Title : Spawning migration of labeobarbus species of Lake Tana across Ribb Dam and Gilgel Abay Weir, Ethiopia: The challenges of fish spawning migration and proposed mitigation measures
Abstract:
In Ethiopia, particularly in the Lake Tana sub-basin, the practice of irrigation development is increasing. However, this development ignored the fisheries, and no information on its impact was available. The sub-basin is rich in fishery resources, including the 18 Labeobarbus spp. The present study examined the impact of irrigation structures and practices on the spawning migration of these species and discussed possible interactions with other human-related impacts, and potential mitigation measures were discussed. Methods include fish sampling below and above the weir, below the Ribb Dam on the Ribb River, interviews with key informants and experts, secondary data collection, and the impact significance matrix method. The data collection period was from July 2019 to May 2020. Qualitative and quantitative data were evaluated. Existing irrigation systems affect fisheries by blocking spawning routes upstream (Gilgel Abay Weir and Ribb Dam). The catch below the Gilgel Abay Weir was significantly higher than upstream of the weir (Shannon Index, P < 0.001). According to our observations, even large-sized species were not able to jump over the Gilgel Abay weir (> 2 m), and the fishes couldn’t swim upstream past the dam. According to local sources, in particular, the Ribb River became seasonal after 2007 due to excessive water abstraction for irrigation, leading to mass die-offs of fish and a failure to recruit juvenile fish to the lake. In one instance, we recorded the deaths of over 837 adults and a number of juveniles on the Ribb River and its tributaries. Due to the small volume of water, non-fishers also collect fish from the pools, and during the spawning season, fishers target spawning migratory species at the weirs, which are exposed to the gears of the fishers at the weir, which is also a problem. The current study also showed a smaller number of species compared to previous records on these rivers. The presence or absence of species in this and other tributaries could be due to changes in reproductive strategy, inefficient sampling and identification, interbreeding, or altered breeding habits. Reversing this situation is crucial, noting the implementation of such measures as habitat restoration and improvement, replacing the weir with environmentally friendly technologies, the prohibition of illegal fishing, and the construction of fish ladders could alleviate the observed negative impacts on the fisheries.
Keywords: Gilgel Abay River, Impacts, Irrigation structures, Labeobarbus spp., Lake Tana, Migration, Ribb River
