They sometimes construct bubble nets around a school of fish and then lunge up through the bubble net to get their food. With very small populations this low rate of reproduction means that it may take 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 animal 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.