Fish Hatchery Management - fisheries & aquaculture

HAl'CHERY OPERATIONS 109vulnerable to many outside influences: a cloudy day that slows photosynthesis;a hot still day that causes stratification; a miscalculated food rationthat is too large for fish to consume before it decomposes.Typically the summer oxygen content in a pond iollows a 24-hour cycle:highest in the late afternoon after a day of photosynthesis; lowest at dawnafter a night of respiration. It is the nightiime oxygen depletion that ismost critical to pond culturists.Pond managers can take several precautions to prevent, or reduce theseverity of, dissolved oxygen problems.(l) Most ponds are fertilized to stimulate plankton production for naturalfish food. Suitable plankton densities allow secchi disk readings of 12-24inches. Fertilization should be stopped if readings drop to 10 inches or less.Special care should be taken if the pond is receiving supplemental fishfood, as this can stimulate sudden plankton blooms and subsequent dieoffs.(2) Because the frequency of dissolved oxygen probrems increases withthe supplemental feeding rate, fish should not be given more than 30pounds of food per acre per day.(3) tr atglciaes are used to control plankton densities, they should be appliedbefore, rather than during, a bloom. otherwise, the accelerated dieoffof the bloom will worsen the rate of oxygen depletion.(4J During critical periods of the summer, the oxygen concentrationshould be monitored. This is most easily accomplished at dusk and two orthree hours later. These two values can be plotted against time on a graph,and the straight line extended to predict the dissolved oxygen at dawn.This will allow emergency aeration to be prepared in advance.Dissolved oxygen problems may arise in spite of precautions. correctivemeasures for specific problems are suggested below.(1) If there has been an excessive kill of pond weeds or plankton thatare decaying, add 20% superphosphate by midmorning at a rate of 50-100pounds per acre. Stir the pond with an outboard motor or otherwise mix orcirculate water to rapidly distribute phosphate and add atmospheric oxygen;I to 2 hours of stirring a r-acre pond should suffice. Dilute theoxygen-deficient water with fresh water of about the same temperature.Dis.tribute, as evenly as possible, 100-200 pounds of hydrated lime,ca(oH)r, per acre in the late afternoon if co2 levels are l0 parts per millionor higher. Then stir for another one to two hours.\2) Low dissolved oxygen may be caused by excessive rooted vegetationand a lack of phytoplankton photosynthesis. If the pond is unstratified, addPto; and stir or circulate as in (l) above. Add fresh water if available. Ifthe pond is stratified, which is the usual case in warm months, aerate thesurface waters by agitation, draw off the cool oxygen-deficient bottom water,or add colder fresh water.