The ecology of the leopard shark Stegostoma fasciatum, with emphasis on a seasonal aggregation in southeast Queensland.

A PhD project undertaken by Christine Dudgeon at the University of Queensland, Brisbane, and supervised by Janet Lanyon

Leopard sharks aggregate annually in the coastal waters of southeast Queensland over the summer months. Leopard shark numbers have been depleted globally due to fishing pressure, except in Australian waters where they have commercial importance for display in aquaria and for the recreational diving industry. However, very little is known about the ecology and biology of this species in the wild.

This project employs a combination of visual tagging, ultrasonic telemetry and genetics to investigate the ecology of the southeast Queensland aggregation of leopard sharks. The primary study site is in the waters just off shore of Point Lookout, North Stradbroke Island in southeast Queensland. The duration of this project is from April 2003 to October 2006.

The specific aims of the study are:

1. To quantify the abundance, sex ratio and size distribution of leopard sharks in southeast Queensland.
   Social organisations such as aggregations are complex and poorly understood in sharks. Various shark species are known to aggregate in different habitats for a range of purposes such as mating, laying eggs or birthing, foraging or resting. Furthermore, habitat partitioning due to size and sex has also been observed in sharks (Hoenig and Gruber 1990). This component of the project will investigate the abundance and demography of the leopard sharks visiting southeast Queensland over the summer months using mark-recapture methods.
2. To assess the seasonality and site-fidelity of leopard sharks in southeast Queensland.
   Anecdotal reports suggest that leopard sharks aggregate in southeast Queensland during the warmer months of November through to April and then head elsewhere for the colder half of the year. This component of the project will use ulstrasonic telemetry to investigate whether the aggregations are truly seasonal by assessing when the leopard sharks begin and end the seasonal aggregation. We will also investigate the site fidelity by quantifying the frequency of visitation by leopard sharks to the major aggregation sites, and to others within the region.
3. To investigate the space-use and foraging ecology of leopard sharks in southeast Queensland.
   Energetic requirements and relationships of marine apex predators such as sharks are little understood but are fundamental to determining the role they play in the ecosystem (Hoenig and Gruber 1990; Wetherbee et al 1990). Leopard sharks have been recorded as feeding on a range of prey items of all trophic levels including crustaceans, molluscs and teleosts (Last and Stevens 1994), indicating an opportunistic feeding regime. To better understand the energetic requirements of the leopard shark and its trophic position in the local region, we will investigate its space use and activity budget during foraging.
4. To investigate the genetic relatedness of leopard sharks within the southeast Queensland aggregation.
   Understanding the relatedness amongst individuals within a population can help elucidate reproductive patterns and life history characteristics of the population. Microsatellite markers that convey both maternal and paternal hereditary information will be used to address questions of paternity and relatedness amongst individuals.
5. To investigate the broadscale distribution of leopard sharks within Australasian waters.
   The potential impact on leopard shark populations in Australian waters from heavy fishing pressure overseas is unknown. This component of the study will firstly identify leopard shark localities throughout these waters and use population genetic analysis to investigate the connectivity of leopard sharks from these different localities.

Preliminary results and progress

• A mark-recapture pilot study was conducted during November 2003 and February 2004 during which 119 individual leopard sharks were photographed and and 49 of these sharks had dart tags inserted
• Preliminary mark-recapture estimates suggest at least 500 individuals in the population.
• There is a strong female bias in the population with a female to male ratio of 3.8 : 1.
• All sharks had total lengths ranging between 1.8 to 2.4m, and this suggests the population is completely comprised by reproductively mature adults.
• Five ultrasonic data loggers were deployed at the primary study site during April 2004 and May 2004, and five leopard sharks had ultrasonic pinger tags attached in early April 2004.
• Two of the five sharks showed repeated visitation to the study site over the two-month period.
• Tissue samples have been collected from 49 adult leopard sharks from the primary study site and another 16 samples been collected from the Cairns region.
• Four leopard shark specific polymorphic microsatellite loci have been identified so far.

Reference

Dudgeon, C L, M J Noad and J M Lanyon (2008). Abundance and demography of a seasonal aggregation of zebra sharks Stegostoma fasciatum. Mar Ecol Prog Ser; 368: 269 - 281.

Figures

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.