Trophic ecology of squids in the Benguela Upwelling System elucidated by combining stomach content, stable isotope and fatty acid analyses

Squids occur worldwide in marine ecosystems and play a major role in pelagic food webs by connecting lower and higher trophic levels. Their high feeding intensities and pronounced diel vertical migrations make squids important components of nutrient cycles in marine ecosystems. This study assessed the trophic position and nutritional ecology of a squid species assemblage in the Benguela Upwelling System (BUS) by combining stomach content, stable isotope and fatty acid trophic marker analyses. Samples were collected in the northern (nBUS) and southern (sBUS) BUS in austral spring 2021. A total of 20 squid species from eleven families were identified. Stomach content analyses showed that squid species preyed on a variety of organisms ranging from crustaceans to lanternfishes (Myctophidae) and flying squids (Ommastrephidae). Stable isotope analyses revealed significant differences in delta N-15 values of small squids (dorsal mantle length < 10 cm) between the two subsystems, but none in those of large squids (dorsal mantle length > 10 cm). Trophic levels ranged from 2.2 to 5.0. Isotopic niche width was widest in the families Ommastrephidae and Cranchiidae. Principal component analyses of fatty acid compositions displayed distinct clusters separating squid families and different prey taxa. This study shows that nBUS and sBUS squids exploit a large variety of pelagic prey organisms and that trophic differences are primarily dependent on squid species and size. The results emphasize the squids' importance as interlinking element in pelagic food webs and their key function in energy transfer between epi- and mesopelagic layers.