WBCHSE HS Fisheries common mistakes
WBCHSE HS Fisheries · Understand the traps and how to avoid them.
Students answer 'capture fisheries' when asked to classify pond carp culture, or describe West Bengal's inland fish production as mainly marine capture.
The terms capture and culture are memorised as slogans without linking them to the source of stock and ownership of the water body; West Bengal's dominance in inland aquaculture is also confused with coastal marine landings.
What to do instead
For every example, state three tests: is stock deliberately introduced, is the water body managed, and is the harvest dependent on natural recruitment? Then classify pond carp, bheri prawn, Hooghly estuarine hilsa and Digha marine catch separately.
Writing a fin formula such as D. 12-13 without separating unbranched and branched rays, or counting all fin rays as a single group.
The formula is copied as a string rather than derived from observation; students do not practise counting the first stout unbranched ray separately from the branched rays.
What to do instead
Use the standard format: fin abbreviation, number of unbranched rays in Roman numerals, number of branched rays in Arabic numerals, e.g. D. iii/14-16. Count one specimen of rohu, catla and mrigal and mark the transition from unbranched to branched rays before writing the formula.
Recommendeding a fixed lime dose, such as 200-300 kg/ha, for every pond without first considering soil pH, pond bottom condition or water alkalinity.
Lime is remembered as a routine 'pond preparation' step; the distinction between agricultural limestone, quicklime and slaked lime, and their different neutralizing effects, is ignored.
What to do instead
State the decision rule: test soil pH and water alkalinity first; acidic soils need higher liming, alkaline soils need little or none. Specify the lime type and calculate the dose from the test result, and note that quicklime is caustic and applied only to an empty pond bottom.
Describing induced breeding as 'injecting pituitary extract into the fish' without naming the gland source, the donor species commonly used, the dose unit or the role of the second injection.
The procedure is learned as a single sentence; students do not link hypophysation to the spawning cycle and therefore omit the priming and resolving doses.
What to do instead
Write the sequence: collect pituitary from a mature donor (commonly major carp), preserve it, prepare extract, give a priming dose and then a resolving dose to the female, and inject the male with a lower dose. State that the dose is expressed per kg body weight and that latency depends on water temperature.
Calculating FCR as weight gain divided by feed given, or reporting FCR without stating whether it is on a dry-feed or wet-weight basis.
The formula is reversed under exam pressure; students also ignore that feed moisture and fish size change the ratio, so values are quoted as universal constants.
What to do instead
Use FCR = total feed offered (dry weight) ÷ total fish weight gain (wet weight). Work one numerical example with 120 kg feed and 80 kg gain, giving FCR 1.5, and state that FCR rises with poor feed, overfeeding and small fish size.
Labelling every skin ulcer, gill discolouration or mortality event as 'bacterial disease' and prescribing antibiotics without checking dissolved oxygen, ammonia or stocking density first.
Disease names are memorised as a list of symptoms; water quality is treated as a separate chapter rather than the common predisposing cause of outbreaks.
What to do instead
Follow a diagnostic order: measure DO, pH, ammonia and temperature; inspect gills and skin; then decide whether the primary problem is environmental, parasitic or bacterial. Recommend antibiotics only with an accurate diagnosis, correct dose and withdrawal period, and state that improving water quality is the first intervention.
Stocking all six composite-culture species at the same density, or omitting the bottom-feeder and weed-feeder groups from the combination.
The six species are listed as a set to be memorised, but their feeding niches and the reason for different stocking ratios are not understood.
What to do instead
Explain the niche: surface feeder (catla), column feeders (rohu, silver carp), bottom feeders (mrigal, common carp) and weed feeder (grass carp). Give a typical ratio such as 4:3:2:1 or 3:4:2:1 and justify it by available natural food and planned supplementary feeding.
Classifying a gillnet as an active gear, or stating that mesh size has no role in selective fishing.
Gear categories are memorised by name; the mechanism of capture - entanglement or gilling versus active pursuit - is not connected to selectivity and bycatch.
What to do instead
Define passive gear as gear that waits for the fish (gillnet, trap, longline) and active gear as gear that is towed or dragged (trawl, purse seine). For gillnets, state that mesh size determines which size class is gilled, so a larger mesh lets juveniles escape and reduces bycatch.
Describing icing as 'putting fish in ice' without layering, draining melt water, or maintaining a fish-to-ice ratio and temperature close to 0°C.
The purpose of icing - removing heat and slowing bacterial and enzymatic spoilage - is not linked to practical handling; students omit that melt water must drain and that uneven icing creates warm pockets.
What to do instead
State the practice: chill fish rapidly, use clean ice at roughly 1:1 fish-to-ice depending on duration, layer fish and ice in an insulated box, drain melt water and keep the box shaded. Explain that temperature abuse, not lack of ice, is the usual cause of rapid spoilage.
In a cost-benefit answer, listing only feed, seed and labour as costs and treating pond lease, depreciation, family labour and interest on working capital as free or nonexistent.
Farm accounts are confused with cash payments; students forget that economic cost includes owned resources and the annual share of durable assets.
What to do instead
Build a cost table with variable costs (seed, feed, fertiliser, lime, labour, fuel) and fixed costs (pond lease or rent, depreciation of pump and nets, interest, insurance). Then calculate gross revenue, net income and benefit-cost ratio, and state the assumptions used.