The Years of the North Atlantic Humpback (YoNAH) project collected photographs of tail flukes of humpback whales to identify individuals for estimating abundance using capture-recapture techniques. Data were collected during the winter breeding and summer feeding seasons in 1992 and 1993. Smith et al. (1999) identified possible violations in three of the assumptions needed for unbiased estimators of abundance: closed population, equal probabilities of capture, and the reporting of all marks on recovery.

To evaluate the assumption that all animals were recognized when photographed again, methods for coding photographic quality and the distinctiveness of whales were developed and applied to the YoNAH photographic database. Analytical methods were then developed to evaluate the effects of photographic quality and whale distinctiveness on the abundance estimates produced from this database. To evaluate the assumptions of a closed population and equal probabilities of capture, a model of the probability of sampling individual humpback whales was developed.

The YoNAH photographic identification database was coded for the quality of the photographs. The error rate of the photographic quality coding process was found to be a function of the quality of the photographs, such that poor quality photographs were less likely to change code on a second coding. Poor quality photographs were found to cause a potential positive bias in the abundance estimates. Analyses indicated that removing photographs of the lowest quality level and images of partial and right or left half flukes provided the optimum balance between precision and bias.

The coding of whale distinctiveness independently of photographic quality proved difficult for some coders. The distinctiveness of whales represented by at least one high quality photograph did not significantly affect their recapture rate.

Simulations of the YoNAH sampling project indicated that estimators that compared data collected during a breeding season to data collect during a feeding season within the same year were more robust than other estimators to violations of the equal probability of capture assumption and the closed population assumption. However, variability in the individual probability of sampling significantly affected all estimators. The probability of whales migrating to the winter breeding area and the time that females spend in the breeding area relative to males significantly affected the estimator that compared two breeding periods. To measure the bias in these estimates of abundance, estimates are needed of the variability in individual sampling probabilities, the proportion by age and sex of the population that does not migrate to the breeding area, and the relative length of time which females spend in the breeding area.