ResearchResearch methodsVisual surveys

Visual surveys

Visual surveys (or sightings surveys) are one of the main research methods for investigating the distribution and abundance of cetaceans. They are valuable because they can be conducted at a wide range of scales: from small studies with a minimum of equipment in a small area to provide local information; to major surveys over large areas of ocean to provide accurate and precise estimates for use in a specific conservation or management context.


Visual surveys have been used to estimate the abundance of minke whales in very large areas of the North Atlantic, North Pacific and Southern Ocean. Other large-scale surveys have been conducted in the eastern North Atlantic for harbour porpoises, in the eastern tropical Pacific for spotted and spinner dolphins, and in the Mediterranean for striped dolphins and fin whales. Smaller scale surveys have generated estimates of abundance for many other species around the world.Sightings surveys use so-called line transect sampling techniques for data collection and analysis. These methods were first developed to estimate the abundance of terrestrial animals but are now widely applied to cetacean populations via shipboard or aerial surveys. In these sightings surveys, the study area is sampled by the survey platform searching along predetermined transects. The distance and angle to sighted animals (or groups) are measured (or estimated) allowing the calculation of perpendicular distance from the sighting to the transect line. These data are used to estimate the effective width of the strip searched so that sample density can be estimated. If transects are placed in the study area to ensure that everywhere in the area is equally likely to be surveyed (so-called equal coverage probability), sample density can simply be extrapolated to the whole area to provide an estimate of abundance.

There is an increasing number of visual surveys which do not achieve equal coverage probability but nevertheless generate good line transect sampling data. For these surveys, the recently developed method of spatial modelling provides a way to obtain estimates of abundance and to investigate the relationships between cetacean distribution and environmental features such as the depth of the sea bed, water temperature and measures of ocean productivity. In this method, the line transect sampling is combined with the spatial modelling. The perpendicular distance data are used to estimate a detection function, that allows abundance to ne modelled aa a function of physical and environmental factors associated to the transects sampled. In this way, abundance can b estimated for thw whole study area through extrapolation and density maps are created.

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