ABSTRACT

White Shark aggregation sites offer some of the best opportunities to gather data on species biology and ecology. Mark-recapture methods are useful for estimating abundance, as long as their underlying assumptions are met. In this chapter, we employed the encounter histories of 113 (67 males and 46 females) photo-identi¢ed White Sharks as the base information for a mark-recapture analysis for the Guadalupe Island aggregation site. We estimated apparent survival, recapture probabilities, permanent entry into the population, and the superpopulation size by using the CormackJolly-Seber (CJS) model and a generalization of the Jolly-Seber (JS) model for the open-population

mark-recapture experiments. We ¢tted several model structures for each of the CJS and JS types and selected the best-¢t model structure using the corrected Akaike’s Information Criterion. Our results indicated that although apparent survival and permanent entry into the population varied with time, the recapture probability varied with sex (pfemale = 0.51 ± 0.044; pmale = 0.79 ± 0.029). The superpopulation size by sex were 51 females (SE = 2.7) and 69 males (SE = 1.6). However, the assumption of equal recapture probability was violated by our encounter histories of males, producing an underestimation of the population size by the model. We discuss all the possible explanations for the violation of the assumption, based on this and another recent study. We recommend the use of this type of analysis as an index of abundance only, which can provide a long-term population trend but not an absolute abundance until methods can be improved.