Pacific herring play a key role in the marine food web of Canada's West Coast. They are prey fish and comprise
30% to 70% of the summer diets of Chinook salmon, Pacific cod, lingcod, and harbour seals in the coastal waters of southern British Columbia. The eggs of Pacific herring are important to the diets of migrating sea birds, gray whales, and some invertebrates. Pacific herring are not mature enough to spawn until age three. Spawning takes place in coastal areas where algae beds are abundant and the water is uncontaminated.
Which of the following factors is an example of a density - independent factor that influences Pacific herring survival and growth?
a. ocean temperatures
b. population of grey whales
c. imposition of fishing quotas
d. algae populations