Shallow raceways as a compact, resource-maximizing farming procedure for marine fish species

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Abstract
To develop a compact farming system, a shallow raceway has been tested on five flatfish, three bottom-dwelling species and three open-water species. The main effort has been on turbot Scophthalmus maximus L., Atlantic halibut Hippoglossus hippoglossus L. and spotted wolffish Anarhichas minor (Olafsen). The system is almost a standard raceway, but with a very low water level (7 mm for fish at about 100 mg; 25 cm for fish above 2 kg). The system is a package with some crucial preconditions. These include high fish density (often 100–500 kg m− 3), no countercurrent in the levelled raceways (no jet currents), adjustment of water intake with the most remote fish in mind and feeding with floating pellets. Self-cleaning is easily obtained in the system. The system has been tested for a wide size range of raceways (0.07–80 m2) and fish sizes (2 mg to 10 kg), normally with growth and survival rates as good as with traditional rearing systems. There seems to be a ‘learning window’; the earlier the fish are introduced to shallow raceways, the better they perform. The results indicate that a variety of fish species can be produced in shallow raceways; these can be stacked in racks, which facilitate re-use of water from level to level. A very compact and cost-effective farming system will be the outcome.