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
N. M. Otway, M. T. Storrie, B. M. Louden and J. J. Gilligan, 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 (PDF - 151 KB) | (RTF - 4,728 KB)
About the report
In the past, the grey nurse shark, Carcharias taurus Rafinesque, 1810, had a broad inshore distribution, primarily in sub-tropical to cool temperate waters on continental shelves off the coasts of eastern USA (Springer 1948, Bigelow & Schroeder 1948), Brazil (Sadowsky 1970, Sadowsky et al. 1989, Amorin et al. 1998), Uruguay (Marin et al. 1998), Argentina (Chiaramonte 1998), in the Mediterranean Sea (Compagno 2002), around the Canary Islands and to western equatorial South Africa (Cadenat 1956), in the Red Sea (Compagno 2002), off the east coast of South Africa (Bass et al. 1975, Compagno 2002), off the east to west coasts of Australia (Whitley 1940, Last and Stevens 1994), and in Japanese waters (Taniuchi 1970). As a result of overfishing, it is doubtful that sustainable populations of grey nurse sharks still exist in the Mediterranean Sea, around the Canary Islands and to western equatorial South Africa, the Red Sea and in Japanese waters. Consequently, the shark’s distribution is now likely restricted to the east coasts of North and South America (i.e. USA, Brazil, Uruguay and Argentina), the east coast of South Africa and, the east and west coasts of Australia.
In Australia, C. taurus has, in the past, been recorded at various sites from Cairns on the Queensland coast around the southern half of the continent and northwards to Shark Bay in Western Australia (Last and Stevens 1994). The species has not been found in Tasmanian waters, but has been caught in the Arafura Sea on a few occasions (Read and Ward 1986). Over the past few decades, C. taurus has been restricted to two, separate and genetically distinct populations (Cavanagh et al. 2003; Stow et al. 2006) on the east and west coasts of Australia. The east coast population extends from Double Island Point (25º 55' S, 153° 12' E) in southern Queensland to Narooma (36º 15' S, 150º 12' E) in southern New South Wales (Cavanagh et al. 2003, Otway et al. 2003). The range of the west coast population is less well known, but the by-catch of commercial shark fisheries indicate that the species occupies sites from North West Cape (21º 46' S, 114º 09' E), around the south-western coastline and then east to the coastal waters south of Cocklebiddy (32º 15' S, 126º 12' E) in the Western Australian section of the Great Australian Bight (McAuley et al. 2002, Cavanagh et al. 2003).
The shark is a slow, strong-swimming species that is often seen hovering motionless near the bottom in or near sandy-bottomed or cobble-filled gutters, overhangs or in rocky caves around inshore rocky reefs and islands at depths between 15 and 40 m, but may extend to 191 m (Pollard et al. 1996, Compagno 2002). Grey nurse sharks aggregate at these sites and feed on a wide range of bony fish, squid and occasionally on crustaceans (Bass et al. 1975, Compagno 2002, Schmid et al. 1990, Smale 2005).
The grey nurse shark population off eastern Australia has had a very chequered history (Otway 2001, 2004). In the late 1800s and early 1900s, they were harvested for their flesh for human consumption, liver oil for powering Sydney’s streetlamps, and skin for leather and sandpaper (Whitley 1940, Roughley 1955). Grey nurse sharks were initially caught in large numbers in the NSW shark meshing program, but catches declined rapidly (Reid & Krogh 1992, Krogh 1994). They were also targeted by gamefishers from the 1930s until the late 1970s (Pepperell 1992). In the 1950s and 60s, they were wrongfully accused of the shark attacks off Sydney’s beaches and again hunted (Stead 1963, Otway 2001). With the advent of the powerhead, numerous grey nurse sharks were killed by spearfishers in the 1960s and ‘70s (Cropp 1964, Byron 2000). Combined, these activities caused a substantial reduction in the grey nurse shark population. After much lobbying by various groups, the shark was declared a protected species in 1984 by the New South Wales government. With their numbers continuing to decline via inadvertent capture by commercial and recreational fishers, the grey nurse shark was declared a threatened species in 1999 and is now listed as “critically endangered” on the east coast of Australia under the New South Wales Fisheries Management Act (1994), the Queensland Nature Conservation Act (1992), the Environment Protection and Biodiversity Conservation Act (2001) and by the IUCN (Cavanagh et al. 2003). In 2002, 10 aggregation sites in NSW waters were declared as critical habitat sites. Many of these sites have subsequently been incorporated within sanctuary zones within the multiple use marine parks declared at various locations along the NSW coast (Breen et al. 2003, Breen et al. 2004, Breen et al. 2005ab).
Like many elasmobranchs, the grey nurse shark exhibits late onset of sexual maturity (6 - 7 yr in males, 9 - 10 yr in females in the USA - Goldman et al. 2006), low fecundity with 2 pups born biennially following intrauterine cannibalistic and oophagous phases (Springer 1948, Gilmore et al. 1983) and longevity of about 35 years. Their habitat requirements and biology have made grey nurse shark populations extremely vulnerable to over-exploitation and they require decades to recover from anthropogenic reductions in their numbers (Smith et al. 1998, Otway et al. 2004).