Marine mammals can be difficult to identify at sea. Even under ideal conditions, an observer often gets little more than a brief view of a splash, blow, dorsal fin, head, flipper, or back, and this is often at a great distance. Rough weather, glare, fog, or other bad sighting conditions compound the problem. Many species appear similar to another, especially in the brief glimpses typical at sea. Animals of some poorly known groups (most notably beaked whales and Southern Hemisphere fur seals) are especially difficult to identify to species, even with a good look at a live animal or an “in hand” specimen. For all these reasons, even experts often must log a sighting as “unidentified” or on an easily confused pair or group of species. In all cases, this designation, accompanied by a detailed description is preferable to recording an incorrect identification.

The species identification sheets in this guide are designed to be the primary tool used in identifying marine mammals observed at sea. A dichotomous key to marine mammals observed at sea would be virtually worthless, because of the lack of useful cues for most sightings and the variability of marine mammal behaviour. Marine mammal identification at sea is something that must be learned through doing. Experienced marine mammal observers, like birders, often will be able to make an identification based on a composite of characteristic features including behaviour, and personal knowledge of the local marine mammal fauna. This ability will come with experience, guided by working with seasoned observers and the use of a proper field guide.

Marine mammals specimens “in hand” can best be identified by using the dichotomous keys to external features. With such specimens, it may be possible to view the entire body and to measure relative proportions of features. Various features of coloration and morphology are often useful in such considerations. We have used geographical information as little as possible to separate the species. This will help to avoid biasing observers toward making an identification based on what they think is “supposed” to be there.

Marine mammal skulls can be keyed out using the keys provided at the beginning of each major section. We have assumed that no geographical information is available, so the key can be used to identify an untagged skull of unknown origin in a museum. It is clear from our own work and discussions with colleagues that is is not yet possible to prepare a reliable skull key for the non-specialist. Published keys and related literature are marred with errors and inconsistencies. Skulls of many species are sufficiently similar that it will be necessary to examine a full series of each to define reliable diagnostic features. Until that exercise is completed for each species, it would be a disservice to prepare a key to the species level. Instead, we provide a key only to family level.

It can sometimes be very difficult, or impossible, to identify marine mammals to species, whether based on sightings at sea , specimens “in hand”, or an unlabeled skull. Great variability in behaviour, coloration, body morphology, and bone structure can occur. Sometimes it may only be possible to label an animal or group as “unidentified long-snouted dolphin”, “unidentified beaked whale,” or “unidentified fur seal.” If this guide helps lead to an identification in some cases and to narrow down the choices in others, then it will have served an important function.