Observations on the population dynamics of Hemisynodontis membranaceus (pisces: mochokidae) in stratum vii of Lake Volta

Abstract

The population dynamics of the mochokid, Hemisynodontis membranaceus (Geoffroy St. Hilaire, 1809), a dominant fish species in purse seine or “winchnet” catches in Stratum VII of Lake Volta, were studied using length frequency data. Using the FAO-ICLARM Stock Assessment (FISAT) Software, the von Bertalanffy growth parameters, L¥, K and to were estimated for the fish as 44.0 cm Standard Length, 0.55 per year, and 0.26 year respectively. The natural mortality coefficient, M was found as 1.12 year-1; fishing mortality coefficient, F= 2.91 year-1; and the total mortality coefficient, Z found as 4.03 year-1. The exploitation rate, E = F/Z was thereby found as 0.72 indicating that the species is over-exploited requiring immediate management attention. At least 4 year groups or cohorts were found to occur on the fishing grounds which have a longevity of about 5 years. The average annual yield of H. membranaceus by winchboats (purse seine) is estimated as 4,000 tonnes. Information available from Thompson and Bell analysis suggest that a Maximum Sustainable Yield (MSY) of 4,800 tonnes per annum of H. membranaceus may be achieved when the present fishing effort of winchboats is reduced by about 20%. Any further increase in the effort is not expected to increase the average yield.

JOURNAL OF THE GHANA SCIENCE ASSOCIATION Volume 1 Number 2, January (1999) pp. 43-52