PHYSICAL FEATURES OF WATER
Information on physical and chemical characteristies of water in which aquaculture is to be practised is of prime importance in deciding the suitability of the water body for successful culture. These characteristics would differ widely depending on the type of water body, such as pond, tank, lake, man-made impoundment, running water or coastal water body chosen for culture. It becomes obvious that when the information on the environmental characteristics of the water bodies are available one could make a proper choice of site only with the information on the tolerance and optimal ranges of these characteristics for the species to be cultured.
For each ecological factor in question the organisms to be cultured has a range or zone within which it can live indefinitely, referred to as the tolerance zone. The area outside this zone is the lethal or resistance zone in which the organisim will die in a fixed period time (survival or resistance time), an information of much value to the aquaculturist, for quite often during the extreme fluctuations of a factor such as oxygen or temperature on a summer day, the organism is exposed to the lethal factor for a short time only and the capacity (length of time) in which it can resist the inclement condition, would decide its survival.
The tolerance zone is further restricted by ranges of the factors for optimal biological activities, growth and reproduction being the most important but perhaps more important are the range for behavioural activities such as feeding and swimming, (see Fig. 8.1)
The physical properties of water appear to be ideal for life, as indeed they should be, for it is the environment which moulded life. Early references to this discussion can be found in Hutchinson (1957), the most important of which is ‘Fitness of the Environment’ by L.J. Henderson (1913). Ice having a lower density floats in water of higher temperature. In waters which freeze in winter thus the floating ice gives a protective cover, enabling the bottom water to be warmer and living things to survive.
Fig. 1. Zones and ranges of tolerance (survival) and other biological functions as controlled by an environmental factor such as temperature or pH. The horizontal double-head arrows indicate ranges and the vertical arrows the optimal ranges of the ecological factor for the specific biological functions such as activity (eg. Swimming in fish), growth and reproduction.
There are only few inorganic liquids existing in nature. They are water, elementary mercury and liquid CO2, apparently present under high pressure in cavities in quarts orystals. The other natural liquids are mixtures of hydrocarbons and petroleums. The perculiarity of water having much higher melting and boiling points, at 0 and 100°C, is known from the fact that other dihydrides such as those of sulfur, selenium and tellurium have BPs below 0°C (As obvious, for the study and measurement of heat, water has served as the base subject - the Celsius scale is set by the freezing and boiling points of water; the measure of heat, the Calorie (large or kilo calorie) as the heat or energy required to raise the temperature of one litre of water by 1°C; the specific heat of water is unity).
In this discussion on the “Physical features of water” we shall take up the following aspects in the sequence shown, indicating their importance and optimal range for aquaculture to the extent possible: Temperature
Temperature changes in small water bodies
Fish pond temperature
Temperature relations of large water bodies
Temperature relations of culturable organisms
Controlling temperature in fish ponds
Water conditions and depth
Light, colour and turbidity