marks makes it possible to use all the methods of mark–recapture (Chapter 3) on populations of
mammals that are never captured in traps or subjected to a tagging or marking procedure. These
types of non-invasive studies are clearly desirable for species of high conservation interest.
Method
A catalogue of photographs of individuals must be maintained for the population of interest,
and, if the public is involved, a mechanism for obtaining and copying photos taken by amateur
observers is needed. The use of digital cameras with data and time stamps can now make the
electronic collection of such photos possible. As in all population studies, good record keeping
is critical to the success of these projects. Typically the species chosen are long-lived, and there
is relatively little turnover from year to year.
Advantages and disadvantages
One advantage of this method is that it can make use of untrained observers to increase sample
sizes. Photos must be of high quality, and a well-defined screening process must be defined in
order to eliminate photos that are too poor to allow certain identification. Because this method
can be very time-consuming, it can rarely be done solely by paid observers, so amateur input can
be essential to successful sampling.
With these methods it is important to define the population being sampled and to cover the area
occupied by the population. Most individuals in the population must be identified to provide the
most precise population estimates. By providing spatial locations of the photographs, it is also
possible to map the movements of individuals when there is good coverage of the areas occupied.
Biases
Care must be taken to ensure that individuals with similar markings are not confused in the
counts. For humpback whales, Blackmer et al. (2000) showed that colouration patterns on the
flukes changed with age, and that dorsal-fin shape was a more stable characteristic for individual
recognition. If a total count is to be achieved, a high level of sampling effort must be expended. If
mark–recapture methods are to be used on these data, all the assumptions of the particular method
360 Mammals
must be evaluated (Chapter 3). In particular, some individuals may be more easily located than
others, so equal detectability cannot be assumed in a mark–recapture model.
Counting calls
Bats make ultrasonic calls that can be recorded by a bat detector. Lions roar, and seals and sea