Abundance and demography of a seasonal aggregation of zebra sharks Stegostoma fasciatum

Seasonal aggregations commonly occur in the marine environment where typically wide-ranging organisms come together to exploit temporary resources or find conspecifics for mating events, The zebra shark Stegostoma fascialum is a demersal carpet shark that aggregates over the austral Summer months in the coastal waters of southeast Queensland, Australia, This study employed photo-identification and mark-recapture methods over a 3 yr period (2003 to 2006) to investigate the population size and structure of this aggregation. In total 327 individual zebra sharks were identified from 570 photographs. Numbered dart-tags on 15 zebra sharks were used to confirm that pigmentation patterns were unique and persistent in wild zebra sharks for up to 810 cl. Pollock's robust design resulted in an annual population estimate of 458 individuals (95% Cl = 298-618). The mean number of zebra sharks observed on a single clay was 8 ( 8 SE) and the maximum number of zebra sharks seen on a single day was 34. In total, 27%, of [he sharks were sighted in more than one summer aggregation period and males had greater re-capture probabilities than females. The aggregation consisted exclusively of large (>1800 mm total length) adults with an overall female sex bias of 3.81 though sex-ratios varied temporally. Predictable visitation of large, presumably mature individuals to the site raises conservation concerns if aggregations of similar size and structure occur in regions where zebra sharks are fished.