Site Selection & Water Quality for Cage Culture

Site Selection

Factors to consider for cage placement:

• at least 1/2 acre or more in surface area
• at least 1 feet to 2 feet of water must be below the bottom of the cage. This allows waste to be flushed away from the cage.
• locate cage in pond where it can receive prevailing winds. This will provide overall mixing and aeration. Want to locate the cage where it will have maximum available water movement.
• be convenient for feeding and inspection
• not located in areas where it can be contaminated with runoff containing high levels of pesticides or large amounts of livestock wastes. Pesticides are toxic to fish. Heavy amounts of manure can lead to dissolved oxygen problems.

Water Quality

Temperature

Water temperature affects fish activity, behavior, feeding, growth, and reproduction. Fish are cold-blooded, and their temperature is approximately the same as their surroundings. The temperature tolerances of common cage cultured species can be found in Table 1. Since fish take on the surrounding temperature, they cannot tolerate rapid changes in temperature. This becomes important when stocking. Fish need to be acclimated to the receiving pond water temperature. The recommended acclimation rate is for every 10 °F change in temperature temper fish for 20 minutes.

TABLE 1. Species, optimum temperature range, temperature extremes (high and low).

Temperature also determines the amount of dissolved oxygen in the water. The cooler the water temperature, oxygen is more soluble. That is, there is more oxygen in cool water than in warm water.

Temperature has a major role in pond stratification. Temperature differences causes density differences. Density differences results in two layers being formed within a pond. Water gets heavier (weighs more) as it cools until it reaches approximately 39 °F. As it cools below this temperature, it becomes lighter until it freezes (32 °F). Water is unusual in that the solid form, ice, is less dense (weighs less) than the liquid form, which is why ice floats.

Oxygen

Dissolved oxygen refers to oxygen gas that is dissolved in water. Fish need oxygen just like land animals do. Fish absorb oxygen directly into their bloodstream using their gills. Fish cannot live without oxygen. Oxygen depletion is the cause of many fish kills. Low oxygen stress is the cause of disease outbreaks. The amount of oxygen in water decreases as temperature and altitude increases. As temperatures increase, fish metabolism will increase so they consume more oxygen. So both the increase in temperature and increase in fish metabolism may cause oxygen depletion in the summer.

Oxygen is produced during the day when sunlight shines on the plants in the water through photosynthesis. Oxygen levels will drop at night because no photosynthesis is occurring and respiration continues. Typically, there is a balance between the oxygen produced and consumed during the day. There are some events that upset this balance:

1. Increased organic wastes that enter the pond. Any organic material such as manure,septic tank waste, and excess fish feed, increase the oxygen demand in the water. As these excess organic materials decay, oxygen is consumed.
2. Die-off of aquatic plants. Since aquatic plants are the primary source of oxygen through photosynthesis, a die-off can result in oxygen depletion. As these dead plants decompose, their decomposition requires oxygen.
3. Excess aquatic plants. Excess plants (phytoplankton and submersed aquatic vegetation) produce more oxygen than can be held in water. We call this supersaturation. The oxygen demand by these plants will be great into the evening hours which results in wide fluctuations in oxygen levels.
4. Turnovers. With stratification, the pond has two layers with a warm surface layer and cooler bottom layer. A layer is created that acts a physical barrier between the two layers. Photosynthesis and oxygen production only occurs near the surface. Water in the deep layer becomes oxygen deficient. During heavy winds or cold rain, the barrier can be broken which causes the two layers to mix. If the oxygen demand is great in the oxygen-deficient layer, the dissolved oxygen that is present will be rapidly removed from the water. This may result in a fish kill.

Signs of Low Oxygen

• Fish swim at or near the surface.
• Fish gulp for air. (Piping at the surface)
• Fish stop feeding.
• There is rapid color change in water to brown, black, or grey.

pH

pH is a measure of how acidic or basic water is. The range goes from 0-14, with 7 being neutral. pH less than 7 is an acid. pH greater than 7 is a base. pH is really a measure of the relative amount of free hydrogen (H+) and hydroxyl (OH-) ions in the water. Water that has more free hydrogen ions is acidic, whereas water that has more free hydroxyl ions is basic. Since pH can be affected by chemicals in the water, pH is an important indicator of water that is changing chemically. pH is reported in "logarithmic units." Each number represents a 10-fold change in the acidity/basicity of the water. Water with a pH of 5 is 10X more acidic than water having a pH of 6. The acceptable range for most fish is from 6.5 to 9.

NCCE Aquaculture Home Page

By: Molly Sandfoss, Area Specialized Agent - Aquaculture
Last Updated: August, 2003