Winter Scale Rings in Lates niloticus (Pisces: Centropomidae) from Lake Chad

Abstract

THE formation of scale rings in tropical fishes has usually been ascribed to some form of physiological stress producing a temporary check in the growth-rate. Holden¹ thus proved that in Tilapia spp. from Lake Victoria ring formation is associated with spawning; Johnels² and Daget³ both correlated the annual rings which occur in many West African savannah species of fish with adverse environmental conditions, particularly food shortage, occurring during seasonal drying-up of the rivers. It appears that scale rings usually formed under tropical conditions are not strictly comparable with those of temperate species, which are laid down as a consequence of differences between summer and winter growth-rates, and where change in temperature is the most important factor.