A PhD project undertaken by Christine Dudgeon at the University of Queensland, Brisbane, and supervised by Janet Lanyon
The leopard shark Stegostoma fasciatum (also known commonly as the zebra shark) is an egg-laying carpet shark that forms temporary aggregations and is distributed within shallow, coastal, subtropical and tropical waters of the Indo-West Pacific (IWP) region. The IWP region encompasses various densities of human habitation leading to differences in the levels of exploitation of sharks and rays, which are targeted for local consumption and export. This is reflected in two current regional IUCN Red List Classifications for leopard sharks: these are ‘Least Concern’ within Australian waters, and ‘Vulnerable’, throughout the rest of their range. Despite the conservation concerns for this species, as well as their importance within the ecotourism industry and collection for aquarium trade, little is known about leopard sharks in the wild. This project comprised the first intensive examination of the population ecology of wild leopard sharks and addressed three primary objectives: (i) to describe the population structure and demography of a relatively unexploited aggregation of leopard sharks in southern Queensland, Australia; (ii) to investigate the environmental parameters associated with the formation of the aggregation in southern Queensland and (iii) to investigate the genetic continuity between leopard shark populations from locations with differing levels of exploitation.
Figure 1: Map showing the location of the primary study site off North Stradbroke Island.
Figure 2: Leopard sharks aggregating off North Stradbroke Island.
Figure 3: Summer residents: leopard shark and manta ray.
Figure 4: Leopard shark with dart tag.
Figure 5: Taking a tissue sample from the tail of a wild leopard shark.
Figure 6: Ultrasonic data logger deployed off North Stradbroke Island.
Figure 7: Taking measurements of a leopard shark brought to the surface.
Results
- The size and structure of an aggregation that occurred at a rocky reef located approximately 500 m offshore from Point Lookout, North Stradbroke Island, was investigated over a three year period (2003-2006) using photo-identification and mark-recapture methods. In total 327 individual leopard sharks were identified from 570 photographs. Concurrent physical tagging showed that pigmentation patterns were unique and persistent in wild leopard sharks up to 4 years. Mark-recapture modelling resulted in an estimate of 458 individuals (95% confidence intervals = 298 – 618) visiting the aggregation annually. In total, 27% of the sharks were sighted in more than one summer aggregation period and the aggregation consisted exclusively of large (>1800 mm total length) adults with an overall female sex bias of 3.8:1, though sex-ratios varied temporally.
- Passive acoustic telemetry methods were used to investigate the seasonality and residency of the sharks and the environmental parameters associated with their presence at the aggregation in southern Queensland. Five acoustic receivers were deployed for a period of 21 months and recorded 10 acoustically-tagged leopard sharks. The sharks showed clear seasonality in their visitation patterns with greatest presence during the austral summer months of November through February and displayed diel periodicity with greater presence during daylight hours, indicating that they may be using the site as a resting location.
- Population genetic methods were employed to investigate whether leopard sharks from different IUCN classification regions comprised one or multiple populations. In total 180 leopard sharks were sampled from 13 locations thoughout their range and a leopard shark specific microsatellite library was developed. Microsatellite and mitochondrial DNA data sets concorded with the differing IUCN regional classifications however, greater subdivision was found within these regions, including subdivision between locations connected by habitat suitable for migration.
Conclusions
This project has demonstrated that leopard sharks form a large, predictable, shallow coastal aggregation comprised entirely of mature adults, as well as display high genetic subdivision in the Indo-West Pacific region with little to no recruitment into over-exploited populations from healthy populations. These findings indicate that if similar aggregations occur in regions of high fishing pressure then, without sufficient protection and management, localised depletion and extirpation will enventuate for leopard sharks populations in heavily fished areas.