The ability to recognize individual animals has substantially increased our knowledge of the biology and behaviour of many taxa. However, not all species lend themselves to this approach, either because of insufficient phenotypic variation or because tag attachment is not feasible. The use of genetic markers ('tags') represents a viable alternative to traditional methods of individual recognition, as they are permanent and exist in all individuals. We tested the use of genetic markers as the primary means of identifying individuals in a study of humpback whales in the North Atlantic Ocean. Analysis of six microsatellite loci among 3,060 skin samples collected throughout this ocean allowed the unequivocal identification of individuals. Analysis of 692 'recaptures', identified by their genotype, revealed individual local and migratory movements of up to 10,000 km, limited exchange among summer feeding grounds, and mixing in winter breeding areas, and also allowed the first estimates of animal abundance based solely on genotypic data. Our study demonstrates that genetic tagging is not only feasible, but generates data (for example, on sex) that can be valuable when interpreting the results of tagging experiments.

Accurate identification of humpback whales from photographic identification data depends on the quality of the photographs and the distinctiveness of the flukes. Criteria for evaluating photographic quality and individual distinctiveness were developed involving judgments about overall quality or distinctiveness and about specific aspects of each. These criteria were tested for the level of agreement among judges. The distinctiveness scheme was tested for the independence of distinctiveness judgments and photographic quality. Our results show that judges could agree when evaluating specific and overall aspects of photographic quality and individual distinctiveness. The level of agreement varied for different pairs of judges, and less adept judges were identified. Ability to agree on evaluations of photographic quality was independent of the experience of the judges. Overall photographic quality and overall distinctiveness were successfully predicted from more specific variables, but the agreement between judges for these was not significantly greater than the agreement for the overall measures judged directly. There was no correlation between individual distinctiveness and photographic quality for four of the five judges, but the power of this rest may be low. Analyses of photographic identification data frequently require evaluations of photographic quality and individual distinctiveness. To obtain reliable results from such analyses, evaluation schemes and judges should be tested to ensure reliable and consistent evaluations.

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The results of a double-marking experiment using natural markings and microsatellite genetic markers to identify humpback whales (Megaptera novaeangliae) confirm that natural markings are a reliable means of identifying individuals on a large scale. Of 1410 instances of double tagging, there were 414 resightings. No false positive and 14 false negative errors were identified. The rate of error increased with decreasing photographic quality; no errors were observed among photographs of the highest quality rating, whereas an error rate of 0.125 was identified in sightings for which only part of the area used for identification was visible. There was also a weaker relationship between error rate and the distinctiveness of markings, which may result from non-independence in coding for image quality and distinctiveness. A correction is developed for the Petersen two-sample abundance estimator to account for false negative errors in identification, and a parametric bootstrap procedure for estimation of variance is also developed. In application to abundance estimates from the North Atlantic, the correction reduces the bias in estimates made using poorer quality photographs to a negligible level while maintaining comparable precision.Résumé : Une expérience de marquage double à l'aide de marques corporelles et de marqueurs génétiques microsatellites confirme que l'utilisation des marques corporelles est une méthode fiable d'identification à grande échelle des individus du Rorqual à bosse (Megaptera novaeangliae). Des 1410 événements de double marquage, 414 impliquaient des animaux vus antérieurement. Aucun résultat faussement positif ou faussement négatif n'a été décelé. Le taux d'erreur augmentait avec la mauvaise qualité des photographies; aucune erreur ne s'est produite à l'utilisation des photographies de haute qualité et le taux d'erreur était de 0,125 lorsque seulement une partie de la surface de l'animal utilisée pour l'identification était visible. Il y avait aussi une relation plus faible entre le taux d'erreur et le caractère distinctif des marques corporelles, qui peut provenir du manque d'indépendance entre le codage de la qualité de l'image et le caractère distinctif de l'animal. On trouvera ici une correction de l'estimateur d'abondance à deux échantillons de Petersen pour tenir compte des résultats faussement négatifs dans l'identification, ainsi qu'une procédure bootstrap paramétrique pour estimer la variance. Appliquée à des estimations d'abondance provenant de l'Atlantique nord, la correction réduit à un niveau négligeable l'erreur due à l'utilisation de photographies de qualité inférieure dans les estimés, tout en maintenant un niveau de précision similaire.[Traduit par la Rédaction] 1870 Stevick et al.