SMALL-SCALE TROUT FARMING

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IT'S THE WATER

Rainbow trout (which are usually preferred for intensive growing over the other varieties) are an amazingly hardy breed, but if you're to have the most productive ponds possible, you'll need to provide your fish with the right environment. Once past the fingerling size, rainbows grow most rapidly when the water temperature stays in the 550 to 580F range. (They can survive down to about 34 or up to around 70', but their enzymatic systems will be upset, and their growth will halt long before those deadly levels are reached.)

Temperature also influences the amount of oxygen that water can hold, so an overly warm pond imposes a double threat. In fact, water at 39°F can hold about 12.9 parts per million (ppm) at saturation, while at 680F it can maintain only 9 ppm. Since rainbows begin to become distressed at oxygen levels of below 6 ppm, and will die at 3 ppm, cooler water provides a much greater margin of safety. Furthermore, in a "stair step" pond system like Rick's, if the oxygen level of liquid entering the course is only 9 ppm, there's a much greater chance that the fish in the upper ponds will use so much oxygen that those in the lower pools will suffocate. This problem can be eased by including aeration devices along the way. For example, Rick installed splashboards made from corrugated fiberglass (with holes drilled in them) at the downstream end of each of our ponds, and made sure that the entry line at the top of each tank was angled up at 45 degrees to create splashing. Finally, altitude is yet another factor that comes into play with oxygen content, since the saturation level of water declines about 0.5 ppm for every 1,000 feet above sea level.

Of course, the ponds used for trout aquaculture must also be kept very clean. Kamloops are sensitive to concentrations of zinc as low as 0.04 ppm, an amount that could result from the use of galvanized piping. For that reason, only plastic or aluminum conduits are used in trout farming. The fish can also be adversely affected by the runoff from any fertilizers or pesticides that might be used by farmers on sites above pond location(s). In general, spring- or well water is considered best-simply because it's much less susceptible to contamination than is a major surface watercourse-but there are cautions to be observed when using liquid from even those relatively pristine sources. Iron bacteria found in the groundwater, for example, can accumulate on the gills of the fish, reducing their ability to breathe.

Hardness and pH are two additional factors that, while both important, can be somewhat at odds with each other. Trout absorb very few minerals through their gills, so the nutritional advantages of hard water are of little benefit to them. Furthermore, water with only a small mineral content has more ability to hold oxygen, so the dissolved oxygen content in soft water is typically greater. Actually, were it not for the lack of pH-buffering capability of mineral-poor water, the low-hardness liquid would always be preferred. However, should a low-pH rain squall (perhaps one that's been acidified by airborne pollution) drain into the stream supplying a pond with soft water, the liquid could become so acidic that the fish would be injured (or killed). Several major kills have, in fact, occurred in the southern Appalachians, and only prompt and thorough liming of the water (to raise the pH level) can ward off the disastrous results of the soft water/low-pH rain combination.

The final environmental prerequisite for successful trout farming is an ample amount of water. Here in western North Carolina, we've found that we can grow about eight pounds of fish for each gallon per minute (GPM) of flow. If we had only one pond, then, we'd be limited to a total of 480 pounds at our minimum flow of 60 GPM. Because we have four tanks linked in series (with oxygen-replenishing spillways in between), however, our total crop could approach 2,000 pounds! As we've already suggested, though, the carrying capacity of a gallon of water depends on temperature, hardness, dissolved oxygen content, and other factors ... so each

region has its own limitations on the number of pounds that can be grown per GPM of flow. (Pennsylvania, for example, can support about four pounds per GPM, while the most successful Idaho ponds have pushed 20 pounds per GPM.) If there's enough flow, however, the total production can still be quite high ... even if the water itself has a low carrying capacity.

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