Credit: Douglas Killam, Calif. Department of Fish and Wildlife.
Winter-run Chinook salmon embryos (like the ones shown above) hatch from their egg capsules after more than a month-long development period buried in gravel nests. After hatching they spend another month or longer as alevins, buried under gravel continuing to grow by feeding off the energy stored in their yolk sacs until finally they emerge from the gravel as fry. Salmon are especially vulnerable to elevated temperatures during their embryonic stages.