Modeling and comparing the distributions and associations of two co-occurring catshark species off South Africa.

Holohalaelurus regani and Scyliorhinus capensis are relatively small catsharks, which both occur off the continental shelf around South Africa and are often caught together as by-catch in demersal trawls. The present study used data collected during annual demersal research surveys conducted between 2009 and 2015 and is a first attempt at modeling the potential intra- and interspecific associations, by maturity stage and depth, of H. regani and S. capensis to elucidate species-specific patterns in their distributions in South African waters. Intraspecifically, both species displayed a wide overlap in distribution between maturity stages, but only H. regani displayed marked changes in distribution with maturity stage, with mature individuals found further eastwards and occupying deeper waters than immature individuals. Interspecifically, the two catsharks displayed an inverse relationship in their distribution, with an increase in the abundance of H. regani and a decrease in the abundance of S. capensis when moving from the south coast to the west coast. Some localized patches of co-occurrence were, however, evident between species and maturity stages, especially in offshore areas. Overall, our results indicated a stronger co-occurrence of mature and immature stages within each species and a rather weak co-occurrence of maturity stages between the two species. The spatial information provided in the present study provides useful information on how sharks with similar morphologies and lifestyles may partition their habitat as a mechanism for potentially reducing competition between them.

Spatial, ontogenetic and interspecific variability in stable isotope ratios of nitrogen and carbon of Merluccius capensis and Merluccius paradoxus off South Africa.

General linear models (GLMs) were used to determine the relative importance of interspecific, ontogenetic and spatial effects in explaining variability in stable isotope ratios of nitrogen (δ(15) N) and carbon (δ(13) C) of the co-occurring Cape hakes Merluccius capensis and Merluccius paradoxus off South Africa. Significant GLMs were derived for both isotopes, explaining 74 and 56% of observed variance in Merluccius spp. δ(15) N and δ(13) C, respectively. Spatial effects (west or south coast) contributed most towards explaining variability in the δ(15) N model, with Merluccius spp. off the west coast having higher (by c. 1.4‰) δ(15) N levels than Merluccius spp. off the south coast. Fish size and species were also significant in explaining variability in δ(15) N, with both species showing significant linear increases in δ(15) N with size and M. capensis having higher (by c. 0.7‰) δ(15) N values than M. paradoxus. Species and coast explained most and similar amounts of variability in the δ(13) C model, with M. capensis having higher (by c. 0.8‰) δ(13) C values than M. paradoxus, and values being lower (by c. 0.7‰) for fishes off the west coast compared with the south coast. These results not only corroborate the knowledge of Merluccius spp. feeding ecology gained from dietary studies, in particular the ontogenetic change in trophic level corresponding to a changing diet, but also that M. capensis feeds at a slightly higher trophic level than M. paradoxus. The spatial difference in Merluccius spp. δ(15) N appears due to a difference in isotopic baseline, and not as a result of Merluccius spp. feeding higher in the food web off the west than the south coast, and provides new evidence that corroborates previous observations of biogeographic differences in isotopic baselines around the South African coast. This study also provides quantitative data on the relative trophic level and trophic width of Cape hakes over a large size range that can be used in ecosystem models of the southern Benguela.