Growth and mortality rates of brown mussels Perna perna (Linnaeus) in Kwazulu-Natal: a comparison between sites and methods using non-parametric length-based analysis

Abstract

Three non-parametric, length-based methods were used to estimate growth and total mortality rates of Perna perna at five sites along the KwaZulu-Natal coastline. Monthly samples were split into quarterly intervals to estimate variances. Pauly's growth performance index (Φ')values obtained using the ELEFAN I method were significantly lower than those using SLCA or PROJMAT. A multiple comparison of group means divided site-specific growth rates into two groups; one group of sites had a mean Φ' of 3.92 (95% confidence interval CI 3.83–4.02) and the other group had a mean Φ' of 4,11 (95% CI 4,02–4,19). Seasonality was marked at three sites and growth was slower during winter (annual monthly sea temperature range 4,7°C). Total mortality estimates were positively correlated with growth rate estimates, which made between-site comparisons difficult; the mean was 1,53·year⁻ (95% CI 1,07–1,99). Variation within regions seems to override any latitudinal trend in growth rate of P. perna along the South African coast.