Use of photographic identification in capture-recapture studies of Mediterranean monk seals.

The use of photo-identification and its reliability in capture-recapture st udies of Mediterranean monk seals were assessed using slides collected in t he colony at Cap Blanc, western Sahara, from 1993 to 1996. Five tests indic ated that researchers involved in photo-identification were proficient in m atching slides of identified seals, consistent in classifying the side of t he seal shown in slides and in assigning the morphological stage of the sea l, and that changes of markings over a period of three years were insuffici ent to affect matching success. The certainty of identifying a seal was not dependent on the number of slides used but on distinctiveness of the marki ngs and the quality of the slides taken. Capture-recapture abundance estima tes were biased upwards when including poor quality slides. The exclusive u se of excellent- and good-quality slides provided the best estimates. The p roportion of distinctive seals varied between morphological stages and was significantly lower in juveniles. When including the identification histori es of juveniles, the heterogeneity of capture probabilities was higher. The refore, abundance estimates were less biased when all juveniles were consid ered as non-distinctive seals. Reliable abundance estimates required a bala nce between duration of capture occasions and time interval between these.