Fish Hatchery Management - fisheries & aquaculture

HA'I'CHERY OPERAI'IONS 105may be damaged by activity of the chemical. Avoid overdoses and spillages.Avoid use near sensitive crops and reduce drift hazards as much aspossible; do not apply herbicides on windy days. Clean all applicationequipment in areas where the rinsing solutions will not contaminate otherareas or streams.Fish culturists must also be aware of the current registration status ofherbicides. Continuing changes in the regulation of pesticide and drug usein the United States has created confusion concerning what chemicals maybe used in fisheries work. Table l4 lists those chemicals that presently possessregistered status for use in the presence of food fish only, a food fishbeing defined as one normally consumed by humans.Special Problems in Pond CultureDISSOLVED OXYGENBecause adequate amounts of dissolved oxygen are critical for good fishgrowth and survival, this gas is of major concern to fish culturists (Figure39). On rare occasions, high levels of oxygen supersaturation-caused byintensive algal photosynthesis-may induce emphysema in fish. Virtuallyall oxygen-related problems, however, are caused by gas concentrationsthat are too low.Tolerances of fish to low dissolved oxygen concentrations vary amongspecies. In general, fish do well at concentrations above 4 parts per million.They can survive extended periods (days) at 3 parts per million, but do notgrow well. Most fish can tolerate l-2 parts per million for a few hours, butwill die if concentrations are prolonged at this level or drop even lower.In ponds that have no flowing freshwater supply, oxygen comes fromonly two sources: diffusion from the air; and photosynthesis. Oxygen diffusesacross the water surface into or out of the pond, depending onwhether the water is subsaturated or supersaturated with the gas. Onceoxygen enters the surface film of water, it diffuses only slowly through therest of the water mass. Only if surface water is mechanically mixed withthe rest of the pond-by wind, pumps, or outboard motors-will diffusedoxygen help to aerate the whole pond.During warmer months of the year when fish grow well, photosynthesisis the most important source of pond oxygen. Some photosynthetic oxygencomes from rooted aquatic plants, but most of it typically comes from phytoplankton.Photosynthesis requires light; more occurs on bright days thanon cloudy ones. The water depth at which photosynthesis can occurdepends on water clarity. Excessive clay turbidity or dense blooms of phytoplanktoncan restrict oxygen production to the upper foot or less of water.Generally, photosynthesis will produce adequate amounts of oxygen for