Trout Farm Site Selection, Design, and Construction
History of North Carolina Production
North Carolina ranks second to Idaho in the United States in commercial production of rainbow trout, producing four to six million pounds per year. Most of the 60 commercial trout arms are cement raceway systems in series, with 20 uses of the water or more in some cases.Earthen ponds and earthen raceways continue to be used at some sites, but are more difficult to manage (North
Site Selection and Evaluation
As is true of any new enterprise, the three most important things are location, location, and location. A trout farm which is developed on a poor site will cause its proprietor unnecessary managerial disasters, and likely will fail regardless of the amount of initial investment. It is advisable to consult with a professional in the location of the trout farm. The following are some criteria which may be useful in evaluating sites for commercial trout farming.
Water flow and quality – surface waters
Minimum “7Q10”, or ten year-seven day average low flow:
*350-500 gallons per minute for supplemental income production
*1000 gallons per minute for primary income
In North Carolina this figure (7Q10) represents approximately 30% of the average stream flow. Estimated point-in-time low flow is generally about 50-60% of the 7Q10 but can vary widely.
Obtaining a reasonable estimate of the water flow from the source(s) to be used for trout production is essential to proceed in design of the facility. Water Flow Estimate can be used to estimate the stream water flow, once a weiror weir-like structure is established.
Temperatures should not exceed 75oF and rarely should exceed 70oF in years with normal amounts of precipitation. Optimum temperatures for trout production are from 55o to 65oF. Cooler waters (48o to 56oF) are best for trout eggs and fry. Dissolved oxygen levels will most likely be at saturation as the water enters the diversion area. Oxygen levels are rarely a problem until after the water enters the system where it is used by the fish.
To operate a trout farm in which water is used three or more times, the alkalinity of the water should be low (<30 ppm) and the pH should be between 6.5 and 7.5. Surface waters in western North Carolina typically have these qualities.
Suitable watersheds and slopes
The watersheds most compatible with trout farming will be completely wooded with little or no cropland. Although it is not essential the entire watershed be wooded, the stream channel definitely should be shaded in order to avoid high water temperatures and fluctuations in the summer months.
The slope of the site for the raceway facility should be gentle enough for ease of access, yet provide a minimum fall of three feet between raceways. Four feet of fall is preferred. On a typical trout farm site in NC, a 4 to 10% grade is suitable for raceway construction. In virtually all cases, it is best to avoid building a trout production facility within a 100 year flood plain.
The design of the production facility and intake is site specific and should involve a specialist or someone familiar with construction and design principles. The following are generalized comments on raceway and intake design.
The water diversion structure is the most important component of the farm. A reliable intake is essential for dependable production. It is advisable to consult with a professional in the location and design of the intake.
Intake Screen Sizing
Proper screen area is essential for planned water flow volumes. If the area is too small for a given volume of water then the force of the water will tend to trap debris on the face of the screen. The suction through the screen should not impede the washing of the screen by the overflow water.
Because of the hydraulic action of falling water, a splash apron should be poured on the downstream side of the dam to prevent structural weakening. Of course if the dam is on solid bedrock, this may not be necessary, but in other situations the apron should extend from the dam downstream about 1 1/2 times the height of the dam. This is only good for relatively flat terrain. On very steep terrain, the apron may have to be extended much farther.
Wing Walls or Riprap
Whenever a dam is built across any stream we essentially raise the bed of the stream at that point equal to the distance of the height of the dam. In so doing, new stream banks are exposed and must be protected from erosion. The banks that are exposed are those adjacent to the dam pool and those just after the dam. Usually, valley streams are much more prone to erosion than rocky mountain streams and therefore must be more thoroughly protected
Three methods have been employed for this protection. One is to build a retaining wall extending in both directions from the dam to protect any vulnerable areas. Another is to lay riprap which may be bonded with concrete to ensure stability under the most severe conditions, and the third is simply that in many cases the natural rocky banks of the stream are stable enough that additional precautions need not be taken. Whatever method is employed is certainly site specific.
A fish ladder must be installed on any dam over 16″ tall to allow for the free passage of native fish up and down the stream. The ladder must be constructed from a suitable material, such as steel or concrete, which can withstand the natural forces in the stream. The ladder is nothing more than two walls, at least 2 feet apart, extending perpendicular to the dam downstream. The length of the walls should allow for a 24″ long pool and a step of no more than 12″. A slot in the dam in front of the ladder should be open to allow sufficient water to flow down the ladder to attract migrating fish.
Source: C. R. Brown Enterprises
As with the intake structure, the design and location of the raceways is site specific. It is advisable to consult with a professional in the location and design of the raceways.
Construction cost estimates approach $1.00 per pound of production. A 100,000 pound facility will cost about $100,000 to build, excluding land cost.
Production Estimates and Economics
Once a water flow estimate is completed, a production capacity estimate can be made. Production estimates range from 30-100 pounds of production per gallon per minute of available flow per year, dependent on the water quality and chemistry. For example: 1000 gallons per minute of available flow will support 30-100,000 pounds of production per year.
About 80% of trout produced in North Carolina are sold to local processors. The net profit from these sales is $.15 – .30 per pound, after direct production costs. A 100,000 pound production facility will have a net return of $15-30,000 per year. To secure the required permits, construct the facility, and harvest the first group of trout will take 18-24 months and will require $150-200,000 for a 100,000 pound facility.
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