Abstract
Effects of high stocking densities (HSDs) were evaluated for Nile tilapia fish (Oreochromis niloticus) under culture to determine its influence on plasma cortisol and whole blood glucose concentration. Plasma cortisol levels (ng/ml) were assayed byEnzyme-LinkedImmunosorbentAssay (ELISA). Whole blood glucose levels were determined using a hand-held one touch ultraglucose meter (MD-300) and test strips. Plasma cortisol and whole blood glucose level determinations were replicated three times forO. niloticusreared under both low stocking densities (LSD) and HSD. One way Analysis of Variance (ANOVA) was performed on the data collected, and comparison of significant differences in means was carried out between LSD and HSD at 0.01%. Plasma cortisol levels revealed statistically (P≤0.01) significant values of HSD at 6.32 ± 1.06 ng/ml than in LSD at 4.62 ± 1.58 ng/ml for theO. niloticusgroups studied. Whole blood glucose analysis revealed a statistical (P<0.05) difference in the means in HSD and LSDO. niloticusgroups (F(df,1; 8) = 7.946 > Fcrit = 4.414;P=0.01). Mean plasma glucose concentration was statistically (P≤0.01) higher for HSD than LSDO. niloticusgroups at mean ± SD, 96.84 ± 5.28 and 76.82 ± 5.92, respectively. The findings of this study demonstrate that high stocking densities increase both cortisol and whole blood glucose concentration in tilapia fish, indicating a marked increase in stress levels. Elevated plasma cortisol and whole blood glucose concentration can be used as biomarkers for acute stress inO. niloticusproduced under aquaculture systems. The findings of this study can help inform policy on the management of stress caused by overstocking ofO. niloticusand other related Cichlids under industrial aquaculture production.
Highlights
Fish and fisheries play an important role in addressing nutritional and livelihood food security, especially of the poor in the developing countries [1]
Healthy Nile tilapia of Lake Victoria were obtained from the fish rearing facility of the Department of Zoology, Maseno University. ey were first acclimatised to the laboratory conditions and progenesis as reported in [13, 26, 28]. e fish were reared under natural environmental conditions by the seining technique [29]
Note that the data marked with an asterisk (∗) were obtained from samples extracted from the second aquaria for high stocking densities (HSDs) and low stocking densities (LSD) groups to bring the data to 10 a piece for better graphing. is was due to the fact that the second aquaria positions in between the first and third aquaria ensured homogeneity in experimental conditions for the HSD and LSD groups
Summary
Fish and fisheries play an important role in addressing nutritional and livelihood food security, especially of the poor in the developing countries [1]. Nile tilapia have a mild white flesh that appeals to customers, making them economically important fish [8] and the most desired by Lake Victoria Community [9]. It is an important fish model for studies on social stress due to overcrowding [10]. Stocking density is considered a key factor in determining the productivity of fish aquaculture systems [11]. High stocking density negatively affects both fish growth and feed utilization [1, 15]. High stocking densities have been shown to reduce feeding activity and growth rates in Coho Salmon (Oncorhynchus kisutch) and rainbow trout (Oncorhynchus mykiss) but have a positive
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