Assessing the distribution and relative abundance of wobbegong sharks (Orectolobidae) in New South Wales, Australia, using recreational scuba-divers

Abstract

Wobbegongs are benthic sharks that are commercially targeted in New South Wales (NSW), Australia. Given a dramatic reduction of more than 50% in landed catch in a decade, there is a clear need to ensure that basic ecological data such as distribution and abundance are available for management use. Opportunistic sightings of wobbe- gongs collected by recreational scuba-divers were used to assess the distribution and relative abundance of wobbegongs in NSW. From July 2003 until January 2005, 304 dives were undertaken by recreational divers and 454 wobbegongs were reported. Larger numbers of wobbegongs were sighted in northern compared to southern NSW. Spotted and ornate wobbegongs were sighted in similar numbers, but species composition was highly variable across locations. Only a few juvenile and newborn spotted wobbegongs were sighted, whereas small ornate wobbegongs were mostly sighted north of central NSW. The latter were possibly the third, cryptic species, the dwarf ornate wobbegong. The paucity of sight- ings of small wobbegongs suggests that juveniles and newborns are inconspicuous to divers or that small wobbegongs are found in areas not visited by divers. Potential species and size segregation suggest that closing areas to fishing may enable populations to sustain current levels of commercial exploitation. The cost-effectiveness of using recre- ational scuba-divers to opportunistically collect distribution and relative abundance data was apparent from this study. However, the lack of spatial and temporal homogeneity in diving effort suggests that future studies should consider incorporating organized surveys and a facilitator, rather than using opportunistic records of sightings.