Environmental conditions within ponds also can cause fish to be stressed. One problem is the accumulation of nutrients from run-off and/or from overfeeding are:
- excessive algal blooms (very dense, pea-green water color),
- surface scums,
- excessive aquatic macrophyte growth,
- strong odors, and
- a rapid change in water color.
These problems may show up in any pond as it ages (a process called eutrophication). In most aquaculture ponds nutrient accumulation is related to the quantity of feed fed rather than the number of fish in the pond.
Excessive algal blooms (those measuring less than 12 inches visibility on a Secchi disk) and macrophytic growth can cause dissolved oxygen depletions at night or on heavily overcast days . Surface scums, such as those caused by blue-green algae, can shade out the rest of the algal population and cause dissolved oxygen depletions; this has been related to certain types of off-flavor issues.
Changes in water color and the strong odors that often accompany decaying plant material—either algal or macrophytic— usually signal a pending dissolved oxygen depletion caused by the aerobic decomposition of organic matter.
Rapid algal die-offs sometimes occur naturally and may be caused by the lack of an essential nutrient, environmental changes, or competition among species. As the algae die, the nutrients in their cells are returned to the water and stimulate the growth of a new algal population.
Herbicide or algicide applications also can cause algal die-offs. Although it may be necessary to control plants chemically before they become excessive, always be careful when applying these chemicals . Supplemental aeration is almost always necessary to maintain the oxygen levels sufficient to sustain fish.
At times of severe phytoplankton or macrophyte die-offs, even supplemental aeration may not maintain the proper oxygen level in the cage. In these cases it may be necessary to release the fish into the pond where they may find enough oxygen to survive. Other options are to move the fish to another pond or to harvest them.
It is better to learn to recognize situations that may cause oxygen depletion and take corrective measures promptly than to try to save the fish after the dissolved oxygen has dropped to dangerous levels. Remember that the stress caused by low dissolved oxygen is one of the major causes of disease outbreaks. Contact your county Extension agent or state fisheries (or aquaculture) specialist for information on diseases or to learn how to measure and predict dissolved oxygen levels in ponds.
The human factor:
A good record-keeping system is an invaluable management tool for the fish farmer. Records should include stocking dates, fish weights, and the number of fish stocked and harvested. The amount of feed fed daily and recorded observations of the feeding activity can help the manager identify trends, both positive and negative.
The dates any treatments are applied or medicated feed is offered should be recorded so required withdrawal periods can be observed before fish are marketed. Financial information, such as the cost of inputs and the price received for fish at harvest, should also be collected. And although profit may not be a person’s sole motive for raising fish, knowing the actual cost of the activity is useful.
The skill of the manager or cage culturist has great influence on the success of the venture. Mismanagement can cause many of the problems encountered, including those water quality problems previously discussed. Other common errors in cage management include:
- choosing ponds with pre-existing problems,
- poor cage construction or location,
- stocking undersized or poor quality fingerlings,
- stocking too many or too few fish per cage,
- stocking too many fish per pond,
- feeding poor quality feed,
- overfeeding,
- disturbing the fish or handling them poorly, and
- failing to observe the fish closely.
These problems are very common, particularly with beginning cage culturists.
Other problems and observations
Biofouling, a common cage problem, is the growth of algae and/or bryozoans (soft-bodied, jelly-like animals) on the sides of the cage or the growth of aquatic macrophytes immediately around the cage. These restrict water flow through the cage and can cause localized dissolved oxygen problems.
Periodically check the sides and bottom of the cage (without lifting it out of the water) and remove any biofouling organisms with a stiff brush or broom. Use slow movements when working around the cage to minimize fish stress.
A few tilapia (if legal in your state) or Koi carp stocked along with the primary cage species may be helpful in controlling algae on the sides of the cage.
The use of smaller-than-necessary mesh size in the construction of the cage also can restrict water flow and cause water quality issues. It is important to use a mesh large enough to allow maximum water movement while retaining the fish and keeping predators and competitors out.
Vandalism, poaching and animal predation can be problems when culturing fish in cages. Cages located in either isolated areas or areas of easy access are prime targets for poachers. While some states place heavy fines on those caught stealing fish from a fish farm, the possibility of an easy meal may entice those willing to take the risk.
To reduce predation and stress caused by otters, raccoons, snakes, turtles and fish-eating birds, locate cages where you can easily control them and fit them with effective tops.
Over-wintering fish in cages:
Some species over-winter better than others, and an in-depth discussion of appropriate species for your area can be found in SRAC Publication No. 163. In general, a good management goal is to produce market-sized fish in a single growing season.
Most fish feed less aggressively, if at all (depending on the species and water temperature), during the winter, but they will usually accept smaller rations offered less often. If you find it necessary to hold fish over the winter because they are too small for market or you lack marketing opportunities, try to feed them only on warm, sunny days when they are most likely to eat.
Be prepared to use medicated feed if bacterial problems develop as the water warms and feeding increases in the spring. Carefully observe fish for signs of parasitic infestation or fungal growth during the winter. Monitoring the fish in the open pond may give you advance warning of these situations in cages.
All fish farming, including cage culture, includes “crisis management.” Visit the pond and observe the fish at least once daily. Plan ahead and be prepared for emergencies. If you are inexperienced and a problem arises, get help fast.
Author:
Michael P. Masser and Peter Woods
