Are hakes truly opportunistic feeders? A case of prey selection by the Argentine hake Merluccius hubbsi off southwestern Atlantic

Abstract

Prey selection and feeding strategy of the Argentine hake Merluccius hubbsi were investigated by analyzing the diet and prey availability in the main nursery area of the Patagonian stock (42.4°–46.4 °S, 70–102 m depth). Stomach contents from fish captured using a bottom trawl net, and zooplankton samples collected with a Bongo net, were analyzed. Acoustic data were examined to identify pelagic aggregations and quantify relative prey abundance. The Argentine hake M. hubbsi fed mainly on crustaceans (92.7%IRI), followed by fish (7.1%IRI) and cephalopods (0.2%IRI). Among crustaceans, the diet was dominated by euphausiids (Euphausia spp.), the lobster krill Munida gregaria, and the hyperiid amphipod Themisto gaudichaudii. The most consumed fish was other hake by cannibalism. Prey availability was also numerically dominated by the euphausiids (Euphausia spp.), followed by the hyperiid amphipod T. gaudichaudii and the lobster krill M. gregaria. Ontogenetic dietary changes were detected: consumption of euphausiids and amphipods decreased with predator´s total length, and the consumption of the lobster krill M. gregaria was higher in adult specimens. The Argentine hake M. hubbsi showed positive prey selection for lobster krill M. gregaria. The consumption of Euphausiids decreased with the increasing acoustic abundance of the lobster krill M. gregaria. The cannibalism was denso-independent. The Argentine hake appeared to be opportunistic when the proportion of prey eaten to the proportion present in the environment was compared, but deeper analyses confirmed that zooplankter with increased abundances and higher energy density was the preferred prey.