Growth and mortality of common octopus Octopus vulgaris reared at different stocking densities in Mediterranean offshore cages

Four ongrowing cycles of common octopus (Octopus vulgaris), each with two stocking densities, were carried out in an offshore area in order to evaluate the effect of the following variables on growth (G, % body weight day⁻¹) and mortality (M, % day⁻¹): weight (W, 0.5-3.0 kg), temperature (T, 14-26 °C), stocking density (SD, 6-46 kg m⁻³), size dispersion, expressed as coefficient of variation (CV, 13-42%) and significant wave height (SWH: 0.4-1.2 m). The assays were performed in an 8 m³ stainless-steel floating cage divided into two compartments of 4 m³ each. For the range of temperatures considered (14-26 °C), G depended significantly on T and SWH, with maximum G values being obtained at 18.5 °C and with an important negative effect of SWH. M depended significantly on T, W and CV, mortality being minimal at 18 °C. The two other variables had an antagonistic effect, mortality increasing with greater size dispersion - suggesting that animals should be graded throughout the process - although the effect diminished as the sizes increased. The results point towards two alternatives for the commercial ongrowing of octopus under the particular conditions of the present study: (A) two ongrowing cycles of 3.5 months to reach a final weight of 2.5 kg or (B) one 5-month cycle to reach a final weight of 3.5 kg.