EFFECTS OF FISH CAGE–CULTURE ON ZOOPLANKTON ABUNDANCE IN THE TROPICAL RESERVOIR OF ITAPAJI, SOUTH-WEST, NIGERIA

Abstract

Zooplankton are important components in the aquatic food web and their abundance are dictated by prevailing environmental conditions. The limited information on the effects of cage culture on zooplankton population in Itapaji geared this study. Four cages located 50m equidistantly and tagged A, B, C, and D with cage D located upstream. Water quality and zooplankton abundance were investigated before stocking (BS), during culture (DC), before harvesting (BH). The mean values of water quality parameters measured were dissolved oxygen (BS= 4.73 ± 0.34mg/L, DC = 3.72 ± 0.13mg/L, BH = 3.40 ± 0.11mg/L), temperature (BS= 28.45 ± 0.10^oC, DC = 27.30 ± 0.13^oC, BH = 27.09 ± 0.10^oC), pH (BS= 6.19 ± 0.12, DC = 6.05 ± 0.10, BH = 5.73 ± 0.10), phosphate (BS= 0.21 ± 0.01mg/L, DC = 0.17 ± 0.02mg/L, BH = 0.12 ± 0.01mg/L), and nitrate (BS= 0.001.19 ± 0.00mg/L, DC = 0.001 ± 0.00mg/L, BH = 0.001.00 ± 0.00mg/L). Zooplankton species identified taxonomically using appropriate keys were a total of 13912 individuals (BS = 49.7% [6914 individuals], DC = 26.9% [3737 individuals] BH = 23.4% [3261]) belonging to 3 taxonomic groups (protozoan = 70.9% [9869 individuals], rotifers =15.2% [2116 individuals] and crustacea = 13.9% [1927 individuals]) with 9 species (Euglena [10.8%], Chilomona [0.9%], Frontonia [21.1%], Coleps [5.3%], Rotaria [15.21%], Cypridopsis [13.9%], Vorticella [3.9%], Paramecium [25.2%], and Chilodonella [3.7%]). Spatial variations in zooplankton abundance between cage sites were observed from the Principal Component Analysis (PCA), BS had PC1 (76.4%) and PC 2 (20.3%), DC had PC 1 (70.0%), PC 2 (19.8%) and PC 3(10.2%) and BH had PC1 (67.3%) and PC 2 (27.8%) accounting for the total proportion of zooplankton species. This revealed that the zooplankton abundance varied due to the prevailing cage activities, and it is therefore essential to maintain a healthy environment for the sustenance of aquatic biota.