Great Fish River: promoting sound fishing practices
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CTA. 2004. Great Fish River: promoting sound fishing practices. ICT Update Issue 16. CTA, Wageningen, The Netherlands
Permanent link to cite or share this item: http://hdl.handle.net/10568/57653
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Margot Collett explains how researchers in South Africa´s Great Fish River project are using acoustic telemetry to help in the design of conservation strategies for the estuary´s fisheries.
In a boat in the Great Fish River estuary, in South Africa´s Eastern Cape province, researchers gather around the screen of a laptop computer to watch a series of moving dots indicating the movements of fish beneath the water. Science (SAIAB) and the Norwegian Institute for Nature Research (NINA), are collaborating in a project using acoustic telemetry, or electronic tagging, to monitor the numbers and the behaviour of two species of fish - the spotted grunter and the dusky kob - as they migrate between the open sea and the estuary. The project´s findings will be used to promote more sustainable fishing practices in order to protect the estuary´s valuable resources. After being spawned at sea, spotted grunter fiction or science gone overboard? Neither - the researchers, from the South African Institute for Aquatic Biodiversity (Pomadasys commersonnii) and dusky kob (Argyrosomus japonicus) enter the nutrient-rich waters of estuaries where they spend the first few years of their lives. Here, juveniles of the two species are heavily exploited, both by local subsistence communities for food, and recreational fishers, to the extent that the sustainability of these fisheries hangs in the balance. Better resource management, based on knowledge of the population biology, habitat and migratory behaviour of the species, is essential. The project team aims to describe the movements of the fish, how long they spend at sea and in the estuary, and the timing of their migration between the two habitats. Answers to these questions will promote the sustainable development of the fisheries, and provide local and national authorities with the information they need to ensure an equitable distribution of the estuary´s resources among the different user groups. The telemetry equipment consists of a battery-powered acoustic transmitter (fish tag) that is either attached externally or surgically implanted. Each transmitter emits unique coded signals on a fixed frequency and allows several individual fish to be tracked simultaneously. The transmitted signals are retrieved in either of two ways. Stationary hydrophones (underwater data-logging receivers), suspended from buoys positioned in the estuary, are used to monitor the presence or absence of fish within a fixed reception range. Alternatively, the researchers use a hand-held hydrophone from a boat to track individual fish more closely. The signals are transferred to a laptop computer that converts the sound signatures into high-resolution spatial data indicating the position, and direction and speed of movement of each fish. Electronic tagging is an appropriate ICT tool for tracking migratory fish like the spotted grunter and dusky kob. In contrast with radio waves, for example, the acoustic (sound) signals are not hindered by the poor conductivity of the estuary´s salty water. The information collected using this technique also allows the research team to collate fishery catch data with calculations of fish movement trends, in order to assess how vulnerable the species are to localized depletion. The team further hopes to explore the effectiveness of various conservation strategies to protect juveniles, such as creating protected areas within the estuary. The project´s methods and findings could be applied far beyond the estuary - they may also assist in the development of sustainable fish exploitation strategies for various fishery sectors at national level. ´The techniques used in this initiative could also be applied to work we´re doing at Kosi Bay [more than 700 km away], where the main recreational and subsistence fisheries target the same species currently being studied on the Great Fish estuary´, says Steven Weerts, research assistant at the University of Zululand. ´Institutional collaborations of this nature are invaluable as they assist in the transfer of knowledge and skills, and help build local capacity.´ In future, the team plans to combine biological and physical data obtained from the fish tags with remote sensing data about, for instance, surface temperatures of the sea and the estuary. They will then be able to establish the relationship between movements and behaviour of the fish to oceanographic processes. This would constitute the final major advance necessary to understand the distribution of migratory fish in relation to their changing physical and biological environments on daily and seasonal time scales. mailto:firstname.lastname@example.org Margot Collett is communications manager at SAIAB. The Great Fish River estuary project is funded by the South African/Norway Programme on Research Cooperation. For further information, visit http://www.saiab.ru.ac.za/story25.htm www.saiab.ru.ac.za/story25.htm.
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