There is very little known about the biology of wobbegong sharks, their importance to reef communities or the impacts of fishing pressure and other anthropogenic factors. A postgraduate study was recently completed in western Australia, incidental records of wobbegongs have been made in beach netting studies and other work and breeding programs have been undertaken at several commercial aquariums in Australia. However, there is limited published work available on wobbegongs.

Preliminary records made during 1998 illustrated some interesting trends in the population of wobbegong sharks on the north coast of New South Wales. These trends included:

• Distinct size partitioning of the two species recorded in Byron Bay (Orectolobus ornatus being predominantly small (in the 50cm to 1m size range) and Orectolobus maculatus generally falling into a 1-1.5m size range (also noteworthy, ornatus of intermediate length were not recorded and smaller maculatus were rare)),
• A significantly higher proportion of females than males were recorded for both species,
• An apparent depth segregation was observed between species, (maculatus occurring in deeper offshore sites more frequently),
• Larger ornatus were only recorded in offshore sites (smaller individuals occured in the same areas), &
• An apparent micro-habitat segregation (ornatus occuring predominantly on algal or coral covered reef while maculaus occured on sand).

Aim

To examine the population structure and dynamics of Orectolobidae shark populations in NE New South Wales and identify directions for future research.

Objectives

Recreational diver based surveys have been used with some success by New South Wales Fisheries in studies of the Grey Nurse Shark and for incidental records of other threatened species. More formal methodologies for diver based surveys of fish populations are well documented along with approaches to training survey participants (English et al (Ed) 1997). This non destructive sampling technique appears to be particularly suited to sedentary demersal species like wobbegongs. Wobbegongs are relatively placid and can be approached at close quaters or even handled. The sharks rarely retreat from divers and can be observed or measured with relative ease.

Cost and return for effort are important considerations when undertaking a study of this kind. To provide a large, continuous database of records with a limited budget an innovative approach was required. The use of recreational divers is cost effective, but unfortunately difficulty in differentiating species and accurately estimating length of individuals dictates the use of trained recorders. For this study the only way to obtain reliable data cost effectively was to conduct surveys while leading recreational dives as a Dive Master or dive leader.

To facilitate rapid data collection simple forms were developed with most variables being recorded as category data (eliminating the need for time consuming measurements and additional equipment). Once the surveyor has familiarised themself with the characteristics of the different species of sharks surveyed, how to determine gender and developed skills in estimating length, the only equipment required is a slate and survey form, SCUBA depth guage and digital temperature guage. Initially lengths were measured with the aid of a 1.5 metre graduated fibreglass measuring rod until they could be reliably estimated. An example of the survey forms used is shown below.

Figure 1: Wobbegong shark survey forms, all fields in a record to be completed each time a shark is observed during a survey dive. Forms are printed on waterproof paper and attached to a slate.

Field work for Bakers project during 1998 provided an opportunity to determine if trends in data collected using the above methodology were supported by a more formalised approach. The project utilised transects (timed swims over set compass bearings) to detect and record data on wobbegongs. Graduated measuring sticks and tape measures were used to accurately record overall length of sharks. Similar gender distributions were found using this technique. However, trends in size distribution with depth were not supported. A likely explanation for this discrepancy may be a difference in sampling effort between inshore and offshore sites. Most offshore sites were surveyed while leading recreational dives where the survey was not the main objective and survey effort less intense. Larger sharks (predominantly maculatus) were more likely to be detected than the smaller, more cryptic, ornatus. At inshore sites more time and effort could be directed at finding sharks hence a higher proportion of ornatus in the sample results.

Study variables which are not considered to be affected by this sample bias include; gender distribution of recorded individuals, micro-habitat preference of sampled sharks and depth distribution of sharks in a given size range. Variables which may be affected by sample bias (and which may require further investigations applying more intense survey techniques) include; overall size distribution within a species (and any changes over time), relative numbers of each species or size of shark and differences in depth preference between species. Records of this kind should, however help to highlight directions for future research.


The story so far!!

Surveys have been undertaken, primarily at Julian Rocks Marine Reserve (Byron Bay, Australia), since May 1998 to the present day. Occasional surveys are carried out at inshore ("Tassie II" and "Wollongbar" ship wrecks, Broken Head and Middle Reef) and offshore (Spot X and the Mackerel bowl) sites in Byron Bay as well as North Solitary Island (Coffs Harbour) and Fish Rock (South West Rocks), as conditions permit. Some gaps exist in the Julian Rocks database due to illness or long periods of inclement weather (also affecting sampling effort for some months, see graph below).

Unfortunately only 1998 data has been compiled to date (too busy, hope to rectify soon), making any detailed analysis impossible. In this section I have tried to briefly highlight some points of interest from the data sofar and where the study is going.

Gender ratio over time

The following graph illustrates changes in gender ratio over a seven month period in 1998 (including preliminary surveys). The predominance of females in records is clearly demonstrated. Overall there is a significant difference between the observed ratio of males to females and an expected ratio at birth of 1:1 (P=1E-16). This does not appear to change between different sites or depths (but further work is required). A number of factors could be responsible for this trend including sexual segregation (gender preference for water temperature, depth or other factors) or differences in the survival rate of males and females. In the 1998 data there is a noteworthy increase in the proportion of males sampled for October. It is hoped that sampling in 1999 and 2000 will allow any trends of this kind to be clearly identified and related back to environmental factors (such as water temperature or time of year). There are limited records of copulation but the occurances will be examined in relation to gender ratio over time.

Figure 2: Showing changes in gender ratio and sampling effort at Julian Rocks Marine Reserve for shark surveys carried out in 1998.

Sexual segregation in some shark species occurs as a result of different temperature preferences between the sexes. Temperature data has been available since the start of 1999 for each survey dive. A plot of monthly temperature for 1999 is shown in Figure 3 (note that particularly high water temperatures were recorded in early 1999 accompanied by coral bleaching at Julian Rocks). Gender ration will be examined in relation to average monthly temperature to identify any apparent trends.

Figure 3: Changes in average minimum water temperature at Julian Rocks Marine Reserve during 1999. Temperature data is downloaded to PC from a dive computer which acts as a data logger.

There is a significant difference (P=2.6176E-22) in size distribution between ornatus and maculatus with ornatus predominantly in the 50-100cm size range and maculatus in the 100-150cm range. Neither species is recorded in great numbers outside these size ranges in any of the sites surveyed.

It is difficult to speculate on a reasons for this size segregation. Depth or geographic segregation of different size classes may be one explanation (there is limited data from other locations and deeper sites, anecdotal evidence suggests similar size ratios). Another possible explanation may be population recover after heavy fishing pressure, although this seems unlikely.

Habitat preference

Orectolobus ornatus are generally observed on algae or coral covered rocky substrait while maculatus are more commonly found on sand in off shore sites. It is believed that colour plays a part in habitat choice (the more darkly patterned ornatus prefering rocky, algal covered habitat). No statistical analysis has been performed on the microhabitat data collected to date. These data will be examined to determine if there is an association between microhabitat (rocky reef or sand, primarily) and species of wobbegong recorded.

Use of cover (in the form of overhangs and holes) will also be compared between the two species. This may be a function of size and suitability of camouflage to a substratum.

Directions for further research

Several directions for further postgraduate and undergraduate research have been identified to date. These include:

• An age growth study of both wobbegong species, including size/age at maturity. Would require the development of an appropriate methodology for aging sharks and determining maturity. A collection of sharks of various sizes would be required for dissection (a potential Integrated Coastal Project once sufficient sharks are collected).
• A tagging study to examine local and regional movements of individual sharks. A more long term study which may follow on from the current work.
• A threat assessment of the banded and spotted wobbegong in north east New South Wales. I have listed this as a potential Integrated Coastal Project topic for spring semester 2001.