` Ulva lactuca from an Integrated Multi-Trophic Aquaculture (IMTA) biofilter system as a protein supplement in gilthead seabream (Sparus aurata) diet

Protein derived from fishmeal is the most expensive ingredient in fish feeds. Any reduction and replacement of this ingredient by a less expensive protein source which will produce the same growth performances will contribute significantly to the reduction of fish production cost. Protein-rich U. lactuca, used as biofilter in an IMTA system, was evaluated as a dietary ingredient for gilthead seabream (Sparus aurata), at moderate (3-8%) and high (up to approximately 30%) replacement of fishmeal ratio. Three main elements were considered: 1. The total Ulva biomass replaced as a proportion of the total feed biomass. 2. The relative reduction of fishmeal sources in the feed, and 3. The economic benefit of the fishmeal replacement. Growth performances similar to those of fish fed the commercial feed used as control were obtained in two ways: a) by reducing ingredients of animal source in the feed from 41% to 29.1% of the total food biomass, and b) by removing 100% of fishmeal and adding up to 14.6% U. lactuca of the total food biomass. When 260 g kg(-1) of fishmeal was removed entirely and replaced with 291 g kg(-1) of poultry meal and 146 g kg(-1) of U. lactuca, the total cost of feed was reduced by $ 0.25 kg(-1). With a FCR of 1.7, the saving was of $ 0.45 per 1 kg fish produced. Since in intensive aquaculture fish feeds represent over 60% of the operating costs, saving close to 10% on the cost of the feed is economically relevant. In addition, reducing nitrogen loads from the effluents by Ulva biofilter will also save on water treatment costs, an additional advantage of IMTA system.