Abstract
It is not possible to derive reliable fishing mortality estimates for the majority of species that are potentially impacted by the trawl fishery from landings data alone. For some species the proportion of the catch that is landed is driven by market forces so that fluctuations in annual landings do not necessarily reflect fluctuations in actual catches or in abundance, whereas many species are not commercially useful and are not landed at all. Although the total annual catch of such species is not known, trends in the trawl survey abundance indices can be used as indicators of the impact of incidental mortality on their populations. The trend in abundance indices for 1985 to 2008 are presented for 100 bycatch species (or species groups) taken during research trawl surveys of the west and south coasts of South Africa. For many species the inter-annual fluctuations are greater than any apparent trend. Trend lines for the majority of species do not have significant slopes.