Ethiopia is endowed with a number of lakes located across the country in different ecologicalrnregions. However, most lakes are found clustered in the Ethiopian Rift valley but there are alsornfew lakes located in the Ethiopian highlands (>1800 m a.s.l.). Lake Hayq is one of the highlandrnlakes located in the north central highlands of the country. The lake fishery provides economicrnand ecosystem services to the local community. However, since 2010, the fishery production ofrnthe lake in general and the population of Nile tilapia, in particular, have been reduced. Inrnaddition to this, the growth of Nile tilapia has been stunted. There is limited information onrnfactors that might have contributed to the low population and stunted growth of Nile tilapia inrnLake Hayq. Therefore, this study aimed at assessing possible internal and external factors thatrncould have contributed to the decline of Nile tilapia fish growth and the fishery. To realize thesernobjectives, I determined physicochemical water quality, plankton community structure, fishrndiversity, relative abundance and some biological aspects of common carp (Cyprinus carpio) andrnNile tilapia (Oreochromis niloticus) and assessed the effect of fishing activities on O. niloticusrnbetween January and December, 2018 using standard methodology. Though significantrndifference (ANOVA, P< 0.05) was observed in Dissolved Oxygen (DO) and water temperaturernbetween dry and wet seasons, variability in most of the physicochemical parameters was low inrnLake Hayq. The low variability in physicochemical parameters could be associated with lessrnclimatic variability (rainfall and water temperature) and higher depth of the lake. The depthrnprofile data showed that the physicochemical parameters including DO and temperature variationrnwere less between the surface and the deeper portion of the lake. Hence, the lake was notrnstratified during our sampling period. In this study, the reduction in the concentration of totalrnphosphorus (TP) and Chlorophyll-a (Chl-a) and increment in Secchi disk depth (SD) wasrnobserved. Thus, the change in these parameters contributed for the change in the trophic state ofrnthe lake from eutrophic to mesotrophic state. A total of 44 phytoplankton taxa grouped under sixrndivisions: Chlorophyta, Bacillariophyta, Cyanophyta, Euglenophyta, Dinophyta, andrnCryptophyta were identified in Lake Hayq. Chlorophyta and Bacillariophyta were the majorrngroups in terms of species composition. Peridinium (Dinophyta) was the most numericallyrnabundant species in most of the sampling seasons and sites and formed blooms. Currently, a totalrnof 28 zooplankton taxa were identified from Lake Hayq. The number of both phytoplankton andrnVIrnzooplankton taxa was higher in this study which could be associated with a higher number ofrnsampling sites and two of them were from the shore of the lake which might have used asrnrefuging and feeding sites. A total of 1980 fish specimens belonging to three species, Cyprinusrncarpio (1055), Oreochromis niloticus (892), and Clarias gariepinus (33) were collected. Therndominant fish species in abundance were C. carpio (53.28 %), followed by O. niloticus (45.05rn%). The length and weight relationships showed nearly isometric growth in both female andrnmale C. carpio. Males (596) were more numerous than females (459) which showed arnsignificant deviation from the 1:1 hypothetical sex ratio in C. carpio. The length at first sexualrnmaturity (L50) of female and male C. carpio were 21.5 and 17.5 cm, respectively. The length andrnweight relationships in O. niloticus showed negative allometric growth in both female and males.rnMale O. niloticus were more numerous (553) than female (339) which was deviated from arnhypothetical 1:1 sex ratio (Females: Males). The size at first sexual maturity (L50) of females andrnmales O. niloticus were 12.8 and 12.9 cm, respectively. Both C. carpio and O. niloticus havernsimilar peak breeding season between February and April. In Lake Hayq, diet overlap wasrnobserved at a smaller size (< 12 cm for O. niloticus and < 16 cm for C. carpio). Fishing activitiesrnin Lake Hayq were fully illegal, fishermen have been used monofilaments of mesh s sizes of 4- 6rncm (below the recommended mesh size, > 8 cm). In addition to this, the selective fishingrnpressure on O. niloticus was higher for Nile tilapia. The change in trophic state from eutrophic tornmesotrophic (low food availability), presence of similar breeding season, diet overlap, andrnfishing pressure (overfishing) especially on O. niloticus might have contributed for stuntedrngrowth of O. niloticus in Lake Hayq. Therefore, monitoring the limnological variables,rnrestocking O. niloticus fingerlings, and closing the lake at least for two consecutive years shouldrnbe done to improve the fishery of Lake Hayq in general and O. niloticus in particular.