Others are skimmers that swim along with their mouths open before straining their prey from the water. One of the more interesting feeding methods has been observed in humpback whales. 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. 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 no longer presents a threat to the survival of the baleen whales in U.S. waters, 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' lives. Commerical fisheries may also affect whales as some whale species often become entangled in fishing gear.