Spawning aggregation and segregation of Labeobarbus spp. (Pisces: Cyprinidae) in the Megech and Dirma River mouths, Lake Tana, Ethiopia

Spawning aggregation and segregation of Labeobarbus species congregating at the mouths of Megech and Dirma Rivers were studied using catch data collected from December 2003 to November 2004. Fish were collected monthly from November to June and twice in July. However, during the main rainy season (August to October 2004), samples were collected weekly at both river mouths. Polyfilament gillnets with mesh sizes: 6, 8, 10 and 12 cm were set overnight. A total of 4280 Labeobarbus specimens were collected. Six Labeobarbus species (L. intermedius, L. megastoma, L. tsanensis, L. truttiformis, L. brevicephalus and L. surkis) formed spawning aggregations in the river mouths during the peak of the rainy season (August to October). Although there was overlap of spawning aggregations between these six Labeobarbus species weekly catch data indicated temporal segregation among all species, except L. intermedius. Four species (L. intermedius, L. megastoma, L. truttiformis and L. tsanensis) were significantly higher in abundance at Megech River than at Dirma River during the peak aggregation months. Labeobarbus surkis, never reported in other previous studies, was found to aggregate at both river mouths. However, three species (L. acutirostris, L. macrophtalmus and L. platydorsus) formed spawning aggregation in other tributaries were rare in the present study. Closing commercial gillnet fishing in both river mouths during the peak aggregation months (August to October) is highly recommended to conserve the already declining endemic Labeobarbus species flock of Lake Tana. Dam constructions for irrigation in Megech River upstream will impede spawning migration of Labeobarbus species.