Cohort analysis on a freshwater fish Osteochilus hasselti C. & V. (Cyprinidae) at Bukit Merah Reservoir, Malaysia

Abstract

An earlier cohort analysis using catch-at-length data (for the period May 1979–April 1980) on the stock of Osteochilus hasselti C. & V. computes statistics on: (1) the total stock at the start of the cohort ( N = 22.7 × 10 5) (these stock numbers decrease progressively at successive lengths, in a steady state and assuming constant natural mortality rate, M = 2.1); (2) catch numbers in a cohort totalling 780 940 individuals; (3) the exploitation rate of the cohort of E = 0.34, suggesting moderate exploitation of the species at Bukit Merah Reservoir, Malaysia. However, the validity of cohort analysis on a species exploited at a very early age depends on the input value of the variables natural mortality rate ( M), fishing mortality rate of terminal largest fish ( F ter), and growth parameters ( L ∞, K). Because of the methodological difficulties in estimation and verification of M, F ter and K values of O. hasselti, a range of values of input variables is adopted for comparison. The effect of the variability of M values (0.5, 1.0, 1.5 and 2.5) on the computation results of smaller size-class fishes N L is significantly different from that based on M = 2.1, showing a much lower population size. The variability of input values of F ter (range 0.5–2.5) and K (0.28–2.62) is tested on their effects on the population size of recruits. The combined effect of M and K in the relationship of M K ranging from 0.8 to 2.5 is varied to decide the possible values of K. In general, the different input values of M, F ter and K result in lower estimates of the population size of recruits, N r This is about 28% lower than N r = 22.7 × 10 5 when M = F ter = 2.1 and K = 1.15. With an inherent computational bias, the population size of N r = 22.7 × 10 5 is probably an under-estimate, but it is both reasonable and biologically possible for O. hasselti at Bukit Merah Reservoir, Malaysia, since the input variables are generated from representative length composition data of commercial catches and tagging data.