The abundance of whales and seals is usually (though not always) estimated using surveys. Surveys are conducted at sea using either ships or aircraft. Generally ships are used for large offshore areas, whereas aircraft are more useful for smaller nearshore areas.
The survey area is usually divided up into smaller areas, called strata. Survey effort—the amount of time or distance the survey vessel will spend in the area—is divided between strata based on how common the target species is expected to be. Generally speaking, more effort is given to areas with high densities, because this produces a more precise estimate. The survey vessel sails or flies a series of predefined transects that cover each stratum in the survey area. The transects are designed in advance of the survey to cover the area evenly.
The Survey Area is divided into "strata". The ship or plane surveys along transects within the strata.
Visual surveys, which use human observers to count animals, are the most common type. Observers concentrate on the area ahead of and beside the vessel. When an observer sees an animal or group of animals, she immediately records the observation, noting such things as species identity, group size and the number of young in the group. The observer then takes a measurement of the perpendicular distance – the distance at 90 degrees to the direction of travel – to the animal or group. These distances are later used in a statistical analysis to estimate the width of the strip that is being surveyed by the vessel.
Aerial photographic surveys are often used for seals, which haul out on land or ice. An area of known size is photographed, and the animals are later identified and counted on the photos.
|Visual surveys employ human observers to detect and count whales.|
|In aerial surveys the perpendicular distance (X) is estimated by measuring the declination angle to the sighting when it is abeam of the aircraft.|
The collection of perpendicular distances in visual surveys allows the analyst to estimate the width of the strip that is surveyed for a particular species. Knowing the area that was surveyed, and the number of animals that were seen, the density of animals in the area can then be calculated as:
For example, if 2 whales are seen in a surveyed area of 2 square kilometres, there is a density of 1 whale per square kilometre. This density is then applied to the entire stratum to estimate the number of animals there. Another example – if there is 1 whale per square kilometre, and the stratum has an area of 10 square kilometres, we would estimate that there are 10 whales in the stratum. Of course the calculations are much more complex than this in reality. The estimate of density has variance- a measure of the uncertainty of the estimate. Abundance estimates are therefore always expressed with a measure of uncertainty, usually a 95% confidence limit. Instead of saying that “there are 10 whales in the stratum”, we would instead say something like "There are about 10 whales in the stratum, maybe as many as 15, maybe as few as 5”.