Understanding selected growth aspects in Redbelly Tilapia, Coptodon zilli (Gervais) and Largemouth Bass, Micropterus salmoides (Lacepède) in Lake Naivasha, Kenya; Fisheries Management Perspective

Coptodon zilli and Micropterus salmoides were introduced into Lake Naivasha in 1929 and 1959 respectively. The reasons for the introduction were to boost the fisheries and for recreational fishing respectively. However, the numbers of the two fish species in the lake have been declining over the years. It is therefore necessary to assess various aspects of their growth such as length-weight relationship, condition factor, sex ratio and length at first maturity to inform management and policy making for enhanced recovery and establishment of their populations. Length-weight relationship, condition factor, sex ratio and length at first maturity of the fish were studied from October 2016 to November 2018. A total of 303 fish samples; 193 C. zilli and 110 M. salmoides were studied. Results showed that C. zilli and M. salmoides had an allometry coefficient value of 2.9 and 3.1 and condition factor (K) value of 1.77 and 1.39 respectively. The sex ratios (male: female) of C. zilli and M. salmoides were 1.1:1 and 1.3: 1 respectively. The shortest total length for mature C. zilli and M. salmoides recorded were 12 cm and 26.5 cm while 21.8 cm and 51 cm were for the longest respectively. The fish growth pattern therefore failed to obey the cube law of b=3 thus exhibiting allometric growth. The two fish species were found to be in good condition with K factor above 1. It can therefore be concluded that the fish are generally in a good condition though the early maturity especially for C. zilli could be a sign of pressure either from environmental factors or human induced. Domination of the species by males, although a common feature in African lakes, is a case for concern since it presents uncertainty on the future of the fisheries. Restocking of Lake Naivasha with the two fish species should be considered and proper fishing practices enforced