Pacific salmon–among the `elite' athletes of the sea–hatch in rivers, spend their adult years in the ocean, and then make one last incredible journey up-river to reproduce in their natal streams. This final trek, or spawning run, returns 10 million salmon to North American west coast rivers annually where, for up to 1000 km, they encounter turbulent currents,narrow deep gorges (particularly the Fraser River's appropriately named Hell's Gate) and fluctuating temperatures. However, humans may have created a new hurdle that salmon may not be able to overcome. Tony Farrell and his colleagues in Canada suggest that increasing river temperatures profoundly affect the salmon's athletic capacity, and are largely responsible for salmon vanishing from their migration routes.
Farrell's team wondered why some salmon populations seemed to be more affected by temperature than others, and so they compared `aerobic scope'between several populations. Aerobic scope is the range of swimming speeds that a fish can sustain before resorting to quick bursts of anaerobic exercise(not requiring oxygen) or suffering complete cardiovascular failure. As expected, athletic fish would possess a greater aerobic scope than couch potatoes. Furthermore, there is a swimming intensity within this range where the fish's body captures and delivers oxygen to the animal's working muscles most efficiently. Because oxygen capture and delivery is a temperature-sensitive process, it makes sense to understand how temperature influences aerobic scope.
The team collected salmon at various points along the Fraser River migration routes and determined their aerobic scope by testing how long the fish could swim at various temperatures and speeds before becoming exhausted. They found that some populations swam most efficiently in warm waters while others swam best at cooler temperatures. The team also noticed that when a wide range of temperatures are available, the salmon opted to swim predominantly at temperatures where they are physiologically most efficient and have the greatest aerobic scope.
To understand how efficiently salmon were exercising in the wild, the next step was to determine temperatures along the spawning routes. Farrell's group equipped some of the migrating fish with tracking devices to monitor their locations and nearby water conditions, and then compared the present day temperatures to historic temperatures. The team discovered that current river temperatures are higher than have been recorded in 50 years! Rising river temperatures mean that some populations are forced to swim at temperatures near the boundaries of their aerobic scope, resulting in enormous energy expenditure. Essentially, migrating salmon are exercising at physiologically inefficient and potentially life-threatening temperatures.
As if the fish don't have enough difficulties in hot water, temperature also signals when the fish should begin migrating. Unfortunately, increasing temperatures have confused fish into hurrying or delaying their journey,usually resulting in death before they reach their spawning grounds. However,the researchers in Canada did report one optimistic discovery. Some populations were observed `out-smarting' thermal barriers, waiting in deep cooler tributaries or lakes until the river temperatures became safe for them to enter. Ultimately, this waiting strategy increases migration success and guarantees a next generation, but is only a short-term fix to the global problem. Farrell's group concludes that temperature is the key to successful salmon migration, and suspects that global climate change may lead to the disappearance of the entire species if the thermal barriers that we have created prevent their epic journey home.