Johannes Iitembu, Some aspects of the trophic relationships of hake and demersal sharks off Namibia

Oral Presentation

Johannes A. Iitembu1,2 and Nicole B Richoux2
1National Marine Information and Research Centre (NatMIRC), Ministry of Fisheries and Marine Resources, Strand Street Box 912, Swakopmund, Namibia
2Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa

Hake (M. capensis and M. paradoxus) has been a major constituent of Namibian commercial fishery for decades. Squalid sharks (C. squamosus, D. calcea and D. profundorum) off Namibia have an overlapping depth distribution with the two hake species. In addition to feeding on hake, these sharks share common prey with hake. Samples of the sharks (n=42), hake (n=106) and prey (n=201) species were collected at the same time and region, during annual hake biomass survey off Namibia. To better resolve their predator-prey relationships, stable isotopes (nitrogen (15N) and carbon (δ13C)) of their muscles, and that of potential prey were analysed. Results indicate that, sharks species had more enriched δ15N than hake species. C. squamosus fed at higher trophic level than all hake species and sharks species considered. The two species of shark from Deania genus were not different from each other in term of δ15N, and fed on similar prey that overlapped with that of M. paradoxus. δ13C indicated that hake and shark species fed on prey from similar basal resources. There was significant difference in δ13C between M. paradoxus and C. squamosus, showing that they fed on different preys. The δ13C range and δ15N range indicated that hake species had higher diversification of the carbon sources as well as enhanced trophic diversity in their feeding pattern than shark species. Feasible contributions to isotopic mixture of sharks indicated that hake species are not major prey of C. squamosus. There are modest contributions of hake species to the diets of sharks from Deania genus .The sharks from Deania genus fed on diverse prey than C. squamosus whose diet was dominated by crustaceans. Overall these results support the hypothesis that hake and sharks have overlapping trophic niches and feeding ranges.

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