John Weinberg, member of the Working Group on Marine Mammel Ecology (WGMME)
'Marine Mammal' is a term used to describe those mammals that spend all or part of their lives in the ocean. It is a mixed term in that it is part taxonomic (mammal) and part geographic (marine).
The taxonomic class Mammalia or more commonly 'mammals' is a group of related animals that have a backbone and, on the whole, show the mammalian characteristics which typically include having lungs and breathing air, being warm blooded, having hair at some point in their life, and providing milk for live born young.
The group 'mammals' contains over 5000 species and is further divided into around 26 orders. Some of these orders such as the Primates (apes, monkeys, lemurs and humans) contain no marine mammals where as others such as the Sirenia (sea cows, manatees and dugongs) are entirely marine.
Altogether there are around 125 species considered to be marine mammals. These include all the Sirenia, the sea cows, manatees and dugongs; all of the Cetacea*, the whales and dolphins; from the order Carnivora and the family Pinnepedia*, the seals, sealions and walruses; from the family Mustelidae just sea otters; and from the family Ursidae just the polar bear.
A variety of conservation issues affect marine mammals today, many of which are related to human activity. They include amongst others fishing, pollution, hunting and the effects of noise from shipping, oil and gas exploration, military activity and tourism. The degree of impact of any human activity, varies considerably between different species and depending on their ecology, distribution and abundance.'
*although there are a few species from these two groups that are only found in freshwater they are usually also included as marine mammals