Presently, no stock assessment for thickskin sharks has been completed in Western Australia. New management arrangements are being proposed for the West Coast Demersal Gillnet and Demersal Longline Fishery, Western Australia’s two Northern shark fisheries and the Commonwealth Tuna and Billfish Fisheries (which could lead to a dramatic increase in longline fishing effort in Western Australia). It is necessary for fisheries managers to have a much better understanding of the catch of thickskin sharks, the status of their stocks and the catch of other long lived carcharhinids, such as the dusky shark, as a basis for future management decisions.
There is also a need for a methodology to be established on which to base future stock assessments for thickskin sharks. The stock assessment process requires a better understanding of the species’ biology in Western Australia than is currently available. With the northern and western fisheries for thickskin sharks separated by a large closed area, studies of stock discrimination and movement are necessary to determine the appropriate geographic scale for management of this species.
Bycatch of (particularly adult) thickskin and dusky sharks by ‘non-shark’ fisheries needs to be quantified and considered in relation to Australia’s international conservation responsibilities. If the exploitation of these species is shown to be unsustainable the future viability of valuable fisheries (eg. Southern and Western Tuna and Billfish) may be threatened, unless this bycatch can be eliminated or minimised.
Final report
The purpose of this project was to collect the biological and fishery information necessary to conduct a stock assessment of the sandbar (known locally as ‘thickskin’) shark, Carcharhinus plumbeus and to improve and update the existing stock assessment for the dusky shark, Carcharhinus obscurus. Results from this project have already been used by the Western Australian Department of Fisheries to determine appropriate management arrangements for the State’s shark fisheries to ensure the sustainable exploitation of these species. Results have also assisted the WA target-shark fisheries in conducting Ecologically Sustainable Development (ESD) assessment in order to meet Department of Environment and Heritage ecological assessment requirements for maintaining the fisheries’ export approval.
During the mid to late 1990s, changes in targeting practices of vessels operating in the west coast zone of the temperate WA target-shark fisheries caused rapidly escalating sandbar shark catches. By 1998, sandbar sharks had overtaken dusky sharks as the primary component of the west coast fishery’s catch and had become the 3rd largest component of the temperate fisheries’ total catch. At the same time, a demersal longline fishery, targeting sandbar shark off the Pilbara and Kimberley coasts began to develop and sandbar catches also began to increase dramatically in the State’s north. Given this species’ demonstrated vulnerability to overfishing, a formal assessment of the status of this stock and sustainable levels of exploitation became imperative.
The current project therefore collected the biological and fishery-related data that was necessary for accurate stock assessment and developed assessment models appropriate for this long-lived species. Extensive sampling was undertaken in the target fisheries, as well as in those fisheries that were identified as having a significant bycatch of sharks. Additional research was conducted through a series of cruises on board the WA Department of Fisheries research vessels Flinders and Naturaliste. Commercial sampling involved the collection of operational data from the various fisheries, e.g. fishing locations, dates, set times, depths, gear characteristics, etc., as well as identifying and measuring catches, collection of biological data and samples and tagging large numbers of sharks. Fishery independent sampling allowed data to be collected from areas in which commercial vessels are prohibited from operating, in areas where commercial fishers choose not to operate and with fishing gear-types in areas where commercial vessels are not permitted to use them.
Previous FRDC funded research into WA’s shark stocks (projects 93/067 and 96/130) focussed on the traditional target species of the State’s temperate target-shark fisheries, i.e. dusky shark, C. obscurus, gummy shark, Mustelus antarcticus, and whiskery shark, Furgaleus macki. One of the key findings of this previous research was that, whilst the exploitation of dusky sharks in the target fisheries was likely to be sustainable, their sustainability was dependent on a very low level of mortality of older sharks outside the temperate shark fisheries. Since this research was undertaken, several potential and developing sources of adult dusky shark mortality were identified. Consequently, the assumption that there was a negligible level of exploitation of older dusky sharks could no longer be relied upon and further advice on the status of this species was required. In addition, it was necessary to update the biological parameters and exploitation rates used in the previous assessment with new data, derived from project 96/130 and the current study.
