The mean condition dynamics of Pachymelania byronensis (Wood, 1828) (Gastropoda: Thiaridae) in the Cross River, Nigeria

Abstract

The monthly mean condition index (Kmean) of Pachymelania byronensis was computed from a unique combination of the body weight–shell height relationship parameters (i.e. a, b). Shell height (H) and weight (W) were measured for each specimen. There was a wide (827.7 –fold) variation, from a minimum of 0.012 in October to a maximum of 9.932 in December. Mean annual value was 8.064. The relatively low values of the condition index in February – May, August -- October were attributed to declines in body weight as the gametes are released during spawning. This gastropod species spawns three times a year, a feature that is responsible for the high density of the gastropod in the river and its capacity to sustain a year-round strong fishing pressure. The monthly condition indices significantly correlated with river hydrological parameters, including rainfall, surface temperature, discharge, transparency, pH, and dissolved oxygen concentration.