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The ichthyofaunal by-catch of penaeid prawn trawlers operating on the Tugela Bank of Natal, South Africa, was sampled at irregular intervals from May 1989 to June 1992. A total of 110 samples was collected and 108 teleost species were identified. Six species, of which four were sciaenids, comprised 80% of overall abundance by number. Four poorly separated main sample groups were identified by a multivariate approach, using dendrograms produced by hierarchical agglomerative clustering and ordinations produced by nonmetric multidimensional scaling, based on Bray-Curtis dissimilarity measures. Species which characterised and distinguished the four sample groups were identified by a SIMPER (percentage similarities) procedure, whereby the contribution of each species to similarity measures within sample groups and to dissimilarity measures between sample groups was determined respectively. Ten or fewer species accounted for 80% of the contribution to each within-group similarity measure and intergroup dissimilarities were low. The ordination was then replotted using the environmental data (season, depth and time of day), with samples being stratified according to these variables, irrespective of how they were grouped according to species composition. The plots of environmental data superimposed on the species ordinations suggested that the grouping of samples was attributable to season and depth. A non-parametric equivalent of MANOVA (ANOSIM) was used to test for interactive effects of all three environmental variables. This procedure confirmed that depth and season were highly significant in determining sample groupings and day/night effects were only slightly significant. It was concluded that many of the Tugela Bank prawn trawler by-catch fish species were permanent inhabitants of the region and that the most abundant species exhibited seasonal and distributional trends in abundance. |
