Handling of brood fish should be kept to an absolute minimum. Gentleness when handling fish is of utmost importance to prevent physical injury and physiological stress. Damage to the slime (mucus) layer, scales, and skin of the fish can result in infection.
It also causes excessive uptake of water by freshwater fish or loss of water from marine species (osmotic stress). Knitted fine-mesh dip nets are recommended for handling fish to minimize injury and scale loss; do not use knotted dip nets. Minimize the number of times the fish are lifted from the water, and work as quickly as possible when transferring fish.
Time spent with the fish out of the water during handling can mean the difference between a good spawn, no spawn, or death.
Transporting brood fish:
Fish crowded in a transport tank can rapidly become stressed due to physical injury, deteriorating water quality, rapid changes in water temperature, and osmotic imbalance. Handling tanks in capture boats and transport trucks should be large enough to allow complete freedom of movement to the brood fish and have no sharp corners or edges that might injure the fish.
Hauling tanks are usually aerated with oxygen (bottled or liquid) with air stones, electrical agitators (12-volt), or both. A high level of dissolved oxygen is crucial for rapid recovery of the brood fish from the oxygen debt incurred during capture and handling.
Oxygen bottled or liquid is recommended for reviving fish immediately following capture. For long hauls, water agitators should be used in addition to oxygen to drive off the carbon dioxide that accumulates in the water. The combination of aerators also provides a backup in case of system failure.
Water in small containers can warm quickly, resulting in temperature shock. Warm water also reduces available oxygen and increases the metabolic rate of the fish, adding further physiological stress. Capture and transport brood fish during cool evening or early morning hours to minimize stress. Ice may be added to the water during hauling to prevent an increase in water temperature.
Salt (0.3 -1.0 percent) may be used in the transport water to minimize osmotic stress and infection. Anesthetics have also been used successfully during transport of fish.
Injecting brood fish:
Females that have eggs in a sufficiently advanced stage of development for successful hormoneinduced spawning (See Determining Sexual Maturity of Broodstock for Induced Spawning of Fish) should be injected as soon as possible.
Any delay in injecting the brood fish greatly diminishes the chance for a successful spawn. While injecting the fish, every effort must be made to minimize stress and injury. It is unnecessary to remove the fish from the water when giving injections. Brood fish are usually captured and gently restrained in a net for injections.
Avoid squeezing or forcefully holding the fish. Fish may be anesthetized with MS-222 if necessary. The fish may struggle less if a cloth is placed over its head. Underwater injections while the fish is stationary or swimming slowly are sometimes used for large, delicate species of fish because it eliminates the stress of forcible restraint.
Holding brood fish at the hatchery:
Environmental factors in the brood fish holding tank such as dissolved oxygen, water temperature and absence of disturbance to the fish following hormone injections are believed to play an important role for successful induced spawning.
The handling stress and the physiological processes of final maturation of eggs and sperm increase the oxygen demand of the brood fish. High temperature accelerates egg maturation, resulting in an even greater oxygen demand by the fish. Elevated temperature will also increase the rate of development of disease organisms.
However, if the temperature is too low, spawning will be delayed or in many cases completely inhibited. Holding tanks should be large enough to allow complete freedom of movement to the brood fish. Round tanks or tanks with rounded corners are preferable because they minimize injury to the fish.
Holding tanks should be covered to provide shading that will help quiet excitable species and prevent the fish from jumping to their death.
Conclusions:
Brood fish must be handled carefully to minimize physical injury and stress. Speed and gentleness during fish capture and handling are of utmost importance. Crowding, dissolved oxygen depletion, rapid changes in temperature, and osmotic imbalance are well known causes of stress and must be avoided when transporting fish.
Females that have eggs in a sufficiently advanced stage of development for successful hormoneinduced spawning should be injected as soon as possible. Any delay greatly diminishes the chance for a successful spawn. Dissolved oxygen content of the water, proper temperature, and absence of disturbance to the fish following hormone injection(s) are believed to play an important role for successful induced spawning.
Authors:
R.W. Rottmann, J.V. Shireman, and F.A. Chapman
