Trophic Relationships Of Shallow Water Cape Hake (Merluccius Capensis) And Cape Horse Mackerel (Trachurus Capensis) In The Northern Benguela Ecosystem

ABSTRACT

Shallow water Cape hake (Merluccius capensis) and Cape horse mackerel (Trachurus capensis) are ecologically and commercially important species in the northern Benguela ecosystem (Namibia). The understanding of their trophic relationships is however still limited. In this study stable isotope measurements [carbon (δ13C) and nitrogen (δ15N)] of their muscles and stomach contents were used to investigate their feeding interactions. Understanding the feeding interactions of these two species is vital, in order to consider trophic dynamics in their fisheries management strategies. Muscle tissues (n = 404) and stomach contents (n = 404), were collected during November 2017 bottom trawl survey in Namibian waters. Results indicated that krill (Euphausiids) was a dominant prey in the diet of T. capensis and M. capensis. The diet compositions of T. capensis remained the same with both total length and latitude but not for M. capensis, an indication of the influence of prey availability. A potential for interspecific feeding competitions between the two species was observed as krill and anchovy were found as their common prey species. The feeding interactions of the two species appear to have not changed much over time. Significant differences were found in both δ15N values and δ13C values of the two species. Although niche overlap was observed, a wider niche for M. capensis than T. capensis was observed; an indication of M. capensis broader trophic diversity and a more specialized niche of T. capensis. This is the first study that has combined stable isotopes and stomach content analysis methodologies, to understand the feeding interaction of M. capensis and T. capensis.

Keywords: Stable isotopes, Stomach content, Trophic relationship, Northern Benguela Ecosystem, Shallow water Cape hake, Cape horse mackerel