Diet and trophic role of octopus on an abalone sea ranch

Abstract

Gastric 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.

Food Web Trophic Structure at Marine Ranch Sites off the East Coast of Korea

“…The slight shift in TL values between sites appeared to reflect a spatially varying consumption of predatory invertebrates. A similar mid-trophic position has been reported for Octopus tetricus consuming large volumes of abalone in a marine ranching food web (Greenwell et al, 2019). Earlier studies reported that GPOs are preyed upon by large fish, marine mammals (such as sea otters, harbor seals, sea lions, and killer whales), sharks, and even other octopuses (Haley, 1978;Hart, 1980;Gillespie et al, 1998).…”

“…Its δ 13 C values were rather close to benthic-affinity prey, displaying a slight spatial variation. This result indicates an opportunistic feeding habit, which primarily exploits the most abundant prey type available (Vincent et al, 1998;Scheel and Anderson, 2012;Greenwell et al, 2019). The δ 13 C values of E. dofleini were similar to some teleosts, indicating an overlap in the primary carbon sources utilized between them (Kang et al, 2008).…”

“…The δ 15 N values of E. dofleini corresponded to the lower limit of the range for teleosts, probably reflecting exclusive ingestion of primary consumers (i.e., benthic and bentho-pelagic invertebrates) with little consumption of teleosts based on other factors such as strong avoidance and predation risk (Vincent et al, 1998). As a result, their foraging behavior seemed to be associated with general prey availability and accessibility rather than preference of prey species, as indicated by their narrow δ 13 C range (Scheel and Anderson, 2012;Greenwell et al, 2019). On the other hand, the inter-individual variability in both δ 13 C and δ 15 N values and trophic fractionation of E. dofleini depending on starvation and/or food limitation can be expected (Doi et al, 2017;Gorokhova, 2018).…”

“…Although the prey size generally increases as the GPO grows (Hartwick et al, 1981), E. dofleini individuals take a wide range of prey sizes, reflecting foraging behavior with no preysize selectivity (in individuals weighing >2.5 kg, for example; Vincent et al, 1998). Therefore, it can be summarized that the dietary composition of E. dofleini might reflect various intrinsic and extrinsic factors in different habitats (Vincent et al, 1998;Greenwell et al, 2019).…”

“…To do this, we examined δ 13 C and δ 15 N values of the GPO and other sympatric species collected from artificial reefs deployed to create nursery and spawning grounds to determine whether its trophic interaction varies among habitats where community compositions differ from one another. Given the documented foraging behavior of GPO on available resources (Greenwell et al, 2019), it is expected that δ 13 C and δ 15 N values of E. dofleini would reflect trophic pathways supporting its nutrition and trophic position within the marine food web. We tested this hypothesis by comparing the isotope composition of E. dofleini and different community components in two octopus marine ranches.…”