Effect of Water Quality, Common Carp (Cyprinus Carpio) Invasion, and Fishing Activities on the Population of Nile Tilapia (Oreochromis Niloticus) in Lake Hayq, Ethiopia

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Addis Ababa University


Ethiopia is endowed with a number of lakes located across the country in different ecological regions. However, most lakes are found clustered in the Ethiopian Rift valley but there are also few lakes located in the Ethiopian highlands (>1800 m a.s.l.). Lake Hayq is one of the highland lakes located in the north central highlands of the country. The lake fishery provides economic and ecosystem services to the local community. However, since 2010, the fishery production of the lake in general and the population of Nile tilapia, in particular, have been reduced. In addition to this, the growth of Nile tilapia has been stunted. There is limited information on factors that might have contributed to the low population and stunted growth of Nile tilapia in Lake Hayq. Therefore, this study aimed at assessing possible internal and external factors that could have contributed to the decline of Nile tilapia fish growth and the fishery. To realize these objectives, I determined physicochemical water quality, plankton community structure, fish diversity, relative abundance and some biological aspects of common carp (Cyprinus carpio) and Nile tilapia (Oreochromis niloticus) and assessed the effect of fishing activities on O. niloticus between January and December, 2018 using standard methodology. Though significant difference (ANOVA, P< 0.05) was observed in Dissolved Oxygen (DO) and water temperature between dry and wet seasons, variability in most of the physicochemical parameters was low in Lake Hayq. The low variability in physicochemical parameters could be associated with less climatic variability (rainfall and water temperature) and higher depth of the lake. The depth profile data showed that the physicochemical parameters including DO and temperature variation were less between the surface and the deeper portion of the lake. Hence, the lake was not stratified during our sampling period. In this study, the reduction in the concentration of total phosphorus (TP) and Chlorophyll-a (Chl-a) and increment in Secchi disk depth (SD) was observed. Thus, the change in these parameters contributed for the change in the trophic state of the lake from eutrophic to mesotrophic state. A total of 44 phytoplankton taxa grouped under six divisions: Chlorophyta, Bacillariophyta, Cyanophyta, Euglenophyta, Dinophyta, and Cryptophyta were identified in Lake Hayq. Chlorophyta and Bacillariophyta were the major groups in terms of species composition. Peridinium (Dinophyta) was the most numerically abundant species in most of the sampling seasons and sites and formed blooms. Currently, a total of 28 zooplankton taxa were identified from Lake Hayq. The number of both phytoplankton and VI zooplankton taxa was higher in this study which could be associated with a higher number of sampling sites and two of them were from the shore of the lake which might have used as refuging and feeding sites. A total of 1980 fish specimens belonging to three species, Cyprinus carpio (1055), Oreochromis niloticus (892), and Clarias gariepinus (33) were collected. The dominant fish species in abundance were C. carpio (53.28 %), followed by O. niloticus (45.05 %). The length and weight relationships showed nearly isometric growth in both female and male C. carpio. Males (596) were more numerous than females (459) which showed a significant deviation from the 1:1 hypothetical sex ratio in C. carpio. The length at first sexual maturity (L50) of female and male C. carpio were 21.5 and 17.5 cm, respectively. The length and weight relationships in O. niloticus showed negative allometric growth in both female and males. Male O. niloticus were more numerous (553) than female (339) which was deviated from a hypothetical 1:1 sex ratio (Females: Males). The size at first sexual maturity (L50) of females and males O. niloticus were 12.8 and 12.9 cm, respectively. Both C. carpio and O. niloticus have similar peak breeding season between February and April. In Lake Hayq, diet overlap was observed at a smaller size (< 12 cm for O. niloticus and < 16 cm for C. carpio). Fishing activities in Lake Hayq were fully illegal, fishermen have been used monofilaments of mesh s sizes of 4- 6 cm (below the recommended mesh size, > 8 cm). In addition to this, the selective fishing pressure on O. niloticus was higher for Nile tilapia. The change in trophic state from eutrophic to mesotrophic (low food availability), presence of similar breeding season, diet overlap, and fishing pressure (overfishing) especially on O. niloticus might have contributed for stunted growth of O. niloticus in Lake Hayq. Therefore, monitoring the limnological variables, restocking O. niloticus fingerlings, and closing the lake at least for two consecutive years should be done to improve the fishery of Lake Hayq in general and O. niloticus in particular.



Chl-A, Diet Overlap, Mesotrophic, Nutrients, Overfishing, Reproductive Potential