Studies on the Limnology, and Biology of Nile Tilapia, Oreochromis Niloticus L., in a Tropical Soda Lake, Lake Shala, Ethiopia
No Thumbnail Available
Date
2021-05-02
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Lake Shala is the deepest, alkaline lake in Ethiopia and is a vital resource having considerable esthetic, economic, scientific and ecological values. However, the lake is under pressure from various anthropogenic activities and climatic change. There is also a plan to expand the Abijata Soda Ash plant to Lake Shala. The purpose of this study was to assess the spatial and seasonal dynamics, and long-term trend of abiotic factors and biological communities in the lake. Physicochemical and biological data were collected from four sites every month from January to December 2018. All physicochemical factors exhibited significant variation among sites and between seasons (ANOVA, P < 0.05), except DO, which showed only seasonal variation. Compared with previous studies, long-term increasing trend in pH, alkalinity, salinity, electrical conductivity, NO3-N, NH3-N, SRP and TP, and decreasing trend in SiO2 were observed. These changes might be due to the high evaporation rate in the region, precipitation, accumulation of solutes and saline surface runoff from its degraded catchments.
The phytoplankton community in Lake Shala showed changes with time. During this study, 72 phytoplankton species belonging to Bacillariophyta (55 taxa), Chlorophyta (7 taxa), Cyanophyta (6 taxa), Euglenophyta (2 taxa), Charophyta (1 taxon) and Dinophyta (1 taxon) were recorded. The previous dominance of Cryptophytes in the lake has been replaced by diatoms. The lake also lacked Arthrospira fusiformis, but now a few specimens were recorded many times. Zooplankton community structure of Lake Shala (32 species, belonging to rotifers, copepods and cladocerans) has also changed over time, benthic harpacticoids (Nitocra lacustris) were replaced by small-bodied and generalist-v
feeders rotifers (genus Brachionus). Such ecological shifts in phytoplankton and zooplankton communities may be associated with physicochemical changes and ‘atelomixis’ brought about by unusual mixing behavior in deep, tropical lakes. Based on RDA analysis, most phytoplankton and zooplankton species were strongly influenced by nutrients, salinity and electrical conductivity.
The macro-invertebrate community of Lake Shala, was constituted by a few (21) taxa belonging to the Diptera (7), Gastropoda (4), Hemiptera (3), Coleoptera (2), Oligochaeta (2), Argulidae, Arachnida and Corydalidae. Compared with previous data, shifts in macro-invertebrate community structure were observed over the years. The shift might be due to the changes in the water chemistry and recession of the lake littoral zone. The macro-invertebrate taxa richness and abundance were significantly affected by locality (P < 0.05), the two shore sites having significantly higher taxa richness and the opposite was true for abundance. Most families were absent from the open water stations and when present, represented by Diptera (Ceratopogonidae and Chironomidae) and Oligochaeta (Tubificidae and Naididae), which were correlated strongly with depth, DO, EC and alkaline-salinity of the lake. These abiotic factors have presumably become altered in the lake due to undetermined stressors.
Previous studies did not report the ecology and biology of fish from Lake Shala and this study is the first to document the biology of tilapia from this lake. Oreochromis niloticus had a curvilinear growth pattern (TW = 0.0104TL3.19, R2 = 0.98, n = 343). The condition factor between sex categories (ANOVA, P > 0.05) and sex ratio from the hypothetical distribution of 1:1 (χ2 = 0.47, P > 0.05) were not significant. The breeding seasons, rainy (July) and dry (February) coincided with the increase in phytoplankton abundance and
biomass (Chl a). The fecundity ranged between 240 and 1,642 eggs per fish, with a mean fecundity of 806 eggs and correlated with the length and weight of the fish. Analysis of the stomach revealed that 226 (65.9%) fish contained different food items, while 117 (34.1%) had empty stomachs. O. niloticus mainly fed on phytoplankton and had phytoplanktivorous feeding habits. Bacillariophyta was identified as the most desired phytoplankton item. The prey items and food habits of O. niloticus varied significantly among size classes (ANOVA, P < 0.05) and seasons (t-test, P < 0.05). Such variations might be associated with seasonal availability of natural food, habitat, and morphological and physiological changes such as increase in the length of the gut and acidity as the fish grow.
This study is the first comprehensive study on the limnology, and biology of O. niloticus in Lake Shala. Lake Shala is located in the center of economic development pressure and the physical environment and the biota of the lake are continually changing. Such ecological changes in this deep lake might be due to anthropogenic activities around the lake catchments and internal limnological dynamics (atelomixis). However, continuous investigation on physical environment and biodiversity assessment in Lake Shala is necessary to evaluate the ecological impacts of the upcoming soda ash factory and other external impacts on the limnology of the lake ecosystems.
Description
Keywords
Atelomixis, Chironomidae, Diatom, Ecological Shift, Lake Shala, Phytoplanktivorus, Rotifers