Seasonal- and depth-dependent growth of cultivated kelp (Saccharina latissima) in close proximity to salmon (Salmo salar) aquaculture in Norway

The interest to develop an industrialized cultivation of several macroalgae in Europe is growing rapidly. Here, we demonstrate cultivation of the sugar kelp Saccharina latissima in integration with salmon (Salmo salar) aquaculture in a Norwegian coastal area. Sporophytes of S. latissima were deployed at 2, 5 and 8 m depths at a salmon farm and at a reference station 4 km away in August, November, February and June (2010-2011). The growth was good in late autumn and in spring, with peak lengths of the sporophytes in June, while being poor in winter and summer. As a result of a faster initial growth at the fish farm than at the reference station from August to November, the August-sporophytes reached a significantly longer length than those at the reference station in 5 out of 10 sampling months at 2 m depth, and in 9 out of 10 months at 5 m depth (p < 0.05) over the year, while no significantly different lengths were found at 8 m depth. The November-sporophytes showed similar growth rates and lengths at the fish farm and at the reference station, while the February-sporophytes grew faster at 5 and 8 m depths at the fish farm than at the reference station, with significantly longer blades at 5 m depth at the fish farm than at the reference station at peak lengths in June (p < 0.05). The sporophytes deployed in June did not survive the summer. Holding the rapid growth of S. latissima in spring and early summer together with the increase in salmon biomass and feed use in late summer and early autumn suggested a seasonal mismatch considering direct recycling of the nutrient input from salmon farming by macroalgae in Norwegian coastal waters.