Timing is everything.
Salmon are always trying to strike the right balance. If fry hatch too early in spring, they will find themselves without food and probably won’t survive. If they emerge too late, they must compete over food with earlier-born, larger fry and once again, survival will be compromised.
It’s a matter of life and death for fry, who have no control over their emergence dates. In fact, there are only two factors that determine when the fry hatch: the date their mother spawned and the water temperature at their nest.
“Water temperature is important because salmon are cold-blooded and development rates vary with temperature; eggs mature rapidly in warm water and slowly in cold water,” William Deacy from the University of Montana told The Science Explorer. This means that salmon must spawn earlier in cold stream waters and later in the warm waters of lake beaches and rivers, he explained.
Though Kodiak brown bears are known to dine on salmon, energy-dense eggs are the preferred fare on the spawning grounds. So by staggering spawning times to ensure fry emerge under the best conditions possible, salmon unwittingly create waves of resources for hungry bears.
Deacy and a team of collaborators recently found that bears surf these resource waves by tightly matching their sequence of spawning site visits to the timing of egg laying at each one. Their study was published in the journal Ecology.
The researchers fitted 40 brown bears on southwestern Kodiak Island, in the western Gulf of Alaska, with GPS collars to track their movements in relation to the resource waves. Location data revealed that the bears were visiting habitats in which salmon spawned early during the summer months, but waited until fall to forage in habitats with later spawning.
The spaced out spawning times afforded the bears a 70 percent longer feeding period than they would have gotten if the salmon all spawned at once.
How the bears figure out when each salmon population will spawn is still a great mystery, but Deacy suggests “it is probably a combination of learned behavior from their mothers, past experience, responses to cues (scavenger bird activity/ smells), and just travelling around and looking for salmon.”
Their heavy dependence on salmon means that bears stand to suffer as a consequence of human activities that reduce the variation in salmon spawning dates. For example, many salmon fisheries favor stocks with early migration, which is associated with early spawning.
The authors propose that “[f]isheries managers attempting to maximize harvest while maintaining ecosystem function should strive to protect the population diversity that underlies the phenological variation used by wildlife consumers.”
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