Spatial dynamics and habitat preferences of juvenile white sharks : identifying critical habitat and options for monitoring recruitment : final report June 2008

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Juvenile white sharks (1.8-2.6 m total length) in eastern Australia show broad-scale patterns of movement ranging from southern Queensland to north eastern Tasmania and across the Tasman Sea to New Zealand. These patterns are the most extensive documented for white sharks of this size anywhere in the world and extend our knowledge of the habitat niche for juvenile white sharks to include the open ocean. Juvenile white sharks showed two main behavioural modes, temporary residency and travelling, patterns similarly described for sub-adult and adult white sharks. Residency sites were clustered into three primary residency regions in eastern Australia (Fraser Island [Qld], Stockton Beach-Hawks Nest [NSW] and Corner Inlet-Lakes Entrance [Vic]) and a single primary residency region east of New Zealand on the northern boundary of the Chatham Rise. Residency regions were areas known for the abundance of suitable prey, however other seemingly similar areas exist along the eastern Australian coast that were not used as residency sites by sharks tagged as part of this study.

Resident juvenile white sharks largely restricted their movements to between the shore and the 100-120 m depth contour. The depth preferences shown by juvenile white sharks while resident and travelling were highly bimodal with time spent primarily at the surface (0-5 m) and in the 60-100 m zones. When seaward of the continental shelf, juvenile white sharks showed bimodal depth preferences of the surface and depth zones ranging between 300 and 1000 m, depending on the individual shark. The 984 m depth recorded by one juvenile shark is the deepest recorded dive for a white shark in Australian waters and one of the deepest dives recorded for this species anywhere in the world.

Juvenile white sharks experienced temperatures ranging from 6-8 °C to 24-26 °C. The most common temperature range occupied (over 45% of their time) was 18-20 °C. However, tracking during this study was conducted primarily over the spring - summer period, the timing being determined by the availability of sharks for tagging at Stockton Beach. Thus, these temperature data may be biased by the time of year the study was undertaken. Tracking during autumn and winter is required to fully elucidate the influence of temperature on the spatial dynamics and distribution of juvenile white sharks in Australian waters. This will require future tagging and tracking in southern Australian waters (eg Victoria or South Australia) commencing during the summer-autumn period.

Juvenile white sharks occur in other Australian states, suggesting that important juvenile habitats may exist in regions other than east coast waters. Such reports include areas of Western Australia, South Australia and western Victoria. The linkages between these various regions and the eastern Australian sites are unclear. Similarly, it is unclear if the juvenile sharks present in these areas are sourced from a common or from different pupping grounds. Identification and investigation of these other juvenile residency regions and the linkages between them is a critical requirement for further research.

No young-of-the-year white sharks were located at tagging sites during the study. This combined with the obvious temporary residency and highly mobile nature of juveniles provides no further indication of the location of pupping by white sharks in Australian waters. The location of pupping grounds and habitat requirements for young-of-the-year white sharks requires further targeted research in Australian waters.

The identification of a limited number of primary residency regions in eastern Australia and the common usage of these regions by tagged sharks suggests that monitoring even one of these sites may offer insight into juvenile abundance and recruitment levels over time. Vessel-based and aerial surveys offer promise for developing an abundance index for juvenile white sharks provided they are seasonally targeted at primary residency regions. Stockton Beach may be an ideal location for such monitoring. However, tagged sharks showed a high degree of site specificity to particular areas of beaches in the region and did not venture to other beaches in the region where juvenile white sharks were reported to occur. This site specificity and the linkages between beaches across the Port Stephens region requires further evaluation to ensure that monitoring of the Stockton Beach site will provide a representative sample of abundance in this overall region.

The range of sizes observed, and site specificity shown, suggests that individual sharks may return to Stockton Beach in subsequent years until they reach sizes that prompt a change in behaviour and occupancy of juvenile habitat. Further monitoring at this site in particular is required to determine the number of years over which individuals do so in order to evaluate the true significance of such residency regions. In addition the return of tagged sharks to Stockton Beach would offer an unparalleled opportunity to retrieve SPLASH tags and download their entire archived data set.

Although sub-adult and adult white sharks have been recorded undertaking open-ocean forays and cross-ocean basin excursions, juvenile white sharks have previously been considered to be a coastal life history stage. Our data now confirms that white sharks as small as 1.9-2.1 m can similarly make both offshore forays as well as cross-ocean basin excursions and, in doing so, dive to depths close to 1000 m. These behaviours may expose juvenile white sharks to incidental capture in pelagic fisheries targeting squid, tuna and swordfish where, due to their size, they may be mistaken for other species such as mako sharks (Isurus oxyrinchus).