Trophic Relationships Of Prey Species (Etrumeus Whiteheadi, Myctophidae And Sepia) And Their Predators (Merluccius Capensis, Merluccius Paradoxus, Lophius Vomerinus And Trachurus Capensis) Of

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.

Key words: stable isotopes, trophic niche, trophic relationships, trophic level, prey

species, carbon, nitrogen, Etrumeus whiteheadi, Myctophidae, Sepia spp.