Abudance,growth and reproductive biology of oreochromis niloticus (Linneaus,1758)compared with tilapiines indigenous to the middle Zambezi

The introduction of Oreochromis niloticus on Lake Kariba has been widely associated with the decline in preponderance of indigenous Tilapiines, especially Oreochromis mortimeri. Comperative aspects of abundances, growth and reproductive biology for O. niloticus, and three known indigeous Tilapiines: Oreochromis andersonii, Oreochromis mortimeri, and Tilapia rendalli were investigated.
The study was conducted at the riparian Sinazongwe District of Southern Zambia from August to September, 2014. Weight, length and reproductive stages of the different Tilapiine fish were determined. The morphometric and age data was used to determine growth parameters for the Tilapiines using Von Bertalanffy Growth Function (VBGF). Whole ovaries from sexually mature females (ripe and/ or ripe-running) were weighed and preserved in 5% formalin solution; fecundity was determined using gravimetric procedures. Analyses of Variance (ANOVA) were applied in comparing statistical significances in abundance, growth and reproductive parameters among the studied Tilapiines.
Detailed contribution by weight and number for each Tilapiine species at Sinazongwe indicated dominance of O. niloticus (IRI=8.62%) whereas T. rendalli accounted for 0.77%. Oreochromis andersonii and O. mortimeri made insignificant IRI contributions (<1%). Investigation into dependence of Tilapiine abundance on intrinsic growth rate, (K) indicated a positive linear relationship defined by a statistically insignificant regression equation: Abundance=92.49K-8.39 (R2 =0.94; p>0.05). Specimens collected ranged in age from 2-6 years.
The intrinsic growth rate, K, was highest in T. rendalli (0.791yr-1), followed by O. mortimeri (0.308yr-1), O. niloticus (0.180yr-1) and least in O. andersonii (0.147yr-1). Growth performance indices (Φ') for the species differed marginally, being highest for T. rendalli (4.98) and O. niloticus (4.58), and moderately lower in O. mortimeri (4.57) and O. andersonii (4.36). There were significant differences (p<0.05) observed on the fecundities among the studied female Tilapiines. Tilapia rendalli had highest average number of oocytes, 5,135. Among the Oreochromis species, O. niloticus had an average 2,923 oocytes whereas O. andersonii had 1,650 oocytes. On the other hand, O. mortimeri had the lowest fecundity, 994 oocytes.
Oreochromis niloticus was the most abundant Tilapiine at Sinazongwe, and indeed an established species throughout Lake Kariba, on the Zambian shoreline. The studied characteristics among the Tilapiines indicated that O. niloticus was not superior in every aspect of its growth and reproductive biology.