Practical Skills, Field Work and Project Work
Core laboratory skills
Water analysis is the backbone of the practical paper. Measure pH with a meter or universal indicator paper; transparency with a Secchi disk lowered until it disappears and then raised until it reappears, averaging the two depths; alkalinity by titrating with standard sulphuric acid to a methyl orange end point; hardness by EDTA titration; and dissolved oxygen by the Winkler method. Soil pH, plankton sampling with a fine net, counting in a Sedgwick-Rafter cell, fish length and weight measurement, gonad maturity staging, gut content analysis, feed preparation and wet-mount disease diagnosis complete the list.
Project work format
A field project should state a title, objectives, the site and period, materials and methods, an observation table, calculations, a discussion and a conclusion, with photographs and the record book signed weekly. Typical projects: effect of liming on pond pH over 15 days, comparison of growth of rohu under two feeding rates, length-weight relationship of a pond population, or a survey of marketing channels in a local fish market.
Worked example: Winkler dissolved oxygen
Fill a 300 mL BOD bottle without bubbles, add 2 mL manganous sulphate and 2 mL alkaline iodide-azide, stopper, mix, and allow the brown precipitate to settle. Dissolve the precipitate with 2 mL concentrated sulphuric acid. Titrate a 50 mL aliquot with 0.025 N sodium thiosulphate using starch indicator near the end point. If the titre is 2.0 mL, DO = (2.0 x 0.025 x 8000)/50 = 400/50 = 8.0 mg/L, which is healthy for carp. Note that the average Secchi depth of 35 cm recorded on the same visit supports the conclusion that the pond is well fertilised.
Traps that cost marks
- Delaying fixation after sampling, which allows oxygen to change before titration.
- Adding starch too early, which gives a poor end point.
- Reporting GSI without the formula (gonad weight x 100 / body weight) or confusing it with condition factor.