The Biology Of A Mormyrid Fish Mormyrus Kannume (forsskal I 1776) In A Tropical Man-Made Lake; Lake Kamburu

Abstract

Four years after inundation1 changes in some limnological parameters of the new man-made lake, lake Kamburu, have taken place. Higher mean values were obtained from the lake for: transparenc1~43.25 em; surface water temperature- 23.50C; and conductivity- 184 ymhos. Comparable values from the river were: transparency- 22.1 em; temperature- 21.5C{:: pH- 7.6 and conductivity- 109 pmhos. On the other hand; oxygen concentration was lower in the lake (4.8 mg/l) than in the river (7.4 mg/l). Analysis of the monthly changes in the condition of Mormyrus kannume indicated a generally high condition factor and the length-weight relationship revealed that growth of fish is isometric for mature males, while allometric for mature females and near isometric for the immature sexes combined. Studies of food and feeding habits revealed that M. kannume is an insectivorous feeder and both immature and mature fish feed mainly on the larval stages of aquatic insects throughout the year. In order of importance the following were recorded: Chironomus spp., Tanytarsus spp.; detritus and Baetis spp. The species is a bottom feeder and its long, tube-like snout is well adapted for bottom feeding. It is nocturnal In its feeding behaviour. The reproductlve patterns studied through analyses of annual cyclical changes In the gonadal weight and maturity stages revealed that M kannume spawns throughout the year with peaks in spawning activities In November/December and April/May. The minimum Size at maturity was 24.0 cm SL In females and 25.0 cm SL In males. spawning takes place In the riverine environments. The ratio of males to females In the population did not deviate significantly from the 1:1 ratio throughout the year with exception of January 1979 when a ratio of 2:1 In favour of females was observed. Fecundity is low and it ranges from 406 eggs In a 22.9 cm female to 3466 eggs In fish measuring 42.5 em with a mean of 1600 eggs, and it increased with length (r = 0.82 P<O.OOl) and weight (r= 0.85 P<O.OOl) of the females. Egg size ranged froro 1.9 mm to 2.9 mm with a mean of 2.4 mm. The size of the eggs did not seem to bear any relationship to the number of eggs spawned (r= 0.01; P)0.09).