Observations of Heterotis niloticus (Cuvier) in the cross river basin: An introduced species

Abstract

Heterotis niloticus (Cuvier) was observed in the Cross River basin for a period of two years from March 2006 to February 2008, having been accidentally introduced previously. Monthly samples indicated that the species showed significant presence (P < 0.05) in the fish landings, during the months of July to October each year. Samples began to appear in the catches from early June, increased progressively in numbers peaking in September and then showed a steady decline to complete disappearance by March each year. Spatio-seasonal observations indicated higher abundance in the upper reach landing sites during the dry season. Analysis of variance showed highly significant seasonal variation (P < 0.05) in abundance of H. niloticus within the basin. Simpson’s Index of Dominance (D) indicated competitive values with the indigenous Chrysichthys species, in the lower reach landing sites, especially during the wet season. The Jaccard coefficient of similarity also showed higher values in the lower reaches, but slightly lower values in the upper reach landing sites. A total of 98 taxa belonging to 54 genera and 5 families of phytoplankton were identified within the basin area. Total phytoplankton cell densities ranged from 104 cells l-1 to 610 cells l-1. Correlation coefficient showed significant positive relationship (r = 0.751; P < 0.05) between H. niloticus numbers and phytoplankton abundance in the Cross River basin.