High density aquaculture has been described by the United States Department of Agriculture as the most intensive form of agriculture practiced on a large scale in this country.
Cage culture is one of the most intensive forms of aquaculture. Because of this, there are inherent problems with cage culture that can prove challenging.
This fact sheet addresses some of them. For additional information see SRAC Publication Nos. 160–166. Signs of fish stress Early signs of stress in fish are difficult to observe because ponds are usually turbid or murky and fish attempt to hide from people.
Fish farmers, especially new ones, like to show off their fish to visitors, but unusual movements and sounds around the cages can cause stress in caged fish.
Sampling fish to examine them may also cause stress or injury and can make fish vulnerable to diseases. For these reasons, it is critical to observe fish when they are being fed because this may be the only time they come readily to the surface.
Cages contain collections of individual fish that often behave differently, not unlike groups of people. Some caged fish will feed readily with people present, while others will wait until everyone is gone and all is quiet.
It is important to watch closely and note the behavior of the fish during feeding so you will recognize behavioral changes. Changes in behavior are usually caused by changes in their environment (i.e., the pond water quality) or their health.
These changes may be the first signs of stress. Learn to recognize the common signs of stress, which may include:
- eating less,
- eating less aggressively,
- not eating at all,
- gulping for air at the surface,
- swimming erratically,
- skin discolorations,
- dead or dying fish, and
- any other unusual behaviors.
If any of these signs appear, investigate immediately. A quick response is critical. Analyze the problem, then try to determine the reason for the stress symptoms and a solution to the problem.
There could be a number of reasons for stress symptoms you observe. Several are discussed in the sections that follow.
Reduced feeding:
If fish are eating less it could signal a disease, a high parasite load, or a water quality problem such as low dissolved oxygen or high ammonia level. The presence of predators such as otters and raccoons also may cause the fish to decrease their feeding.
Has there been unusual activity in or around the pond? Has the weather or water temperature changed? If it is a heavily overcast and windless day, it may simply be a temporarily low, but not critical, dissolved oxygen problem. Has the pond water colour changed? Water color changes or the appearance of surface scum may also lead to low dissolved oxygen. A sudden halt to feeding usually suggests oxygen problems or disturbance.
Diseases, parasites and other water quality problems usually affect fish over several days and are reflected by a gradual reduction of feed intake.
Gulping for air:
If fish are at the surface gulping for air (most often observed at or before dawn), when the day before everything seemed fine, it is a sign that the fish aren’t getting enough oxygen.
Skin discoloration, erratic swimming and death
Skin discolorations, open wounds or lesions, spots, fin erosion, and erratic swimming or other strange behaviors are usually signs of diseases or parasites. If that is the case, fish will begin dying soon. If you see a few dead fish each day, some type of slowly spreading disease or parasite problem is probably affecting the fish.
But finding progressively more dead fish each day is a sign of a very serious disease problem. Dying fish or fish with suspected diseases or parasites should be diagnosed immediately.
Take or ship a live sample of affected fish and a water sample to your nearest fish disease diagnostic lab. Many state fisheries, Extension aquaculture programs or state conservation fisheries departments offer disease diagnosis.
Contact your county Extension office or state fisheries (or aquaculture) specialist for information on how to properly package, transport and ship fish samples.
Any disease diagnostic lab will be able to tell you the best treatment alternatives for diseases or parasites. The most practical treatment for an internal bacterial disease is usually to offer a medicated feed, but medicated feed works only on fish willing and able to eat it.
Purchase medicated feed only when you need it because antibiotics can loose their efficacy over time; it is illegal to use medicated feed as a prophylactic treatment.
If external bacteria or parasites are the problem, a therapeutic water treatment may be necessary. The diagnostic lab or other professional will base recommendations on the details you provide about fish behavior and recent pond management activities.
Some diseases and parasites can be controlled only by treating the entire pond, while for others treating the water in and around the cage may be sufficient. Another option is to place a plastic bag around the cage and treat just the water and fish in the cage. Problems such as snails, which serve as intermediate hosts for a number of parasites, may be addressed by treating just the margins of the pond. During any therapeutic water treatment the dissolved oxygen level must be monitored closely so that supplemental aeration can be provided if necessary.
Author:
Michael P. Masser and Peter Woods
