Food and foraging ecology of Limnothrissa Miodon in the Sanyati Basin, Lake Kariba
Madzivanzira, Takudzwa Comfort
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The aim of the research was to study the feeding behaviour of Limnothrissa miodon (kapenta) with reference to food items in the Sanyati Basin of Lake Kariba. Sampling was carried out from June to December 2015. Fish samples for dissection were collected per haul in three nights per month at three different sites. Plankton and physicochemical variables were also taken on these sites. Food items in kapenta stomachs were identified and quantified for frequency of occurrence and electivity index. Sardine stomachs were also classified according to fullness. Zooplankton dominated by rotifers and ostracods had the highest frequency of occurrence in the stomachs of sardines. Macroinvertebrates also highly occurred in the stomachs of sardines and had the highest electivity index in all sampled months. Frequency of occurrence of unpalatable prey was significantly different between sampled months (ANOVA, p<0.05) whilst that of most macroinvertebrates, palatable phytoplankton and all zooplankton was not significant different between months (ANOVA, p>0.05). Of the 2970 sardines that were analysed, 69% had empty stomachs, with the remaining 31% stomachs that had food being constituted by 19% ¼-full, 5% half full, 3% ¾-full and 4% full stomachs. There were no significant differences between sampled months with respect to all the stomach classes (ANOVA, p>0.05). The number of kapenta with empty stomachs significantly outnumbered those that had stomachs which were at least ¼-full in all months (χ2 = 376.7, df = 6, P < 0.003). Fullness index ranged from 0.32 in June to 0.14 in November and was significantly different among months (ANOVA, p<0.05). Fullness index also showed a significant difference with respect to the interaction between month and site (ANOVA, p<0.05) and no significant difference between the sampled sites (ANOVA, p>0.05). Total length of fish and weight ranged between 32-70 mm and 0.12-2.63 g respectively. Body condition index for Limnothrissa miodon ranged between 0.67 and 0.73 recorded in July and November respectively. Body condition was not significantly different (ANOVA, p>0.05) with both site and month and however significantly different with respect to the interaction between site and month. Body condition had close relationships with the highly preferred prey groups found in the sardine stomachs. Temperature had a negative correlation with green algae (r = -0.9). All the measured lake water physical and chemical properties except dissolved oxygen displayed significant differences (p<0.05) among months. A total of 44 phytoplankton species were recorded in the Sanyati Basin, which comprised of 6 Bacillariophyta, 21 Chlorophyta, 8 Cyanophyta, 3 Dinophyta, 3 Euglenophyta, 2 Chrysophyta and 1 Xanthophyta. Overall, Cylindrospermopsis raciboskii dominated Cyanophyta had the highest cellular concentrations making up 85% of the total phytoplankton concentrations followed by Chlorophyceae which had 7% contribution. Dinophyceae contributed 5% and the rest of the phytoplankton classes contributed less than 5% to the total population. The high densities of Cyanophyta affected the species diversity of phytoplankton in the Sanyati Basin which ranged between 0.84 and 1.42 recorded in November and June respectively. A total of 26 zooplankton species were recorded which comprised of 3 Cladocera, 7 Copepoda and 16 Rotifera. Keratella cochlearis dominated the in the rotifer taxonomic group and had the highest cellular concentrations making up 83% of the total zooplankton concentrations. Copepoda contributed 12% whilst Cladocera contributed the least of 5%. The density of palatable prey was very low due to the strained environment which is a possible cause of the starvation of Limnothrissa miodon.