Movements and habitat utilisation of nembwe ( Serranochromis robustus ) in the Upper Zambezi River . Implications for fisheries management

Økland, F., Hay, C. J., Næsje, T. F., Chanda, B. & Thorstad, E. B. 2002. Movements and habitat utilisation of nembwe (Serranochromis robustus) in the Upper Zambezi River. Implications for fisheries management. NINA Project Report 20: 1-25. During 4-15 November 2000, 15 nembwe (Serranochromis robustus Günther, 1864) (32-49 cm) were tagged with radio transmitters in the Zambezi River in Namibia. The objective was to analyse the behaviour of nembwe for management purposes. The movements and habitat utilisation were recorded and compared during the three periods 1) low water level immediately before the rainy period, 2) increasing water level during the rainy period, and 3) high water level after the rainy period. The fish were tracked on average every 3.7 day during 23 November-18 May, and individuals were tracked up to 47 times. Mean total distance moved by individual fish was 3,183 m (range 233-11,886 m). The nembwe showed no directional movements upor downstream, but stayed within defined home ranges. Home ranges were generally small, with a 95% probability of localisation within an average area of 184,563 m2 (range 621-566,597 m2). On average, the fish stayed within a river stretch of 1,330 m (range = 24-3,787). Fish were obviously only recorded in permanently water covered areas during low water. During rising and high water, 67% and 71% of the fish utilised temporary flooded areas, respectively. Most fish were recorded both in permanently and temporary water covered areas during rising and high water. Nembwe did not undertake long-distance migrations onto the floodplains, but utilised the adjacent temporary water covered areas. The utilisation of temporary water covered areas during the spawning period may have been connected to spawning and nursery, but knowledge on the breeding behaviour of nembwe is not yet sufficient to support this. All the fish were recorded in the mainstream of the river. However, 62% of the fish were recorded in one or more additional main habitat type; 54% of the fish were recorded in side channels, 46% in permanent swamps, 15% in backwaters and 8% in the mouth of backwaters. Although often recorded in the main river channel, nembwe rather stayed closer to shore than in the middle of the river. The fish were recorded on average 58 m from the nearest shore (range 2-416), which constituted 15% of the total width of the river. The fish were also likely to be associated with vegetation, as on average, 78% of the fixes were near or inside/under vegetation. The most frequently recorded habitat type was marginal aquatic anchored vegetation, followed by marginal aquatic floating vegetation. Water depth where the fish were recorded varied between 1.2 and 7.3 m, and was on average 3.7 m. Water depths where fish stayed were larger during high water than during low and rising water. The fish were mainly recorded on sandy substratum, which is the main substratum type in the Upper Zambezi River. This is the first study where the behaviour of individual nembwe is followed over time, and much of the data are supplementary information to what is previously known about the species. Based on these results, nembwe seem locally vulnerable to overfishing due to their small movements. Nembwe may potentially be locally overexploited if the local exploitation pressure is high, in contrast to species moving about more widely. The management and regulations are, therefore, important for the local populations of adult nembwe. In rivers bordering to several countries like the Upper Zambezi River, multilateral management regulations are necessary even for stationary species to avoid fish being protected in one country and overexploited in the neighbouring country. The small movements of nembwe also imply that sanctuaries probably will protect adult fish, because they will be staying within the protected area. Three fish were released more than 1,400 m away from the catch site, and did not show homing to the catch site. Inability to home when displaced over some distance opens the possibility of re-introduction of species in areas with extinct or reduced populations and relocation of fish from surrounding areas to sanctuaries.