Diet and trophic role of octopus on an abalone sea ranch

Abstract

AbstractGastric tract (crop and stomach) and stable isotope analyses (SIA) were used to evaluate the dietary composition of resident, commercially important Octopus aff. O. tetricus Gould, and its importance as a predator of Haliotis laevigata Donovan, on artificial abalone habitats. Taxonomic diversity and resolution of diet were higher in crops (13 taxa) than stomachs (10 taxa), with the latter dominated by unidentifiable material (%Volume = 74%). Crops contained mainly molluscs (~54%, including 31% abalone) and crustaceans (33%), suggesting octopus are important predators of abalone on the sea ranch. Dietary composition of crops did not differ with increasing octopus weight (<300 g, 300–999 g, ≥1,000 g wet weight), paralleling the findings of SIA and reflecting the high stocking densities of abalone that are able to be consumed by octopus of all sizes. The SIA also revealed that octopus occupy a mid‐trophic level, slightly below teleosts and loliginid squids.