VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

PHYSICAL PARAMETERS

Temperature

• Of all the physical environmental factors, temperature influences a number of other variables in the environment.
• Temperature above or below the tolerance limits of the host animal cause stress. For example, increased metabolic rate caused by high temperature results in higher oxygen demand.
• Dissolved gases, including oxygen generally decrease in solubility with increasing temperature. Further, the solubility of toxic compounds are also known to increase with generally accelerates all the biological processes of the host to a certain limit above which its viability is lowered. Most of the finfish and shellfish can withstand water temperature upto a maximum of 32⁰C.
• Temperature is measured by using a thermometer. There are several types of thermometers. But the most commonly used one is the glass thermometer which measure temperature by means of the expansion and contraction of mercury.
• Procedure
  • Dip the thermometer in a water sample for which the temperature have to be measured. According to the temperature of the water sample, the mercury level will increase or decrease on the graduated scale of thermometer. The mercury levels in the thermometer will show the actual temperature of the water. The temperature is normally expressed in ⁰C (Celsius) or ⁰F (Fahrenheit).

Light penetration

• The visibility of any water depth as a measure of light penetration is determined by using a Secchi disc. A Secchi disc is a circular metal plate with a diameter of 20 cm. The upper surface is divided into four quadrants and is alternatively painted in black and white. The disc is tied with graduated rope at the centre. In the centre of the lower surface, a small weight is attached to help in the sinking of the Secchi disc to the depth.

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• Procedure
  • To obtain the Secchi disc visibility, lower the disc into the water column until it just disappears out of sight and on just lifting the disc it should reappear. With the help of the graduated rope, the extract depth of the reappearance of the disc is noted. While making these measurements, view the disc from directly above. Generally, measurements must be made between mid-morning and mid-afternoon. The water should be calm without disturbance and the sun should be behind you while taking the readings

• The light extinction coefficient (k) of any water column is determined using the formula of Poole and Atkins (1929)

K = 1.7 / Secchi disc reading (m)

• Higher k value indicate higher turbidity of the water column
• Inference

Secchi disc reading	Inference
Between 35 and 40 cm	Ideal
Below 35 cm	Excess algae
Above 40 cm	Less algae

• However, one must remember that plankton blooms are not always green. Plankton blooms may also impart yellow, red, brown or black colouration to water.

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Total dissolved solids and suspended solids

• Total dissolved solid (TDS) is determined as the residue left after evaporation of the filtered sample. 150 ml of water sample is filtered through a glass fibre placed in a funnel. Then 100 ml of the filtrate is transferred to a pre weighed dish. The contents are evaporated in an oven at 103 ⁰ C for 1 hour. Afterwards the dish and the residue are collected in a dessicator and are weighed.

TDS g/L = ( A-B / V ) x 100

• Where,  A= Final weight of the dish (g)

    B= Initial weight of the dish (g)

    V= Volume of the sample (ml)

• Total solids (TS) is determined by weighing the residue left after evaporation of the unfiltered sample and by using the following formula:

TSg/L = ( A-B  / V ) x 100

• Where,  A= Final weight of the dish (g)

    B= Initail weight of the dish (g)

    V= Volume of the sample (ml)

• Total suspended solids (TSS ) is the difference between the values of total solids and total dissolved solids

TSS = TS – TDS

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