Cohort Analysis of Macrobrachium vollenhovenii in the Lagos-Lekki Lagoon System, Nigeria

A cohort analysis was carried out on a 24 month length-frequency data of Macrobrachium vollenhovenii (African river prawn) from the Lagos-Lekki lagoon system, via modal progression analysis using Bhattacharya s method in FISAT (FAO-ICLARM Stock Assessment Tools) software. A total of 11,033 specimen of M. vollenhovenii. was used for the analysis. Three cohorts were decomposed from the composite length frequency sample. The residual of the Gulland and Holt plot, from the length at age derived from the linking of cohort means gave the value of the amplitude of growth oscillation C as 0.40 and the winter point WP as 0.75 or September. The amplitude of seasonal growth oscillation and the winter point derivable from the cohort analysis are of paramount importance in the stock assessment and ultimately management of the resource as they revealed the strength of growth oscillation along with the period of slowest growth which can be inferred as the most susceptible period for the stock. Therefore, resource sustainability require that fishing mortality be minimized in this period (September) in the Lagos-Lekki lagoon system.