Introduction
Zooplankton are important components in the aquatic food web, serving as a vital link between primary producers and the higher trophic level, their abundance is dictated by prevailing environmental conditions, making them a key indicator of environmental change. Over the years, there have been growing concerns about the impact of cage culture on plankton communities, associated with the increasing adaptation of cage aquaculture. The limited information on the effects of cage culture on zooplankton population in Itapaji geared this study. The study assessed the impact of fish cage culture on zooplankton abundance and water quality in Itapaji Reservoir, southwestern Nigeria.
Materials and Methods
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). Water samples for zooplankton identification were collected from the points using a Juday net as described by Vinogradov et al., (1989), individual species were then examined under an inverted microscope at a magnification of x40 and x100 as described by Harris et al., (2000) while counting and taxonomic identification were done using a Sedgewick Rafter Cell under an inverted electron microscope (Conway et al., 2003; Kyewalyanga & Malesa 2024).
Water samples collected per point and period were measured for Dissolved Oxygen (DO), pH, temperature, phosphate, and nitrate. DO and pH were measured in situ using a CTD multi-probe (Sea-Bird Electronics, Model 19-03), and phosphate was measured in the laboratory as described by APHA (1998). Analyses were carried out using ANOVA to access spatial and temporal variation, the result expressed as mean ± standard error.
Results and Discussion
Zooplankton species identified taxonomically using appropriate keys were a total of 13,912 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%], Chilomonas [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.
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 ± 0.10oC, DC = 27.30 ± 0.13oC, BH = 27.09 ± 0.10oC), 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). The result reveals that dissolved oxygen and phosphate levels decreased during culture (DC) and before harvesting (BH), although all measured water quality parameters remained withing the recommended limits for aquatic life (p > 0.05).
This study revealed that the zooplankton abundance varied due to the prevailing cage activities, and it is therefore essential to maintain a healthy and environmentally sustainable fish cage culture practice for the sustenance of aquatic biota.
Acknowledgements
The authors would like to thank the Ekiti State Water Cooperation for granting access to the Dam for the study.
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