Distribution, reproduction and population structure of three gulper sharks (Centrophorus spp., Centrophoridae) in south-east Australian waters
Graham, K.J. and Daley, R.K., 2011. Distribution, reproduction and population structure of three gulper sharks (Centrophorus spp., Centrophoridae) in south-east Australian waters. Marine & Freshwater Research, 62: 583–595.
Gulper sharks are mainly bottom-dwelling dogfishes that grow to between 1.0 and 1.6 m in length. Five species occur in deepwater off NSW and are caught mainly by trawlers and occasionally by dropline. They are commercially valuable but, because of depleted numbers and trip limits, NSW landings are now less than 10 t annually. Three species (Harrisson’s, Endeavour and Southern Dogfishes) were caught in relatively large quantities during the early years (1975-85) of the upper slope trawl fishery but surveys by the NSW Fisheries’ research vessel Kapala in the 1990s showed that, after 20 years of trawling, their numbers had been reduced to less than 1% of their initial levels. In 2003, the three species were red-listed by the IUCN with Harrisson’s Dogfish and Southern dogfish assessed as ‘critically endangered’; currently, they are being considered for listing as ‘threatened’ under the Commonwealth Government’s Environmental Protection and Conservation Act 1999. This publication brings together distributional, catch composition and biological data collected mainly during Kapala trawl surveys off NSW between 1976 and 1997, supplemented by data recorded during an auto-longline survey for gulper sharks off NSW and eastern Bass Strait in 2009. The results clearly show the reasons that make gulper sharks particularly vulnerable to fishing pressure. Like most deepwater sharks, gulpers are slow growing and long lived, and the three study species all matured at sizes greater than 80% of their maximum observed lengths, probably around 20 years of age. They have breeding cycles of at least two years, with Harrisson’s and Endeavour Dogfishes producing a maximum of two pups (and frequently only a single pup) and the Southern Dogfish giving birth to only a single pup. The young of all three species are born at sizes between 35 and 45 cm total length, making them vulnerable from birth to trawls and hooks. The early Kapala surveys (1976-77) caught Harrisson’s and Endeavour Dogfishes along the entire NSW coast, and Southern Dogfish as far north as Crowdy Head; these catches from lightly-fished stocks comprised mainly mature males with relatively few mature females or small juveniles. So few were caught 20 years later that no size comparisons could be made, and the southern distribution of the Endeavour Dogfish had contracted to north of Jervis Bay. The more recent longline survey confirmed that there were very few gulper sharks on the regularly fished trawl-grounds between Broken Bay and Bass Strait, but did take moderate catches of Endeavour and Harrisson’s Dogfishes off northern NSW, and on (now) lightly trawled grounds off Port Stephens. Several adult Harrisson’s Dogfish were also caught at the Taupo Seamount, 230 nautical miles off central NSW, and although Southern Dogfish were only caught south of Newcastle (suggesting a southward contraction of their range), relatively high numbers were taken in an area off Sydney recently closed to trawling by AFMA to protect gulper sharks. In summary, the biological observations confirmed that gulper sharks are possibly the least productive of any shark, making them extremely vulnerable to even low levels of exploitation, and the recent longline survey confirmed that their numbers are still very low on regularly trawled grounds although moderate numbers were found on lightly fished grounds off northern NSW and in some protected areas.