Trophic relationships of hake (<i>Merluccius capensis</i> and <i>M. paradoxus</i>) and sharks (<i>Centrophorus squamosus</i>, <i>Deania calcea</i> and <i>D. profundorum</i>) in the Northern (Namibia) Benguela Current region

Abstract

The trophic relationships of two hake species (Merluccius capensis and M. paradoxus) and three shark species (Centrophorus squamosus, Deania calcea and D. profundorum) were investigated using nitrogen and carbon stable isotope signatures (δ¹⁵N and δ¹³C) of their muscle tissues. The sharks were more enriched in 15N than the hake, an indication of the apex predator status of sharks. Among the sharks considered, C. squamosus occupied the highest trophic level and fed primarily on benthic prey. The two species of shark from the genus Deania were not different based on δ¹⁵N or δ¹³C, so they had similar diets. The δ¹³C signatures indicated that M. capensis and sharks fed on prey derived from similar basal resources. However, there was a significant difference in δ¹³C between M. paradoxus and all other species examined, suggesting that they occupied different feeding niches. Isotope-based population metrics showed narrower trophic ranges in sharks than M. capensis. Carbon and nitrogen ranges indicated that hake fed on a more diverse pool of carbon sources and had generally more enhanced trophic diversity in their feeding patterns than sharks. Among the species considered, C. squamosus occupied a unique isotopic space. Our results supported the hypothesis there are trophic overlaps among these species, but some interesting differentiation was revealed.

Keywords: diet, ecological role, food web, stable isotopes, standard ellipse area, trophic niche, top predators