Population density dynamics of Pachymelania byronensis (Wood, 1828) (Gastropoda: Thiaridae) in the Cross River, Nigeria

Abstract

The ecology of an exploited population of the commercially important molluscan species, Pachymelania byronensis (Wood, 1828) was studied (January-December, 2018) in the lower Cross River, Nigeria, using density [number per hectare (N. ha^-1)] as measure of stock abundance. The results revealed that the river supports a considerable stock of the gastropod (density range: Drange = 508000 – 12510000 N. ha^-1; mean: Dmean = 4849833N.ha^-1). The density decreased by 12002N. ha^-1 (i.e. 10001667N. ha^-1 mo^-1) between January and December, a reflection of the increasing human exploitation as the year progressed. However, the monthly density changes scored low stochasticity (randomness), a product of the resilience of the population to human exploitation. The monthly densities were significantly correlated with river limnological parameters, including surface temperature, discharge, water level, transparency, dissolved oxygen concentration and hydrogen ion concentration. The coefficients of determination showed the decreasing order of the
importance of these parameters as: transparency (67.8%), water level (57.0%), surface temperature (43.5%), pH (40.5%), dissolved oxygen concentration (36.1%) and discharge (34.3%). The monthly values of biotic potential (rmax) were used to show trends in the population dynamics. Major density decreases were noted in February (rmax = -5.456) and December (rmax = -2.931) whereas notable density increases were recorded in January (rmax = + 3.204) and November (rmax = + 3.052). The value of the annual mean biotic potential (mean: rmax(mean) = -0.191) portrayed the general effect of heavy human exploitation on the mollusc. Hence, the gastropod total mortality rate was high (Z = -3.204 yr^-1) The river carrying capacity of the gastropod was estimated at 12510000N ha^-1. The monthly biotic potential correlated significantly with habitat temperature and dissolved oxygen concentration, two important life support parameters.

Keywords: Pachymelania byronensis; density; biotic potential; exploitation; Cross River; Nigeria.