We have already referred to the temperature change with depth and thermal stratification. Closely related to this are the changes in pressure, compressibility, density, viscosity, buoyancy ect. Some of these features of water are of interest in the study of bottom installations which are to be used for aquaculture and here we shall refer to primarily pressure, density and viscosity. See discussion on ‘Light’, given separately.
Hydrostatic pressure increases by 1 atmosphere for every 10 metre depth and thus at 20 metres depth the effective pressure is 3 atmospheres and at 100 metres, 11 atmospheres. (1 at= 1 kg.cm-2 = 0.80665 × 104 Newton m2, See EIFAC, 1986 for convertion factors). The effect of pressure is so much that some wooden beams taken to deep water have been crushed by the effect of pressure, the phenomenon referred to as ‘implosion’, as opposed to ‘explosion’. Quite often when fishes are hauled up from bottom waters their stomachs become everted, as they are not able to adjust to the sudden changes in pressure. Many fishes, however, have the air bladder or swim bladder which helps in secreting or absorbing gas to adjust the body density and then can move up or down.
The most remarkable change in density of water is that caused by temperature. Pure water has maximum density at 4°C, and as it cools to 0°C the density decreases from 1 to 0.999868, and as it warms to 30°C also it decreases to 0.995673 (Fig. 8.4). While the change might appear negligible this physical phenomenon has profound influence on the freshwater bodies, especially those in the temperate region. In winter when the ice-cover is formed the water at 4°C being heavier sinks to the bottom and would not freeze - otherwise the whole lake would be a solid mass of ice (see earlier discussion also).
Fig.8.4. Density in relation to temperature in pure water
During spring and fall overturn the density changes, which and wind action again cause the mixing of water. We have referred already to the thermal stratification of lake water during summer.
Density of water changes due to pressure (unity at 1 atmosphere; 1.0001 at 20 atmospheres) and also due to dissolved solids (sea water: 1.03). Salinity effects will be discussed separately.
Viscosity of water: This property changes greatly with temperature, from unity at 0°C to 0.5 at 25°C. This is of biological interest in that fish can move through at much less frictional drag in warmer waters, a saving in energy of locomotion and also perhaps in pumping of water for respiration. This point is mentioned more as an example where the aquaculturist has to be vary of every aspect of culture where energy saving is involved, for further scientific developments in fish culture might depend on such facts.
