Recapture of manure from open fish farming cages – the missing link for a circular system

The overall aim of the project was to develop a technical system for recovering manure and feed residues (particulate waste) from the cage-based aquaculture in Sweden. This can eliminate the loss of nutrient-rich sludge to the surrounding environment and the risk of sediment accumulation during cultivation while at the same time utilizing the nutrients as fertilizer in plant cultivation.

The project also included a biological evaluation of the waste as a fertilizer, to study legislation, calculate investment and operating costs and to make recommendations to growers and authorities. Another part of the project was to evaluate whether different feed technology solutions can increase recapture rate by changing the structure of the faeces.

An important side effect was that the project, through its design, ensured future competence and collaboration in both the cultivation industry and the academy and creates opportunities for a more objective dialogue between growers, authorities and the public while contributing to a more environmentally friendly circular food production.

The project was part of SLU's work on circular food production for fish, where the work in this project is linked to our work on new and local feed raw materials. The goal is to grow fish without importing more nutrients into the Baltic Sea watershed/catchment area. In the process the project produce knowledge in support of new work opportunities as well as a way to enrich unnatural oligotrophic hydropower dams of Northern Sweden, while producing a world class food commodity in the form of farmed rainbow trout and Arctic charr.

In addition to reduced leakage of nitrogen, phosphorus and carbon, several positive so-called "spin-off effects" were anticipated, such as a reduced risk of transmitting infectious diseases between farmed and wild fish, where particles from the farm can work as vectors for patogenes. The work was done in close collaboration between academia, the aquaculture industry and consultancy expertise.

The work was divided into 5 integrated work packages led by a steering group consisting of SLU (project manager), Egersund Group AS, Sweco and Vattudalen's fish. An advisory group consisting of cultivation expertise from Sweden (Matfiskodlarna) and technical and biological expertise from NMBU (Norwegian University of Life Sciences). Through the main partners the project had access to a global network and the broad competence of Norwegian salmon industry.

A key question for the project was whether a recapture of particular waste can create room for expansion of the farming industry in Sweden. Properly sized and located fish farming contributes to both jobs, production of good and healthy food, while the natural production in surrounding water can be stimulated where it is currently decreasing. This without changing the water status classification or violating applicable standards. In addition, the waste can be recycled and the industry used for, for example, fertilization of forest and agricultural production.

The project also aimed at increased economic efficiency through improved feed utilization as well as a reduced infectious risk both inside and outside the farm. All in all, this could increase the competitive advantages for fish farmers in Sweden. With increased knowledge and a reduced environmental impact, there is also great potential for simplified decision-making permit processes and increased production volumes for both new start-ups and already established farms.

This project was an innovation project and a collaboration between academia, authorities and the fish farming industry. The project ran during the years 2014-2021 and was financed by the European Maritime and Fisheries Fund, together with industry.

Partners:

SLU

Sweco

Matfiskodlarna

VCO

Vattudalens fisk AB

Egersund Net

Egersund Markleen

Fjord Solutions