The biology, ecology, distribution, abundance and identification of marine protected areas for the conservation of threatened Grey Nurse Sharks in south east Australian Waters
- The Grey Nurse Shark, Carcharias taurus belongs to the Family Odontaspididae and occurs in coastal waters off the NSW and southern Queensland and south-west WA. The shark is also found off South Africa and North America where it is respectively known as the "spotted ragged-tooth shark" and the "sand tiger shark". The sharks are often found in or near gutters and caverns around inshore rocky reefs and islands.
- The Grey Nurse Shark is a slow, strong swimming shark with a large body and attains a maximum length of 320 cm. Little is known about the biology and ecology of the species in Australian waters and what is understood has been inferred from studies in South Africa and North America.
- Male Grey Nurse Sharks mature at approximately 190 cm and females mature at approximately 220 cm. The Grey Nurse Shark exhibits an ovoviviparous reproductive strategy where there is no placental connection between the mother and embryo, instead, the two most advanced developmentally embryos eat the remaining embryos and then ovulated eggs (a phenomena known as intra-uterine cannibalism and oviphagy). The gestation period is approximately 9 to 12 months with two pups, occasionally one and rarely four born per litter. Females reproduce biennially and thus only one pup is born per female per year on average. Very little is known about the precise timing of mating and pupping activities and the migratory habits of the Grey Nurse Shark populations in Australian waters.
- The Grey Nurse Shark was afforded protected status in New South Wales waters in 1984 as a result of: (1) a reduction in numbers observed by recreational scuba divers, (2) declining catches be spear fishermen, (3) declining catches in the beach meshing programs, and (4) the realisation that the shark was not a "man-eater". Scuba diving and commercial/recreational fishing are the main human activities that occur in the habitats utilised by Grey Nurse Sharks. More recently, Grey Nurse Sharks have been inadvertently caught on demersal setlines.
- The current project was set up to: (1) quantify the distribution and abundance of Grey Nurse Sharks along the entire NSW coast, (2) identify map sites important to the shark in the Manning Bioregion, and (3) investigate the potential for declaring the important sites as Marine Protected Areas to facilitate the recovery and long-term conservation of the species.
- At the same time as the project commenced, the Grey Nurse Shark was declared a Threatened Species with "Vulnerable" status by the NSW Fisheries Scientific Committee under the Threatened Species provisions of the Fisheries Management Act 1994.
- The abundances of Grey Nurse Sharks in the survey were quantified using underwater visual counts of sharks over a 15 minute period at a range of sites along the NSW and southern Queensland coastlines. The surveys were carried out for a period of approximately 4 weeks and at each site divers recorded the number, sex and size of any Grey Nurse sharks present. They also recorded the presence of hooks, mating scars, etc. Volunteer recreational scuba divers along the NSW and southern Queensland coast also participated in these surveys.
- Numerous sites in the Tweed-Morton Shelf, Manning Shelf and Hawkesbury Shelf bioregions were mapped. In doing so, it became apparent that caves, sandy-bottomed and boulder-filled gutters and large overhangs were crucial habitats utilised by Grey Nurse Sharks.
- The 3 coastwide surveys were done in November/December 1998, March/April and June/July 1999. In survey one, 136 Grey Nurse Sharks were observed across 61 sites along the coast. A total of 106 (78% of all sightings) sharks was observed at 9 of the 61 sites (15% of all sites), and no sharks were sighted at 37 of the 61 sites (61% of all sites). Between 1 and 4 sharks were seen at the remaining 15 of the 61 sites (24% of all sites). Sixty of the 136 (44%) Grey Nurse Sharks observed in survey one were of a reproductively mature size. For individuals of known sex, only 12 males were of a reproductively mature size, and 34 females were of a reproductively mature size.
- In Survey two, 129 Grey Nurse Sharks were observed across 51 sites along the coast. A total of 114 (88% of all sightings) sharks was observed at 6 of the 51 sites (12% of all sites), and no sharks were sighted at 35 of the 51 sites (69% of all sites). Between 1 and 4 sharks were seen at the remaining 10 of the 51 sites (20% of all sites). Sixty-one of the 112 (54%) Grey Nurse Sharks observed in survey two were of a reproductively mature size. For individuals of known sex, only 14 males were of a reproductively mature size, and 42 females were of a reproductively mature size.
- In survey three, 207 Grey Nurse Sharks were observed across 50 sites along the coast. A total of 180 (67% of all sightings) sharks was observed at 13 of the 50 sites (26% of all sites), and no sharks were sighted at 25 of the 50 sites (50% of all sites). Between 1 and 4 sharks were seen at the remaining 12 of the 50 sites (24% of all sites). One hundred and twenty-seven out of the 204 (62.3%) Grey Nurse Sharks observed in survey 3 were of a reproductively mature size. For individuals of known sex, 63 (30.9%) males were of a reproductively mature size, and 42 (20.6%) females were of a reproductively mature size.
- The results of the surveys showed that the total numbers of sharks were very low suggesting that the Grey Nurse Shark population in NSW waters has not recovered since it was made a protected species in 1984. These results also support the initial declaration of the shark as a threatened species.
- Analysis of the size frequency distributions from surveys 1-3 showed that the Grey Nurse Shark population exhibited segregation by size and sex. Proportionally more male 1 - 2 m and >2 m TL Grey Nurse Sharks occurred at Foster and sites to the north and proportionally more females 1 - 2 m and >2 m TL occurred at Seal Rocks and sites to the south. The sex rations of Grey Nurse Sharks were significantly biased towards females in surveys 1 and 2. In contrast the sexes were not biased in survey 3. The biased sex ratios in surveys 1 and 2 is most likely due to segregation of the sexes rather than an actual difference in the abundance's of males and females.
- On subdividing the coastline into northern (i.e. Forster - N. Stradbroke Is.) and southern (i.e. Seal Rocks - Eden) sections, proportionally more males occurred in the northern section in surveys 1-3. In contrast proportionally more females were observed in the southern section in surveys 1 and 2, but in survey 3 there were more females in the north than the south.
- The size and sexual segregation of male and female Grey Nurse Sharks evident during the three surveys suggests a hypothesised pattern of movement comprising: (1) a movement of sexually mature males into shallower water in early autumn (April) presumably to mate. They then move northwards and appear at the northerly most sites in southern Queensland in July/August; (2) the movement of sexually mature females and immature sharks of both sexes to the south in spring and early summer followed by a return to sites north of Forster in the autumn and winter months.
- The number of pups observed (i.e. 6 - 14) was less than expected (i.e. 34 - 42) based on the numbers of reproductively mature females. This is cause for concern because: (1) it suggests that the pups were not observed using the existing sampling techniques, or (2) it is possible that a reproductive failure may have occurred giving an average fecundity of less than 1 pup per annum: a rate that is clearly insufficient to sustain a population yet alone enable it to recover.
- Information from surveys in 1991, 1995 and this study has shown that there has been a significant increase in the rate of incidental capture of Grey Nurse Sharks on bottom setlines.
- As a result of the entire study nine recommendations were made and are listed below:
- That the status of the Grey Nurse Sharks be reviewed with a view to upgrading the status from VULNERABLE to ENDANGERED.
- That Green Island, Fish Rock, Cod Grounds, The Pinnacles, Big Seal Rock, Little Seal Rock and Broughton Island be considered for declaration as aquatic reserves to assist in the long-term conservation of the Grey Nurse Shark.
- That further surveys at the various sites along the coast be done to document the short-term spatial and temporal fluctuations and inter-annual variability in abundance.
- That the location, timing and number of pups born be quantified to estimate the average rate of recruitment.
- That estimates of the number of sexually mature females be further quantified and the location of mating sites be identified.
- That the timing and direction of migratory movements of Grey Nurse Sharks be quantified using tagging techniques.
- That the proportions of Grey Nurse Sharks repeatedly observed in the surveys be quantified using tagging techniques as this will provide an independent assessment of the population status and an estimate of the total population.
- That estimates of the rates of inadvertent capture on bottom setlines be further quantified.
- That acoustic tagging techniques be used to document the localised, short-term movements as these will determine the efficable size of Marine Protected Areas.
The project has 6 main objectives and these are listed below.
- To review all available information on the biology and ecology of the Grey Nurse Shark, the habitats utilised by the shark, the human uses of the main habitats and to highlight those uses that are contrary to the long-term conservation of the species.
- To repeat the distribution and abundance survey of Grey Nurse Sharks at Seal Rocks and extend it to South West Rocks. To analyse statistically the results of this and previous surveys and evaluate the design and efficacy of the survey.
- To assess the feasibility of using Marine Protected Areas to assist in conserving "migratory" species such as Grey Nurse Shark, and
- To map the habitats utilised by the Grey Nurse Shark in the region surrounding Seal Rocks and South West Rocks, and identify sites suitable for declaration as Marine Protected Areas.
The scope of the project was expanded after additional funds were provided by Environment Australia to allow sampling of the entire NSW coast. The objectives of the expanded project are listed below.
- To provide more detailed information about the distribution and abundance of Grey Nurse Shark on the east coast of Australia by surveying recreational dive sites where Grey Nurse Sharks had previously been sighted, from Stradbroke Island in southern Queensland to Eden in southern NSW, and
- To undertake additional mapping of the habitats utilised by Grey Nurse Sharks to encompass the sites sampled in the Tweed Moreton Shelf and Hawkesbury Shelf Bioregions.
The report presents the findings associated with each of the main objectives. Chapter Two documents a review of the available literature on the biology and ecology of the Grey Nurse Shark, including the habitats utilised by the shark and the main human uses of those habitats; Chapter Three presents maps of the sites surveyed displaying the habitats utilised in the Tweed Morton Shelf, Manning Shelf and Hawkesbury Shelf Bioregions; Chapter Four presents the results and analysis of the data collected from the distribution and abundance surveys conducted at Seal Rocks in 1991, 1995 and 1998 as well as the data collected for the statewide surveys conducted on November/December 1998, March/April 1999, and May/June 1999; Chapter Five discusses the feasibility of using Marine Protected Areas to assist in the conservation of the Grey Nurse Shark; and Chapter Six presents the recommendations and final conclusions of the project.