Estimating the abundance of marine mammals: A North Atlantic perspective

This paper investigates how studies to estimate the abundance of marine mammals have used the characteristics and behaviour of the target population to help determine the most appropriate method for any specific study. It also addresses how these studies have attempted to overcome the problems associated with ensuring that important assumptions of the method are not violated. Three basic methods used for estimating marine mammal abundance are described: extrapolating counts; mark-recapture analyses of photo-identification data; and sightings surveys. Recent studies in the North Atlantic to estimate abundance include: harbour seals and bottlenose dolphins in the Moray Firth, Scotland; grey seals in the North Sea and the breeding population in Britain; North Atlantic humpback whales; and the NASS and SCANS sightings surveys for whales and small cetaceans in the North Atlantic and North Sea, respectively. The paper demonstrates the wide range of applicability of the available methods, the way that the example studies addressed and (in some cases) overcame potential problems, and the modification or extension of “standard” methodology to fit particular circumstances.