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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Date
2020-06-05
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Addis Ababa University
Abstract
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.
Description
Keywords
Chl-A, Diet Overlap, Mesotrophic, Nutrients, Overfishing, Reproductive Potential