University of St Andrews
Sea Mammal Research Unit
SMRU > Research > Abundance Estimation

Abundance Estimation

Marine mammals are difficult to count as they spend most of their lives in or under water.  Seals must return to land to rest, moult and to reproduce but cetaceans remain at sea for their entire lives.  A primary goal for many studies is simply to know how many animals there are and where they are to be found. 

For most marine mammals, it is difficult to obtain a precise estimate of the total population size.  Instead, some component of the population is surveyed and statistical models are used to convert this component into an estimate of overall population size. 

Results from abundance estimation are used in population modeling to understand how and why animal population sizes changing.  Such changes may have important conservation implications.  Abundance is an essential factor in assessing Industrial impacts / interactions and Fishery interactions


The methods used to estimate marine mammal abundance depend upon the target animal’s life history.  For many cetaceans the standard method is ship-based or aircraft-based line transect sampling.  Seals are normally counted during the periods they come ashore to breed or to moult. 

Where seals are distributed over large areas, they are usually surveyed from the air, by light aircraft or helicopter.  Where a species is not too numerous or widely distributed and where individuals can be individually recognised through natural markings or tags, then mark-recapture techniques may be used.


NERC, acting through SMRU, has a statutory responsibility under the Conservation of Seals Act 1970 to provide information on the number and distribution of seals in the UK. Annual grey seal pup counts have been made since 1960 and counts of moulting common seals since 1988.  These surveys are conducted by Callan Duck and Dave Thompson and Beth Mackey.  Annual estimates of grey and common seal abundance estimates are provided, through NERC’s Special Committee on Seals, to the Scottish Government and the Department of Environment, Food and Rural Affairs [see SCOS reports]. 

Phil Hammond and Kelly MacLeod have co-ordinated three major international surveys of cetaceans in European Atlantic waters and the North Sea: SCANS in 1994; SCANS-II in 2005 and CODA in 2007. Together these surveys have generated a comprehensive picture of the abundance of the major species of cetacean in shelf and offshore waters of western Europe, especially harbour porpoise, and highlighted changes in distribution during the last decade.

Phil Hammond has also been involved in a number of photo-id studies that have been used as a basis for mark-recapture estimates of animal abundance.

Observers searching for cetaceans on the CODA survey in July 2007



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