmon must repeatedly overcome rapids and other obstacles that require exhaustive bouts of exercise. A longer recovery period would delay their upriver journey and offset their timing, which is key when the next generation depends on everyone meeting at the same place, at the same time. Combined, these findings stoke the fears of conservationists that fewer salmon will be able to reproduce in a warmer future, putting populations at risk.
However, the main question of the study still remains a mystery: why do female salmon experience higher migration mortality rates? Kraskura and the team showed that elevated river temperatures affect both sexes and, if anything, female salmon are more efficient at metering out their limited energy. All things considered, female salmon are the real Olympians, which not only match the athleticism of males but also raise the stakes by carrying the future salmon brood up the rivers.