Fish Hatchery Management - fisheries & aquaculture

,2,2FISHIIAI.CIIERYMANAC;EMEN'l(which could clog the filter). It is important that the water be pathogenfree,because an antibiotic or other drug that has to be used in thehatchery can kill the nitrifying bacteria as well.Settling chambers and clarifiers can extend the life of biofilters and reduceclogging by removing particulate matter. Filter bed material with large voidspaces also can reduce clogging, and foam fractionation will remove dissolvedorganic substances that accumulate. These foaming devices are also called,,protein skimmers," which refers to their ability to remove dissolved organicsubstances from the water. The foam is wasted through the top of the deviceand carries with it the organic material. In a small system, air stones can beused to create the foam. The air produces numerous small bubbles that collectthe organic material onto their surface. Because foam fractionation doesnot readily remove all particulate organic material, it should follow the settlingor clarifying unit in a reconditioning system.Nitrite (ttO;) is an intermediate product of nitrification, and a poorlyoperating biofilter may release dangerous amounts of this toxic ion to thewater. A more rapid growth rate of Nitrosomonas in the biological systemcan lead to accumulation of nitrite, which is highly toxic to freshwaterfishes. Nitrite oxidizes blood hemoglobin to methemoglobin, a form whichis incapable of carrying oxygen to the tissues. Methemoglobin ischocolate-brown in color, and can be easily seen in the fish's gills'Yearling trout are stressed by 0.15 part per million and killed by 0.55part per million nitrite. Channel catfish are more resistant to nitrite, but 29parts per million can kill up to 50'lo of them in 48 hours. Nitrite toxicitydecreases slightly as the hardness and chloride content of water increases.ION T]XCHANGE RI]MOVAI- OF AMMONIAIon exchange for removal of ammonia from hatchery water can be accomplishedby passing the water through a column of natural zeolite. Zeolitesare a class of silicate minerals that have ion exchange capacities (they areused in home water softeners) . Among these, clinoptilolite has a particularlygood affinity for ammonium ions. It is increasingly being used inhatcheries, where it effects 90 !)7'1,, reductions in ammonia (Figure 9).Clinoptilolite does not adsorb nitrate or nitrite, nor does it affect waterhardness appreciably. It can be regenerated by passing a salt solutionthrough the bed. The ammonia is released from the salt solution as a gasand the solution can be reused. Any ion exchange unit can develop into abiofilter if nitrifyingbacteria become established in it. This may lowerexchange efficiencies and cause production of nitrite, so periodic disinfectionmay be necessarY.