Influence of environmental variables on the relative abundance and habitat use of two sympatric notobranchiid fishes in a tropical stream

Effect of environmental variables on the population dynamics of two sympatric notobranchiid fishes, Epiplatys bifasciatus and E. spilargyreius in the seasonal Monai Stream of the Kainji Lake Basin in Nigeria were studied for 24 months. In all, 2,544 and 937 specimens of E. bifasciatus and E. spilargyreius were collected respectively. E. bifasciatus was more abundant of the two species throughout the study period. For both species, monthly abundance followed the same pattern; May to October (rainy season) was a period of low abundance while November to April (dry season) was a period of high abundance. Relative abundance was correlated with physical, chemical, and biological factors using regression analyses. The relationship between 12 environmental variables (temperature, transparency, conductivity, hydrology, dissolved oxygen, pH, nitrogen, phosphate, potassium, sodium, CaCO3, chlorine) and abundance of the two species showed that E. spilargyreius abundance was strongly correlated with water conductivity (Pearson’s coefficient, r = 0.884, P<0.01) but correlated negatively with temperature (Pearson’s coefficient, r = -0.559 at P< 0.05). E. bifasciatus abundance had a slight positive correlation with alkalinity (r = 0.501 at P< 0.05). Three habitat types (vegetated pool, vegetated riffle, and marsh) were preferred by both species, whilst unvegetated habitats were avoided. E. spilargyreius was fairly specific in its habitat preference, with a significant positive correlation (r = 0.65, P < 0.05) to marshy habitat, whereas E. bifasciatus showed some flexibility in habitat-use.   Key words: Epiplatys bifasciatus, Epiplatys spilargyreius, population, abundance, killifish, stream fishes, environmental variables, habitat-use.