Models of seasonal growth of the equatorial carp Labeo dussumieri in response to the river flood cycle

Synopsis The phenology of Labeo dussumieri, an omnivorous carp common to South Asia, was investigated in a population inhabiting a flood plain anabranch of the Mahaweli Ganga, Sri Lanka. The Mahaweli Ganga exhibited a bimodal discharge pattern typical of many equatorial rivers, with a minor peak during the S.W. monsoon and a major peak during the N.E. monsoon. Seasonal changes in several lotic variables were measured in an attempt to correlate changes in environmental conditions to reproduction and growth in L. dussumieri. The onset of gonad recrudescence and spawning were synchronized with the increased river discharge during the S.W. and N.E. monsoons: gonad development followed one monsoonal discharge peak and spawning took place at the beginning of the other. Most fish spawned at the beginning of the major discharge peak in October and November, following the September dry season. Increased discharge was concomitant with a fall in temperature, light intensity, pH and conductivity. Growth was shown to be seasonal, exhibiting an annual bimodal pattern with peaks coincident with S.W. and N.E. monsoonal rains. Seasonal changes in growth were expressed by two models in terms of: (a) change of somatic weight or fork length with time, (b) change of specific growth rate in response to river discharge, modified by somatic weight. Gonad recrudescence and spawning stress did not appear to influence growth rate.