Raceway and Flow-Through Aquaculture: A Guide
How flow-through raceways work: linear tanks fed by continuous cold clean water, design and slope, oxygen and self-cleaning, carrying capacity, and the trade-offs of trout and salmon culture.
Overview
A raceway is a long, narrow channel or tank through which water flows continuously, and flow-through culture is a form of intensive aquaculture in which fish are held at high density in such units. FAO describes the flow-through system as one in which fish are stocked densely in a long, narrow pond or tank with an abundant continuous water flow. The incoming water supplies oxygen and flushes away metabolic wastes, so production depends directly on a reliable supply of clean water. The method is the traditional facility for trout farming.
Design and layout
Raceways are typically rectangular basins, often of reinforced concrete, arranged so water enters one end and leaves the other. A length-to-width ratio of at least about 6:1 is used so that water moves along the channel rather than allowing fish to swim in circles. A slight gradient, on the order of 1–2 percent, helps water flow by gravity and assists self-cleaning by carrying settled solids toward the outlet. FAO notes that water velocity should be kept below a level that stresses the fish.
Water supply and turnover
Flow-through culture requires large volumes of cold, clean, well-oxygenated water, usually drawn from a spring, river or other reliable source. The flow rate is sized to the fish biomass and water temperature. FAO gives specific flow rates for trout-type culture of roughly 0.200 cubic metres per day per kilogram of fish at 15 °C and pH 7.5 when ammonia is the limiting factor, with the requirement rising at higher temperature. A common operating target is to replace the water in a raceway on the order of once per hour.
Oxygen and self-cleaning
In a conventional flow-through system the oxygen demand of the fish is met by the inflowing water rather than by mechanical aeration. As water passes along the raceway, oxygen falls and dissolved wastes rise, creating a gradient from inlet to outlet. The current also keeps solids moving toward the drain, which provides a degree of self-cleaning at adequate stocking and flow. Dissolved oxygen is the key constraint; when it drops below about 5 mg/L fish become sluggish and may stop feeding.
Carrying capacity
Carrying capacity is set by how much oxygen the flow can deliver and how much waste it can carry away. Reported densities for trout reach roughly 30–50 kg per cubic metre at the end of the rearing cycle, while marine species such as sea bass and sea bream are held at lower densities. Because oxygen and waste limits accumulate along the channel, total production per raceway is bounded by the water flow available.
Advantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| Simple design and high stocking density | Depends on a large, reliable supply of clean cold water |
| Continuous oxygen from inflow, little aeration needed | Generates concentrated effluent that must be managed |
| Current provides partial self-cleaning | Disease can spread along serial raceways |
| Easy observation and harvest of fish | Production capped by available water flow |
Species
Flow-through raceways are most associated with cold-water salmonids, especially rainbow trout (Oncorhynchus mykiss), and are also used for juvenile salmon (Salmo salar and Pacific species) and other species tolerant of fast-flowing, well-oxygenated water. The cold, clean, high-flow conditions that raceways require match the natural habitat of these fish.