Spatial behaviour and management of greenhead tilapia (Oreochromis macrochir) in the Zambezi River, Namibia

Thorstad, E.B., Hay, C.J., Økland, F., Nickanor, N. & Næsje, T.F. 2007. Spatial behaviour and management of greenhead tilapia (Oreochromis macrochir) in the Zambezi River, Namibia. - NINA Report 287. 26 pp. The large cichlids in the Upper Zambezi River are valuable species in commercial and subsistence fisheries, and are popular among recreational anglers. However, reports of reduced catches are a major concern for the management authorities. To be able to develop local and regional management strategies, information on fish movements and habitat use is fundamental. The aim of this study was, therefore, to provide information on behaviour and habitat use of one of the most important cichlids, the greenhead tilapia (Oreochromis macrochir). Twenty-two greenhead tilapia (body length 25-44 cm) were captured and radio tagged 27- 60 km downstream from Katima Mulilo, Namibia, during 18-25 November 2003. They were subsequently tracked on average every 3.3 day (a total of 74 tracking surveys) during a period of rising water level (1 December - 28 March), high water level (29 March - 2 May) and decreasing water level (3 May - 5 August). Five transmitters were returned from local fishermen, giving a minimum exploitation rate of 23% during the study. Total distance moved by individuals during the study was on average 4,076 m (range 334- 19,666 m). Mean distance moved between tracking surveys was 100 m (range 14-518 m), and did not differ among periods. Home ranges were relatively small, with a 50% probability of localisation within an average area of 0.5 km2 and a 95% probability of localisation within an average area of 3.1 km2. Home range size did not differ among periods. The length of the river stretch used was on average 2,554 m (range 171-13,697 m). The fish utilised permanently water covered areas only (42% of the fish), alternated between permanently and temporarily water covered areas (50% of the fish), or utilised only temporarily water covered areas (8% of the fish). During rising water, 42% of the fish utilised temporarily flooded areas, during high water 40% and during decreasing water 50%. The proportion of fixes in temporarily flooded areas did not differ among periods. During the study, 67% of the fish were recorded in the main channel of the river, 63% in side channels, 63% in backwaters, 63% in swamps, 25% on the floodplain and 8% in the mouth of backwaters. (Note that percentages add up to more than hundred because some fish are recorded in more than one habitat type.) The fish were recorded in different positions related to vegetation; 83% of the fish were recorded at no vegetation (> 5 m away from vegetation), 75% near vegetation (≤ 5 m away from vegetation) and 83% inside/under vegetation. Only one fish was never recorded near or inside/under vegetation. Water depth where the fish were recorded varied between 0.3 and 12.7 m, and was on average 4.2 m. The fish were mainly associated with sandy substratum; 92% of the fish were recorded on sandy substratum, 58% on clay and 50% on muddy, soft bottom. Total width of the river where the fish were positioned varied between 30 and 2,000 m, and was on average 322 m. Distance to nearest shore varied between 1 and 1000 m, and was on average 51 m. Southern Africa, Namibia, Caprivi, Upper Zambezi River, Cichlidae, greenhead tilapia, Oreochromis macrochir, telemetry, radio tagging, tracking, movements, home range, habitat use, exploitation, flood plain, behaviour, management, fishery, catch-and-release angling, NINA Rapport