Fish spawning migration, Young-of-the-Year (YOY) fish survival, and food web modeling in Lake Tana, Ethiopia

Abstract

Under the current anthropogenic pressures on the ecosystem, the Lake Tana fish species suffer from pollution, habitat destruction, and excessive fishing. It is crucial to develop an effective conservation and management strategy for the lake and its tributary rivers. This requires an understanding of the feeding and reproductive ecology of the fish taxa of concern, namely the species belonging to the genus Labeobarbus, which includes an endemic but threatened fish in the Lake. This thesis aims to investigate the reproductive and feeding ecology of endemic Labeobarbus species in the LakeTana sub-basin, which is addressed in four different chapters. Radio-tagged individuals from Labeobarbus platydorsus, L. megastoma, and L. truttiformis were studied in the Gumara River and its tributaries during the spawning seasons between July and October of 2018 to 2021. The habitat use of other Labeobarbus species was also assessed at the spawning habitats in the main river channel and tributary streams using fyke net and cast net fishing and data from fishers’ catches (chapter 2). The tagged fishes had moved upstream up to 41.0 to 44.4 river kilometers (rkm) within 9 to 27 days to spawn. The estimated ground speed used by the tagged fishes during upstream and downstream movement was not statistically different. Several small-sized individuals from L. megastoma were found spawning in the Wonzuma and Dukalit tributaries. In contrast, bigger-sized L. platydorsus and L. megastoma spawned in the main river channel at the riffles on the gravel/pebble size substrate. Labeobarbus truttiformis primarily used the Kizen tributary with low turbidity and gravel-size substrates. In the Gumara River, a breeding hotspot area was situated between ~ 41 rkm to 45 rkm. In these areas, fishing intensified between August and October, distracting the breeding adults. The result of this chapter support fishery managers to better understand the breeding habitats of Labeobarbus species and develop a strategic plan to monitor and regulate the breeding habitats, including seasonal closing at breeding hotspot sites. In Chapter 3, point abundance electrofishing sampling was used to examine how juveniles of those migratory species used nursery habitat and factors related to habitat changes in their natal River, Gumara. Habitat use shifting was observed among Labeobarbus size classes from weak or no flow pool to fastflowing riffle mesohabitats. The Young-of-the-Year (YOY) were mainly found in microhabitats with a weak to moderate water velocity (i.e., < 0.3 m s-1), while 1+ juveniles (>7.0 cm fork length (FL)) occurred in sites with velocities of more than 0.3 m s-1 . Juveniles had limited access to run and riffle habitats during the peak dry season (March-April). They were subsequently exposed to resource competition and predation risk, which led to lower catches. Maintaining appropriate flow that permits juveniles to access preferred foraging and nursery habitat is crucial for the sustainable management and conservation of the Labeobarbus population in Lake Tana. The feeding ecology of Labeobarbus species was investigated for juveniles in the Gumara River (Chapter 3) and adults and sub-adults of six dominant species in Lake Tana (chapter 4) using a stable isotope technique and Bayesian mixing models. Juveniles of different size classes used various food sources. C1 (< 3.0 cm) and C3 (5.1 – 7.0 cm) size classes mainly relied on algal sources, while C2 (3.0 – 5.0 cm) and C4 (> 7.0 cm) were on zooplankton. Insect larvae were a relatively important source for larger juveniles. The piscivorous L. megastoma and L. platydorsus showed similar feeding habits, with small variations in the proportion of consumed sources between pelagic and littoral sites. Non-piscivorous species, L. brevicephalus, L. intermedius, and L. tsanensis dominantly fed on zooplankton, sediment organic matter (SOM), and benthic macro-invertebrates (BMI), respectively. The contribution of algal sources was estimated to be higher in the littoral habitat for these three species. L. tsanensis exhibited a significant spatial difference in feeding habits, primarily feeding on algae (22.8%) in the littoral but on BMI (41.1%) in the pelagic. L. truttiformis consumed macro-invertebrates (12.0%) in the littoral, but E. tanapelagius (29.4%) in the pelagic. Thus, their estimated trophic level was higher in the pelagic than littoral habitats. A mixing model based on both δ 13C and δ 2H revealed that allochthonous carbon sources from terrestrial C3 plants were an important primary energy source supporting fish production indirectly via detrital pathways, indicating that detrital-based food chains are a vital component in the food web of this large, shallow, and turbid lake system. This study is the first attempt to characterize the trophic interaction and food web structure of the Lake Tana cyprinids using the stable isotope approach, which can serve as a baseline for future stable isotope-based studies and ecosystem-integrated fisheries management in the lake. Chapter 5 focused on the level of pesticide contaminants in selected fish species; Labeobarbus megastoma, L. tsanensis and Oreochromis niloticus. The pesticide concentrations in fillet and liver tissue were quantified using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Meanwhile, potential risks to the normal development of fish species and risks to the health of fish-consuming human populations were evaluated. Ten pesticides were detected in the fillet and liver tissue of the study species. Pyrimethanil was the most frequently detected pesticide in the three studied species, and its concentration significantly differed between L. megastoma and L. tsanensis. The amounts of pesticides found in the three fish species under study pose no direct threat to consumer health. However, the results show the necessity for immediate attention and close monitoring of pesticide use throughout the catchments of the lake (SOM), and benthic macro-invertebrates (BMI), respectively. The contribution of algal sources was estimated to be higher in the littoral habitat for these three species. L. tsanensis exhibited a significant spatial difference in feeding habits, primarily feeding on algae (22.8%) in the littoral but on BMI (41.1%) in the pelagic. L. truttiformis consumed macro-invertebrates (12.0%) in the littoral, but E. tanapelagius (29.4%) in the pelagic. Thus, their estimated trophic level was higher in the pelagic than littoral habitats. A mixing model based on both δ 13C and δ 2H revealed that allochthonous carbon sources from terrestrial C3 plants were an important primary energy source supporting fish production indirectly via detrital pathways, indicating that detrital-based food chains are a vital component in the food web of this large, shallow, and turbid lake system. This study is the first attempt to characterize the trophic interaction and food web structure of the Lake Tana cyprinids using the stable isotope approach, which can serve as a baseline for future stable isotope-based studies and ecosystem-integrated fisheries management in the lake.