Trophic relationships of prey species (Etrumeus Whiteheadi, Myctophidae and Sepia) and their predators (Merluccius Capensis, Merluccius Paradoxus, Lophius Vomerinus andTrachurus Capensis) off Namibia
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Date
2015
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Publisher
University of Namibia
Abstract
This study was conducted in Namibian waters that forms part of the Benguela current ecosystem. This upwelling-driven ecosystem supports high abundances of fish species, some of which have been the backbone of Namibian commercial fishery for decades. The study of the trophic relationships of prey species of commercial fish is important, as it improves understanding of the ecology of their predators and considers multi-species interactions in fisheries management. The objectives of this study were to assess the trophic levels of Sepia spp, Etrumeus whiteheadi and Myctophidae, to determine the trophic relationships among these species, to identify their potential trophic roles in the marine ecosystem and to determine the likely contributions of these prey species to the diet of Merluccius capensis, Merluccius paradoxus, Lophius vomerinus and Trachurus
capensis, using stable isotopes. Muscle tissues were sampled from E. whiteheadi,
Myctophidae, Sepia spp, M. capensis, M. paradoxus, L. vomerinus and T. capensis. Isotope analyses revealed that all the prey species analysed are on the same trophic level except for L. hectoris that fed at a slightly higher trophic level. δ15N values of prey species varied among all prey species. Symbolophorus boops had the most depleted δ15N, while L. hectoris had the most enriched δ15N values. Significant differences were noted in δ13C', with D. hudsoni having the most depleted and E. whiteheadi the most enriched δ13C'. Isotope-based population metrics showed over lapping of trophic niches of all species, with S.boops having a significantly wider trophic niche. All prey species analysed are important in the ecosystem since they all contributed to the diet of the four predators, although their contribution varied. A Bayesian isotope mixing model showed no significant difference in relative contribution of prey an indication that prey availability is possibly a greater determining factor of prey dietary contribution. Etrumeus whiteheadi was a dominant prey item in the diets of the predators with an
exception of that of M. paradoxus. This study contributes towards understanding of prey trophic interactions, which can aid the implementation of an ecosystem approach to fisheries management in Namibia.
Description
A thesis submitted in fulfillment of the requirements for the Degree of Master of Science
Keywords
Stable isotopes, Trophi niche, Trophic relationships, Trophic level, Prey species, Carbon