Characterising the trophic relationships between cuttlefishes, myctophids and round herring in the Northern Benguela

Abstract

Round herrings, myctophids, and cuttlefishes are a crucial part of the trophic interactions in the northern Benguela, as they form important trophic linkages between macrozooplankton and predators like hake, horse mackerel, and monk. The main objective of this study was to understand their trophic relationships using stable isotope measurements of their tissues. Tissues from round herring [Etrumeus whiteheadi], Myctophid species [Symbolophorus boops, Lampanyctodes hectoris, Diaphus hudsoni, Lampanyctus australis, Diaphus meadi], and cuttlefishes [Sepia australis and Sepia elegans] specimens were collected off Namibia. There were little variations in nitrogen stable isotopes (δ15N) values among these species, an indication that they fed at similar trophic positions. The trophic levels ranged from 2.51 to 3.00, an indication of their zooplanktivorous feeding habits. Among these species S. boops fed at a relatively higher trophic level, while L. hectoris fed at the lowest trophic level. Diaphus hudsoni had the lowest carbon stable isotope (δ13C) values and E. whiteheadi the highest. Significant differences were observed in δ13C values between most species, suggesting significant variations in their carbon sources. Isotope-based metrics showed overlapping trophic niches, with S. boops having a significantly broader niche. Our observations support the hypothesis that although these forage species have overlapping trophic niches, there are pronounced differences in the carbon sources of their prey.