Because most species of large whale are depleted, there is considerable interest in methods of population census for these animals. Each species of whale produces distinctive sounds, much as with bird song. Using passive recording systems in the ocean we can make long term (1 year or more) recordings which are then processed to note the number of occasions whales of each species and/or each population were heard. The large baleen whales use low frequency sounds, mostly less than 500 Hz, making computer storage and processing time for the recordings a relatively easy task. Analysis of a one year duration recording from a single hydrophone requires several weeks of effort by one scientist, reviewing the sounds both visually as spectrograms and with automated detectors. Journal articles for this type of study have been published for fin whales near Hawaii, and for fin and blue whales in the Antarctic and off California (Hawaii acoustic density abstract)
(Antarctic acoustic density paper )
Traditional census methods rely on line transect surveys with observers searching for the roll of a dorsal fin or the plume of a blow. Large binoculars (as seen in the photo) are part of the standard procedure. Aircraft surveys cover more miles but miss more animals per mile because the animals may not surface during the time that area of the ocean is in view. Strong winds and rough seas limit visual survey effectiveness. In general, abundance estimation efforts stop if the wind exceeds about 22 knots.
Population estimates are also made using mark-recapture methods where the animals are individually identified, either genetically with a skin sample or with photographs. Skin samples are sometimes obtained by a sample collection dart fired from a rifle or crossbow (see photo). The orange foam visible in the photo floats the dart for pickup.