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The Izak catshark Holohalaelurus regani and the yellow-spotted catshark Scyliorhinus capensis both occur over the continental shelf around South Africa and are often caught together as bycatch in demersal trawls. Yet our understanding of their diet is fragmented, which makes determination of their place in ecosystem models difficult. Using material collected along the coastline and at a variety of depths, we analysed the diet and assessed the trophic ecology of different size classes of each species, using analyses of stomach contents and stable isotopes. The most important prey items consumed by both species were crustaceans, cephalopods and teleosts. Higher-trophic-level prey were more important for H. regani than for S. capensis, and these prey were also more common in the diet of each species on the west coast than on the south coast. Differences in the prey species consumed, compared by coast, size class and depth, were also detected, with the catshark species feeding on the (presumed) most-abundant prey on each coast. Individual catsharks consumed larger, higher-trophic-level prey items with increasing size and depth. Dietary differences were also reflected in the stable isotopes: generally higher δ15N values were noted from samples on the west coast than on the south coast; a significant interspecific difference in δ15N values was detected; and δ15N values increased with increasing size in both species. The present study provides a multidisciplinary framework for a better understanding of the diet and trophic ecology of these catshark species and emphasises the important structuring role (through food-web effects) that these fish play in benthic communities off South Africa's coast. [ABSTRACT FROM AUTHOR]

Research survey data collected over 24 years (1986-2009) were used to explore long-term changes in demersal fish assemblages on the west coast of South Africa. Differences in spatial (latitude and depth) and temporal (seasonal and annual) factors were examined using multivariate analyses. Fish assemblages are clearly influenced by depth, with a distinct change in the region of the shelf break between 300 m and 400 m. There are also geographic differences in fish assemblage from north to south. Multivariate analyses show two clear temporal changes in assemblages, first in the early 1990s and second in the mid-2000s, although the latter change may be confounded by a concurrent change in survey trawl gear. The abundance of three fast-growing, early-maturing species increased over the study period whereas that of two slow-growing, long-lived species decreased, supporting the hypothesis of an increase in fast-growing, early-maturing species and a decline in slow-growing, long-lived species in fished systems. Shifts in demersal fish assemblages coincide temporally with spatial shifts observed in West Coast rock lobster Jasus lalandii and with regime shifts in the pelagic ecosystem. The changes in demersal fish assemblages detected are probably a reflection of long-term indirect effects of fishing in combination with environmental changes. [ABSTRACT FROM AUTHOR]