Growth of juvenile common octopus (Octopus vulgaris) in Korean waters is affected by shelter type

This study evaluated the effects of shelter pipes (PS), tube shelters (TS), and no shelter (NS) on the growth of juvenile common octopus (Octopus vulgaris). The grow-out was studied in situ by conducting a passive follow-up survey using integrated transponder tags. The growth and survival rates were evaluated in two experiments. Each experiment included two replicates. Both experiments ran for 60 days, for which each trial included 24 octopuses that were fed once a day a diet of frozen sardines (Sardinops melanostictus) and swimming crabs (Portunus trituberculatus), at 3–8% body weight. We investigated the relationship between the beak microstructure of the common octopus and possible stress events caused by culture conditions at 15 °C for 15 days. The nine juveniles were separated into three groups (NC: natural conditions, PS: culture conditions as a group, and TS: culture conditions with individuals kept separately) to analyze the amount of stress caused by rearing conditions. Stress marks were observed using a high-resolution stereo fluorescence microscope. In Experiment 1, the specific growth rates (SGRs) were 0.20 ± 0.15% per day in the PS group, 0.24 ± 0.10% per day in the TS group, and 0.68 ± 0.48% per day in the NS group. The survival rate was 62.5 ± 6.25%, 37.5 ± 18.75%, and 12.5% in the TS, PS, and NS groups, respectively. The experimental conditions of Experiment 2 were the same as those in Experiment 1, except that Experiment 2 involved different individuals and occurred during a different season. In Experiment 2, the SGRs were 0.54 ± 0.20% per day in the PS group, 0.59 ± 0.22% per day in the TS group, and 0.44 ± 0.10% per day in the NS group. Survival rate was 87.5 ± 6.25%, 75 ± 6.25%, and 75% in the TS, PS, and NS groups, respectively. Regression analysis showed that the water temperature (from 5.4–23.1 °C) affected the growth of octopuses in Experiments 1 and 2. Capture stress was detected in six of the nine individuals (NC, PS, and TS combined). Statistically significant differences in the stress levels were detected between the PS and TS groups (P ≤ 0.1), with the NC group exhibiting no stress lines after initial adaptation, unlike PS. In conclusion, octopuses kept in TS were not affected by stress after initial adaptation.