Reducing the Environmental Impact of Sea-cage Fish Farming Through Cultivation of Seaweed

Cultivation of Laminaria saccharina (Linnaeus) Lamouroux and Palmaria palmata (Linnaeus) Kuntze was trialled at three fish farm sites in north-west Scotland. Results show that seasonal yields of L. saccharina were enhanced by 50 % and P. palmata by 63 % when cultured adjacent to fish farm cages compared to environmentally similar sites away from the farms. Yields of P. palmata were further enhanced under conditions of optimal water movement.

Ammonium concentrations in the seawater 0 to 50m away from the fish cages were found to be 2 - 3 μM greater than ambient. Enhanced concentrations of ammonium could be detected 200 to 300 m from the cages although the distribution is heavily influenced by local hydrography.

Nitrogen content of L. saccharina and P. palmata cultured adjacent to the salmon cages in summer was higher than in seaweeds cultured at reference sites away from fish cages.

Stable nitrogen isotope analysis indicates that the nitrogen in seaweeds grown next to salmon cages is derived from the fish farm and farm derived nitrogen is likely to be widely dispersed in the lochs where cages are situated.

A preliminary economic analysis showed that growing seaweeds commercially, in particular P. palmata, may be at worst cost neutral, with profitability increasing as a result of enhanced production through increased nutrient availability adjacent to fish farms. A one hectare seaweed farm producing 600 tonnes wet weight over two years (300 tonnes per year) of P. palmata could potentially absorb up to 30 % of nutrients generated from a 500 tonnes salmon production unit.

As farm origin nitrogen is evident in biota at distances of up to one kilometre away from the cages, cultured macroalgae would not have to be sited close to cages to result in net nitrogen removal facilitating the siting of algal farms in areas more suited for individual species requirements while still maintaining bioremediation benefits.