There are many physical and chemical aspects in the water that we must consider for our trout biomass. We will review these aspects in this chapter.
Water quality determines the success or failure of any fish culture operation to a great extent. Physical and chemical characteristics such as suspended solids, temperature, dissolved gases, pH, mineral content, and potential danger of toxic materials should all be considered for managing water quality and running an efficient operation.
Figure 1. Suggested water quality criteria for optimum health of salmonid fishes. Concentrations are in parts per million (PPM)1. Source: Wedemeyer, 1977.
Figure 2. Suggested chemical values for hatchery water supplies, concentration are in parts per million (PPM) 1. Source: Howard n. Larsen, unpublished
1 Source: Fish Hatchery Management
No other single factor affects the development and growth of fish as much as water temperature. Metabolic rates of fish increase rapidly as temperature rises, and many biological processes such as spawning and egg development, and hatching are geared to annual temperature changes in the natural environment. Each fish species has a temperature range that it can tolerate, and within that range, it has optimal temperatures for growth and reproduction. For Rainbow Trout, this tolerable range is 0.5 – 25.5°C, with the optimum being 10-16°C.Within a hatchery, temperatures that become too high or low for fish impart stresses that can dramatically affect production and render fish more susceptible to disease. Most chemical substances dissolve more readily as temperature increases; in contrast, and of considerable importance to hatchery operations, gases such as oxygen and carbon dioxide become less soluble as temperatures rise.
Nitrogen and oxygen are the two most abundant gases dissolved in water. Although the atmosphere contains almost four times more nitrogen than oxygen in volume, oxygen has twice the solubility of nitrogen in the water. Therefore, freshwater usually contains about twice as much nitrogen as oxygen when in equilibrium with the atmosphere. Carbon dioxide is also present in water, but it usually occurs at much lower concentrations than either nitrogen or oxygen because of its low concentration in the atmosphere. All atmospheric gases dissolve in water, although not in their atmospheric proportions; as mentioned, for example, oxygen is over twice as soluble as nitrogen. Natural waters contain additional dissolved gases that result from erosion of rock and decomposition of organic matter. Several gases have implications for hatchery site selection and management. Oxygen must be above certain minimum concentrations. Other gases must be kept below critical lethal concentrations in hatchery or pond water. As for other aspects of water quality, inappropriate concentrations of dissolved gases in source waters mean added expense for treatment facilities. It should also be noted that the solubility of gasses and the amount of gas dissolved in water vary with the temperature of the water.
Oxygen is the second most abundant gas in water – Nitrogen is the first – and the most important as fish cannot live without it. Concentrations of oxygen, like other gasses, are typically expressed by either parts per million by weight (ppm) milligrams per liter (mg/l) or as percent saturation. In the latter case, saturation refers to the amount of gas dissolved when water and atmospheric phases are in equilibrium. This equilibrium amount (for any gas) decreases-that is less oxygen can be dissolved in water at higher altitudes and, more importantly, at higher temperatures.
For this reason, the relationship between absolute concentrations (parts per million) and relative concentrations (percent saturation) of gases is not straightforward. Special conversion formulae are needed; these can be depicted as nomograms in graphical form. Dissolved oxygen concentrations in hatchery waters are depleted in several ways, but chiefly by respiration of fish and other organisms and by chemical reactions with organic matter (feces, waste feed, decaying plant, and animal remains, etc.).
As temperature increases the metabolic rate of the fish, respiration depletes the oxygen concentration of the water more rapidly, and stress or even death can follow. Good hatchery management must consider fluctuating water temperatures and the resulting change in available oxygen. In ponds, oxygen can be restored during the day by photosynthesis and at any time by wind mixing of the air and water.
In hatchery troughs and raceways, oxygen is supplied by continuously flowing freshwater. However, oxygen deficiencies can arise in ponds and raceways, especially when water is reused or reconditioned. Then, chemical or mechanical aeration techniques must be applied by culturists.
In general, water flowing into hatcheries should be at or near 100% oxygen saturation and a level of > 7.0 mg/l. In raceway systems, where large numbers of fish are cultured intensively, oxygen contents of the water should not drop below 80% saturation. In ponds, where fish densities are lower (extensive culture) than in raceways, lower concentrations can be tolerated for short periods. But in either type of holding unit, if the fish are subjected to more extended periods at concentrations below 5.0 mg/l growth, survival will be severely compromised. A continual saturation of 80% or more for Trout provides a desirable oxygen supply.
Some aquatic bacteria and algae may fix molecular nitrogen (N2), but it is biologically inert as far as fish are concerned. Dissolved nitrogen may be ignored in a fish culture if it remains at 100% saturation or below. However, at supersaturation levels as low as 102%, it can induce gas bubble disease in fish.
Theoretically, gas bubble disease can be caused by any supersaturated gas, but the problem is almost always due to excess nitrogen in practice. When water is supersaturated with gas, fish blood tends to become so as well. Because oxygen is used for respiration, and carbon dioxide enters the physiology of blood and cells, excess amounts of these gases in the water are taken out of the solution in the fish body. However, nitrogen, being inert, stays supersaturated in the blood. Any reduction in pressure on the gas, or localized increase in body temperature, can bring such nitrogen out of solution to form bubbles; the process is analogous to "bends" in human divers. Such bubbles (emboli) can lodge in blood vessels and restrict respiratory circulation, leading to death by asphyxiation. In some cases, fish may develop apparent bubbles in the gills, between fin rays, or under the skin, and the pressure of nitrogen bubbles may cause eyes to bulge from their sockets.
Gas supersaturation can occur when air is introduced into water under high pressure, subsequently lowered, or when water is heated. Water that has plunged over waterfalls or dams, water drawn from deep wells, or water-heated from snowmelt is potentially supersaturated. Air sucked in by a water pump can supersaturate a water system.
All waters contain some dissolved carbon dioxide. Generally, waters supporting good fish populations have less than 5.0 parts per million carbon dioxide. Spring and well water, which frequently are deficient in oxygen, often have a high carbon dioxide content. Both conditions can be corrected easily with efficient aerating devices. Carbon dioxide over 20 parts per million may be harmful to fish. Lower carbon dioxide concentrations may be detrimental if the dissolved oxygen content drops to 3- 5 parts per million. It is doubtful that freshwater fishes can live throughout the year with an average carbon dioxide content as high as 12 parts per million.
In very low concentrations, hydrogen sulfide (H2S) and hydrogen cyanide (HCN) can kill fish. Hydrogen sulfide derives mainly from the anaerobic decomposition of sulfur compounds in sediments; a few parts per billion are lethal. Hydrogen cyanide is a contaminant from several industrial processes and is toxic at concentrations of 0.1 part per million or less.
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