Mytilus galloprovincialis is one of recent bivalve species produced in Portugal offshore. The growth, condition index, and the fatty acid composition of the mussel produced at 2 and 13 m depth was compared in longline on the south coast of Portugal. For each depth, a monitoring of several water quality parameters such as temperature, turbidity, nutrients, and chlorophyll-a was carried out. There were no significant differences between mussel growths at different depths. On the first sampling, shell length mean was 24 mm, reaching 50 mm 9 months after. High dispersion of mussel length was observed due to slow growers. The condition index shows a typical seasonal variation, with high values in autumn and late spring and minimum values in January. Analysis of fatty acids did not reveal differences at both depths and DHA levels (8.71 mg/g) were generally higher than EPA (5.90 mg/g). The water quality revealed few differences in the environmental conditions in the water column, which may explain the similarities between the quality of mussels produced at different depths. Based on these data, the mussel offshore production in that area can be done at this range of profundities 2 and 13 m, without affecting growth and fatty acid composition.