Fish Hatchery Management - fisheries & aquaculture

FISH HAfCHERYMANAGEMENTdecreases-that is, less oxygen can be dissolved in water-at higher altitudesand, more importantly,at higher temperatures. For this reason, therelationship between absolute concentrations (parts per million) and relativeconcentrations (percent saturation) of gases is not straightforward.Special conversion formulae are needed; in graphical form these can bedepicted as nomograms. A nomogram for oxygen is shown in Figure 1.Dissolved oxygen concentrations in hatchery waters are depleted inseveral ways, but chiefly by respiration of fish and other organisms and bychemical reactions with organic matter (feces, waste feed, decaying plantand animal remains, et cetera). As temperature increases the metabolic rateof the fish, respiration depletes the oxygen concentration of the water morerapidly, and stress or even death can follow. Fluctuatingwater temperaturesand the resulting change in available oxygen must be considered ingood hatchery management. In ponds, oxygen can be restored during theday by photosynthesis and at any time by wind mixing of the air andwater. In hatchery troughs and raceways, oxygen is supplied by continuouslyflowing fresh water. However, oxygen deficiencies can arise in bothponds and raceways, especially when water is reused or reconditioned.Then, chemical or mechanical aeration techniques must be applied by culturists;these are outlined below for raceways, and on pages 108-110 forponds. Aeration devices are shown in Figures 2 and 3.In general, water flowing into hatcheries should be at or near 100% oxygensaturation. In raceway systems, where large numbers of fish are culturedintensively, oxygen contents of the water should not drop below 80%saturation. [n ponds, where fish densities are lower (extensive culture) thanFtCUnn 2. A simple aeration device made of perforated aluminum can add oxygento the water and restrict fish from jumping into the raceway above. (FWSphoto.)