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 δ13C and δ15N 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 δ15N 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 δ13C with size was observed in MC but not MP. In all size classes (except ≤20–29 cm) MC expressed significantly higher δ13C values, generally matching their respective shelf‐slope adult distributions. Relative to zooplankton and nekton prey, smaller hake of both species (20–39 cm) were trophically indistinguishable at a trophic level (TL) of 3.3, indicating predominant zooplanktivory. The largest MC of 60–70 cm 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.

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

“…paradoxus is consistent with a nearshore–offshore depletion in 13 C of pelagic production. This was also observed by Iitembu et al () for the two Merluccius species off Namibia, and has been seen in other ecosystems (Miller et al , ). The higher δ 13 C in both Merluccius species off the south coast observed in this study, however, was contrary to what would be expected, given the higher upwelling and diatom‐based production off the west coast.…”

“…capensis by Parkins (). Similarly, Iitembu et al () reported significant positive relationships between δ 15 N and size for both M . capensis and M .…”

“…stable isotope ratios off the south coast, which appears likely and which should be attempted if additional data are collected. There is a relatively short shelf and shelf break to deeper water off Namibia, and Iitembu et al () observed a strong delineation in δ 13 C between M . capensis and M .…”

“…capensis trophic ecology using stable isotope ratio analysis of carbon (δ 13 C) and nitrogen (δ 15 N), conducted on shallow water Merluccius sp. off South Africa (Parkins, ), and more recently on both Merluccius species off Namibia (Iitembu et al , ).…”

Biology and fisheries of the shallow‐water hake ( Merluccius capensis ) and the deep‐water hake ( Merluccius paradoxus ) in Namibia

“…However, stable isotope-based results indicated that small hake (20-39 cm) of both species were trophically indistinguishable at about 3.3, indicating predominant zooplanktivory. The trophic levels of M. capensis and M. paradoxus of 60-70 cm were estimated at 3.4-3.6 and 3.7-3.8, respectively (Iitembu et al, 2012).…”