Carcharias taurus is listed as Critically Endangered along the east coast of Australia and there is concern about their status globally. The use of traditional tag–recapture methods to monitor the east coast C. taurus population have been discontinued due to tag–biofouling and injuries that relate to tag attachment. In the current study, captive and wild C. taurus were used to assess whether spots present on their flanks were suitable natural tags for individual shark recognition. Photographic images of seven captive sharks taken at monthly intervals for 13 months and at three years after the start of the study indicated that their spot number, position and relative size did not change over this period. Similarly, eighty–nine wild sharks photographically re–identified at least 23 months after their initial identification (and in one case after 14 years) confirmed long–term spot–pattern retention. Photographic recaptures of individual C. taurus provided information about their temporal and spatial distribution and movement patterns along the Australian east coast in relation to maturity, sex and reproductive condition. A total of 930 sharks were photo–identified between 2004 and 2008 at 23 aggregation sites between Wolf Rock and Montague Island. Of these, 479 were females (271 mature, 208 immature) and 452 were males (288 mature, 60 sub–adults, 104 juveniles). The distribution of pregnant C. taurus was seasonally and temporally distinct from all other sharks. Visibly pregnant C. taurus were recorded at Wolf Rock (the most northern site) from February until October, although many sharks left during July. Pregnant C. taurus were also observed at North Moreton Island, Flat Rock and Fish Rock between June and November. Resting (mature, non–gravid) females and mature males were mostly observed at mid–southern sites from December to June, with northern counts increasing from June to November. The majority of immature sharks were recorded at mid–southern sites. Of 930 sharks identified between 2004 and 2008, 149 were identified at more than one site. On average, mature females moved 338 km (SD ± 465), mature males 340 km (SD ± 299), immature females 147 km (SD ± 98), sub–adult males 185 km (SD ± 216), and juvenile males 271 km (SD ± 237). The maximum rate of movement per day was 18.5 km for a mature female shark, 20.7 km for a mature male, 4.3 km for an immature female, 86 km for a sub–adult male and 4 km for a juvenile male. Mature males and mature non–pregnant females tended to move north from mid–winter and mate in late spring/early summer in warmer waters. From about mid–winter, pregnant females began to move from the warmer waters of their gestation areas to cooler southern waters to pup (probably from late spring to mid–summer). The movement patterns of immature sharks varied temporally, and were more limited spatially. Underwater censuses, photo–identification and acoustic tracking of individual C. taurus were used to investigate their reproductive periodicity, localised movements and behavioural segregation at Wolf Rock – the most northerly aggregation site on the east Australian seaboard. A biennial reproductive cycle was indicated for 18 of 28 females for which re–identifications spanned at least two mating and/or pregnancy events. Nine of the 28 sharks appeared to exhibit a triennial reproductive cycle. Male C. taurus were observed between July and January, but were absent between February and April. Scuba divers reported seeing some mating scars on individuals from mid–October, however fresh mating scars were predominately observed on photographs of individual C. taurus taken in November and December. Four acoustically–tagged mature female sharks remained within 500 m of the Wolf Rock aggregation site within a marine sanctuary zone for 78 – 90 % of the 11–15 day study period. A minimum population estimate of 930 individuals is provided from all sharks identified between 2004 and 2008. In addition, a Jolly–Seber (open model design) mark–recapture analysis on data obtained during four scheduled photo–identification surveys (conducted between July 2006 and February 2008 at 25 aggregation sites along the east coast of Australia) was used to estimate the size of this population. A maximum of 272 sharks (143 females and 129 males) were identified during any scheduled survey period. Model averaging across the highest rated JS models (Popan data formulation) resulted in an estimate of 756 males (95% CI = 590 – 922) and 1185 females (95% CI = 901 – 1469). The mark–recapture abundance estimate is considered preliminary and requires further model development to incorporate the heterogeneity in distribution and migration patterns within the C. taurus population. The occurrence of retained fishing gear and gear–related jaw injuries were quantified from the four scheduled photo–identification surveys along the east coast of Australia. A total of 673 sharks were identified with 119 occurrences of retained fishing gear or jaw injury recorded from 113 individual sharks. For sharks that were known by spot–patterns on both flanks, 29 % of females and 52 % of males were seen with retained fishing gear or a gear–related jaw injury. The largest numbers of identified sharks (222) during the surveys were seen at Fish Rock (off the New South Wales coast): 48 % of all sharks identified with retained fishing gear were first identified at this site. Fish Rock is a designated critical habitat for C. taurus, but most forms of line fishing, except fishing while anchored or moored with bait and/or wire trace line are permitted. Results from the surveys clearly demonstrated that C. taurus is susceptible to a large variety of fishing gear and fishing methods. Current protection measures for C. taurus appear insufficient at this site, particularly as large aggregations that include immature and mature sharks occur consistently throughout the year.
| Identifer | oai:union.ndltd.org:ADTP/279133 |
| Creators | Carley Bansemer |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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