Insights into feeding interactions of shallow water cape hake (Merluccius capensis) and cape horse mackerel (Trachurus capensis) from the Northern Benguela (Namibia)

Abstract

Shallow water cape hake (SWCH) (Merluccius capensis) and cape horse mackerel (CHM) (Trachurus capensis) are ecologically and commercially important species in the northern Benguela ecosystem (Namibia). The understanding of their feeding interactions is however still limited. In this study, stable isotope measurements [carbon (δ13C) and nitrogen (δ15N)] of their muscles and stomach contents were used to understand their feeding interactions. Muscle tissues and stomach contents (n= 404), were collected during bottom trawl surveys in Namibian waters (November 2017). Results indicated that krill (Euphausiids) was the dominant prey in the diet of CHM and smaller SWCH, while the importance of CHM in the diet of larger SWCH was observed. Jacopever (Helicolenus dactylopterus) dominated the diet of SWCH that were larger than 51 cm. The diet compositions of the two species changed with latitude, 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. Significant differences were found in both δ15N values and δ13C values of the two species. A significant positive relationship between δ13C values and size were observed for both species. A negative relationship between δ15N values and size was observed in CHM. The length ranges of isotopic intersections were 34 to 36 cm for δ15N and 32 cm to 40 cm for δ13C, an indication that young SWCH possibly interacts more with larger CHM. Although there was niche overlap, a wider niche for SWCH than CHM was observed. This is the first study that has combined stable isotopes and stomach content analysis methodologies, to understand the feeding interaction of SWCH and CHM.