Counting whales 2. Mark-recapture methods

Mark-recapture studies require multiple visits to an area over time and for individuals to be recognisable.

Recording how many known individuals you recapture on each visit allows you to estimate the size of the population. In the case of gray whales, ‘recapturing’ a whale simply means recognising the individual, usually from pigmentation and scarring patterns on their flanks and the shape and pigmentation on the undersides of their tail flukes. These studies are often referred to as photo-identification studies because photos of the whales are later used to identify and catalogue the individuals. Photo-identification catalogues of gray whales have been compiled for the western North Pacific, from northern California to south-eastern Alaska, and in Baja California.

Mark-recapture studies result in longitudinal datasets that can also reveal valuable information such as lifespan, age at first reproduction, inter-calf interval, survival and movement patterns, as well as being used to estimate the size of the population.

Photo-identification has revealed the movement of a small number of gray whales between the western and eastern North Pacific populations, provided estimates of the number of gray whales in the Pacific Coast Feeding Group (a group of whales that feed in the more southern waters between northern California and south-eastern Alaska over the summer) and revealed changes in distribution between years.