Design of a fishery independent longline survey for chondrichthyans in Northern Australia
Catch Per Unit Effort (CPUE) from commercial logbooks is often used as an index of abundance in stock assessments. However, the use of commercial CPUE as an indicator of relative abundance can be problematic as the underlying assumption that commercial catch rates change linearly with abundance is often compromised. Although some factors that are reported in logbooks can be used to standardise CPUE, there are other sources of variation including:
• Modified fishing practices to target or avoid species to suit quota availability, meet market demands, or to comply with management arrangements.
• Differences in selectivity of fishing gear and use of bycatch-reduction devices.
• The combined impacts of multiple management restrictions on a fishery.
For example, gillnet operations in what was once the Northern Territory’s Shark Fishery now almost exclusively target Grey Mackerel. This change in practice has resulted in an index that is losing its relevance in assessing shark species.
Over 140 elasmobranchs are listed on CITES Appendix II, with the likely-hood that more species will be added in the future. Of the 11,082t of shark landed by the fishery since 2000, CITES listed Hammerheads comprise 17.25% of the shark catch. Other sharks caught by the fishery that were recently added to CITES Appendix II at the Nineteenth meeting of the Conference of the Parties Panama City (Panama), 14 – 25 November 2022 include: Grey Reef Shark, Dusky Whaler, Sandbar Shark, Lemon Shark, Whitecheek shark and all other members of the family Carcharhinidae (which include the Blacktip Shark complex that is the main shark species caught by the fishery - 4688t or 42% since 2000). These species will require a positive Non-Detriment Finding (NDF) and CITES export permit in order to be exported following the 12-month delay in implementation (i.e., December 2023). The fishery also catches Threatened, Endangered and Protected Elasmobranchs, some of which are also CITES listed.
Gillnet and longline effort has decreased since 2000 and there is significant latent effort in the fishery. There is a desire to utilise this latent effort, however, the CITES listing of the majority of sharks species caught in the fishery will increase scrutiny from State, Commonwealth and International environmental agencies, as well as NGOs. This heightened scrutiny will provide greater impetus to demonstrate that shark stocks are at sustainable levels and that fishing is being undertaken sustainably.
There is a strong need for independent survey methods to gain a better understanding of the abundance patterns of shark species over time that can contribute to the development of appropriate management of these species that meets environmental, fisheries, and conservation needs.
Surveying Northern Australia using longlining methods would provide a fishery independent estimate of relative abundance for sharks that would improve economic security and public confidence in sustainability. However, to make an informed decision of the feasibility of a survey a full understanding of the scope of work and cost required is needed. There is a need to look at existing long line surveys undertaken worldwide (e.g. in the USA and Bahamas), to understanding the methods undertaken and the time period required to develop accurate abundance estimates.
