If you like salmon, here's another reason to be worried about climate change. Over the past six decades, the peak summer temperature of the Fraser River in British Columbia, Canada has risen by 2°C, coinciding with higher death rates in sockeye salmon during their migration to freshwater spawning grounds. Yet pink salmon still manage to arrive unscathed at their spawning grounds, despite their migration period including summer months, notes fish physiologist Timothy Clark. Teaming up with colleagues Ken Jeffries, Scott Hinch and Tony Farrell at the University of British Columbia, Clark decided to investigate whether pink salmon have physiological tricks up their sleeve that allow them to cope better with heat than their counterparts (p. 3074).
The team suspected that the key to pink salmon's success during a summer heat wave is an ability to keep their muscles supplied with oxygen while struggling upstream to their spawning grounds. ‘A marathon runner can dramatically increase oxygen transport during exercise,’ explains Clark, ‘while a couch potato can't.’ In other words, pink salmon might simply be fitter than other salmon species. To see how sporty pink salmon really are, the team caught fish from a tributary of the Fraser River and brought them back to the lab.
First, the team needed to show that pink salmon sufficiently boost oxygen transport when they get hot. Placing the salmon in a doughnut-shaped swim tunnel filled with cool water, they increased the water flow until the fish were swimming at top speed, then sealed the tunnel and used oxygen electrodes to measure how quickly the fish were using up oxygen. Then, to see how the fish cope with heat, they ramped up the temperature in 3–5°C increments and repeated the swim tests. Sure enough, the fish steadily increased their oxygen consumption as the mercury rose. ‘It turns out that pink salmon are very athletic,’ says Clark. ‘They can increase oxygen uptake during exercise when the temperature rises, and do so better than previously reported for other salmon species.’
Next, the team investigated whether pink salmon are better at pumping blood around their body while working out in warm water. They measured how much blood the fish pumped out with each heartbeat by fitting fish with blood-flow probes around the ventral aorta, the main vessel carrying blood from the heart. When the fish were again subjected to swim tests, the team saw that the pink salmon's maximum heart rate and blood flow increased as the water got warmer. ‘A combination of exceptional metabolic and cardiovascular capacity means that pink salmon can exercise at higher temperatures than other salmon,’ Clark concludes. ‘This could help pink salmon cope with future climate change. They may even fill the niches of species that aren't able to adapt as well.’
But at temperatures above 21°C, even pink salmon start to suffer. When Clark plotted maximum oxygen consumption and blood flow against water temperature, he saw that both measures clearly start to level off or decrease above 21°C. ‘This means that longer exposure to high temperatures would be problematic for pink salmon too, and they might succumb to heat stress,’ says Clark. Nevertheless, he adds, if their athleticism can help them get through sporadic warm patches in the river as they're heading upstream to reproduce, it might be enough to give pink salmon a competitive edge over other salmon species.