Comparison of ontogenetic trophic shift in two hake species, Merluccius capensis and Merluccius paradoxus, from the Northern Benguela Current ecosystem (Namibia) using stable isotope analysis

Abstract

Two species of hake, Merluccius capensis (MC) and Merluccius paradoxus (MP), together account for most of Namibia's fisheries catch and they are collectively the most important secondary consumers in the Benguela Current ecosystem. To better resolve their feeding behavior in the northern Benguela Current ecosystem, we examined the size-specific ontogenetic shift in MC and MP using stable isotopes of δ 13C and δ 15N as measures of relative source production and trophic level. We also compared hake isotope values to those of their major zooplankton (four species) and nekton (12 species/groups) prey. Results from δ 15N showed a significant positive relationship with size in both MC and MP; however, the slopes of the two species were significantly different, with MP displaying a steeper trophic shift. A significant increase in δ 13C with size was observed in MC but not MP. In all size classes (except ≤20-29cm) MC expressed significantly higher δ 13C values, generally matching their respective shelf-slope adult distributions. Relative to zooplankton and nekton prey, smaller hake of both species (20-39cm) were trophically indistinguishable at a trophic level (TL) of 3.3, indicating predominant zooplanktivory. The largest MC of 60-70cm had TLs of approximately 3.5-3.6, whereas MP of the same size were slightly higher, at 3.7-3.8 TL, indicating greater piscivory. This is the first spatially and ontogenetically extensive stable isotope analysis of MC and MP in the Benguela Current. Further extension of this analysis throughout the hake range would provide considerable insight into the trophic dynamics of this commercially and ecologically important group. © 2012 Blackwell Publishing Ltd.