Documentation of depth-related migratory movements, localised movements at critical habitat sites and the effects of scuba diving for the east coast grey nurse shark population.
Otway NM, Storrie MT, Louden BM and Gilligan JJ (2009) Documentation of depth-related migratory movements, localised movements at critical habitat sites and the effects of scuba diving for the east coast grey nurse shark population. Report to Department of Environment, Water, Heritage & the Arts. Industry & Investment NSW - Fisheries Final Report Series 112. Port Stephens, NSW, Australia. ISSN 1837-2112. 90pp.
|Documentation of depth-related migratory movements, localised movements at critical habitat sites and the effects of scuba diving for the east coast grey nurse shark population.
This project analysed data obtained from research carried out over six years. The study involved using the South-East Australian Coastal Acoustic Monitoring System (SEACAMS), active acoustic tracking studies and tagging studies with acoustic and pop-up archival satellite transmitters to document the localised movements of grey nurse sharks at NSW critical habitat sites and their migratory movements along the NSW coast, and to assess the impacts of scuba diving on the species.
Localised movements were documented by using dive boats, equipped with acoustic receivers, to actively track 9 grey nurse sharks tagged with continuously transmitting acoustic tags. The resulting information was then augmented with data from the SEACAMS acoustic listening stations. Analyses of the nine tracking studies showed that the tagged sharks spent almost 94% of their time within 500 metres of the critical habitat site and predominantly within 100 metres. The excursions away comprised only 6% of their time spent at a critical habitat site and occurred during the day and night. Data from 30 grey nurse sharks tagged with acoustic tags showed that the timing and duration of occupation of critical habitats by grey nurse sharks varied throughout the year. The timing of site occupation was correlated with water temperature, the sexual maturity of the shark and, in females, the stage of the reproductive cycle. Grey nurse sharks occupied critical habitat sites for varying lengths of time ranging from less than a day to in excess of six months, but averaged around 11 days.
Analyses of the data from 30 sharks tagged with acoustic transmitters and their associated detections on the SEACAMS acoustic listening stations, combined with the data from 15 sharks tagged with pop-up archival satellite tags identified the timing and duration of migratory movements; the migratory routes taken; the duration of occupation of sites in shallow and deep waters along the migratory routes and the duration of occupation of waters with particular depths and temperatures. Tagged, sexually-mature, male grey nurse sharks migrated north over the autumn and winter months reaching sites in the Capricorn Channel off southern Queensland. This was followed by a southerly migration during spring and summer months during which they swam to sites in southern NSW waters including the Tollgate Islands and Wasp Island. Sexually-immature tagged grey nurse sharks migrated from southern NSW in autumn and winter months, occupying sites off the mid-north coast of NSW and did not participate in the lengthy migration north to southern Queensland waters. These migratory patterns are very similar to the results of previous studies by NSW DPI and CSIRO. During their migratory movements, the tagged sharks spent, on average, 74% of their time in waters not exceeding 40 metres and 95% of their time in waters less than 80 metres in depth. The tagged sharks also spent 95% of their time, on average, in waters with temperatures ranging from 17 to 24°C.
The effects of scuba diving on the localised movements and behaviour of grey nurse sharks was assessed using three field experiments across 15 sites from Julian Rocks (off Byron Bay) to the Tollgate Islands (off Batemans Bay). The sites used reflected the annual variation in abundance and distribution of the shark along the NSW coast. The data used in these experiments were obtained from the 30 grey nurse sharks tagged with acoustic tags and the SEACAMS network. Importantly, the acoustic tags used transmit at 69 kHz which is well outside the range of frequencies that can be heard by sharks. The numbers of detections received per hour by the listening stations were sorted according to tag number and then used to document the movements of the grey nurse sharks in the gutters at the various sites and at times when scuba divers were present and absent. To ensure that the behaviour of the scuba divers was not modified, no information concerning the aims, methods, sites used and/or timing of the experiments was provided to divers or the owner/operators of dive shops before or during these experiments.
None of the tagged grey nurse sharks left any of the sites in response to scuba diving and all remained for the duration of each experiment. Detailed statistical analyses of the acoustic detections showed that there were no effects of scuba diving on the movements or behaviour of the acoustically-tagged grey nurse sharks in the gutters across the 15 sites from Julian Rocks to the Tollgate Islands in each of the 3 experiments. Preliminary analyses of additional data from several sites indicate that there are unlikely to be any cumulative effects of more intense scuba diving on grey nurse sharks.
The results of this study have clear management implications for the recovery and long-term conservation of the grey nurse shark along the east coast of Australia. The localised movements of grey nurse sharks were restricted spatially and mostly occurred within the 200 metre wide critical habitats declared at key aggregation sites. Grey nurse sharks exhibited extensive migratory movements occurring predominantly along inshore coastal waters less than 80 metres in depth. The timing of these movements appears predictable and should be considered if any additional measures are taken to mitigate fishing-related interactions in the future. Scuba diving has negligible effects on the shark and this may be due, in part, to the observance of the grey nurse shark scuba diving code of conduct developed in cooperation with recreational divers.