Fisheries: Culture Systems, Water Quality and Fish Biology
Core concepts
Fisheries questions reward clear classification. Separate capture fisheries (wild stock) from culture fisheries (farming). Within aquaculture, know extensive, semi-intensive and intensive systems by stocking density and inputs; monoculture vs polyculture (polyculture uses complementary species occupying different niches, e.g., a column feeder, a bottom feeder and a surface feeder); and composite fish culture with Indian major carps - catla (surface), rohu (column) and mrigal (bottom).
Water quality is central: dissolved oxygen (higher is better; low DO causes gasping and mortality), temperature, pH (near neutral to slightly alkaline is ideal), ammonia and nitrite toxicity, and turbidity. Understand plankton as the base of pond food webs - phytoplankton for primary production, zooplankton for fry. Distinguish fertilisation (adding nutrients to boost plankton) from supplementary feeding (adding feed).
Fish biology: know brackish vs freshwater vs marine species; catfish (air-breathing, e.g., magur, singhi), exotic vs indigenous species (tilapia, silver carp are exotic); and breeding terms such as induced breeding using pituitary extract, hatchery, and nursery/rearing/stocking ponds. Disease basics: fin rot, dropsy, and the role of stress in outbreaks.
Traps
Questions confuse fertilisation with feeding, or label a surface feeder as a bottom feeder. Watch for 'which is exotic' versus 'which is indigenous'. Do not assume low oxygen is caused by low temperature alone - high temperature and organic load both reduce DO.
Rule of thumb
For polyculture questions, use the niche rule: surface (catla), column (rohu), bottom (mrigal) - complementary feeding zones reduce competition and raise total yield.