Spawning migration of Labeobarbus species of Lake Tana across the Gilgel Abay River Weir, Ethiopia: The challenges to fish spawning migration and proposed management solutions

Abstract

Lake Tana is one of the largest freshwater bodies in East Africa, with its 18 Labeobarbus species being the world's last remaining Cyprinid species flock. Many of them feed and grow in Lake Tana and migrate to the tributaries of the lake to reproduce. The construction of irrigation structures is increasing in the tributary rivers, however, without due attention being given to the spawning migration routes and critical spawning habitats. The present study examined the impacts of the Gilgel Abay Weir on the spawning migration of these species and discusses possible interactions with other human-related impacts and potential mitigation measures. Fish samples were taken twice monthly from July to November 2019, both below and above the weir with gillnets. No spawning migration to the upstream of the weir has occurred since its construction, with the weir being a barrier to their spawning migration route of Lake Tana fisheries. Of the 11 species caught, only Labeobarbus intermedius (a habitat generalist) was observed above the weir, with the species diversity being significantly higher below the weir (p < 0.05) than above it. The total number of species observed during the present study was also lower, compared with previous reports. The fisheries impacts may also have been exacerbated by other contributing factors such as habitat destruction, river system degradation, destructive fishing practices and weed infestation, a situation which may lead to the collapse of Lake Tana fisheries and ultimately the lake's ecology. Reversing this situation is crucial, noting the implementation of such measures as habitat restoration and improvement, replacing the weir with environmentally friendly technologies, prohibition of illegal fishing, and construction of fish ladders could definitely alleviate the observed negative impacts of the Gilgel Abay Weir on the fisheries.