The profitability of octopus ongrowing remains limited because low cost artificial diets have not yet been effectively developed. Therefore, alternative, low cost diets are worth exploring. In this study, we have investigated the growth and mortality of octopuses fed strictly on mackerel (Scomber colias), a low cost fish, readily available in the Azores Archipelago. Different weight classes (150–300; 300–600; and 600–1,000 g) were maintained at two stocking densities; 5 versus 12 individuals (ranging between 0.38–2.72 kg m^-3). The trial lasted 72 days and was performed in an open system composed of large rectangular tanks. Each individual was tagged, and growth was followed weekly. Mean instantaneous growth rate (G) varied between 0.67 and 1.47 (% increase in body weight day^-1), depending on initial body size. Survival was 100% for octopuses held in tanks at low density. On the other hand, increased aggressive interactions in high density tanks resulted in higher mortality rates (MR), especially for the smallest size class (MR = 66.7%). The results of this study suggest that with a monodiet of mackerel, subadult octopuses of 300 g can reach commercial size (750 g) in 2.5 mo. However, high mortality under elevated stocking densities will hinder commercial profitability with such diet. A proposed approach to increase survival would be to periodically supplement the monodiet with crustacean flesh.