Most of the larger whales have a very low reproductive rate: females only have a single calf every 2-4 years. With very
small populations, this low rate of reproduction means that it may takes decades for some species to recover to their former
population levels. Even under the best conditions, it will take over a hundred years for the right whale to recover.
For some species of whales, the calves are born during the part of the annual cycle when the animals are in warmer waters,
and the adults are not feeding. Unlike dolphins and porpoises, a calf spends a relatively short period of time with its mother.
Calves usually become independent within a single year. During that time, a calf gains weight very rapidly. Whale milk is
very rich, and a blue whale calf may gain almost 200 pounds per day.
Although commercial whaling currently does not present a threat to the survival of the baleen whales, loss of habitat and
other human activities may make recovery more difficult. Collisions with vessels, oil spills and other changes in water
quality, coastal development, and increasing noise created from the use of oceanic resources may all affect the whales.
Fisheries may affect whales in two ways. First, whales may become entangled in fishing gear. As an example, each year
several humpback whales are entangled in fishing gear along the east coast of the United States and Canada. Second, fisheries
may compete with whales for food, such as herring.
Increased noise or boat traffic may cause whales to alter their behavior. There is evidence that humpback whales in Hawaii
may have changed their use of near-shore waters where calves are raised by their mothers because of increasing human
activity. Migrating bowhead whales may move further offshore to avoid human-caused noise.
Although we do not have a full understanding of the possible impacts, pollution could also affect whales. Many contaminants
are stored in a whale's blubber for long periods of time. Pollutant loads are usually lower in baleen whales than in dolphins
and porpoises. Deterioration of the environment could possibly affect the whales in another way: if pollution and other
factors reduce the number of fish and crustaceans, the food available to the whales could also be reduced.