Fish Biology, Taxonomy and Anatomy
Classification you should be able to recite
Finfish are phylum Chordata, subphylum Vertebrata. Chondrichthyes (sharks, rays) have placoid scales and no swim bladder; Osteichthyes have cycloid or ctenoid scales and a swim bladder. Indian freshwater food fishes sit mainly in Cypriniformes, family Cyprinidae (catla, rohu, mrigal, silver carp, grass carp, common carp), Siluriformes (Clarias, Heteropneustes, Mystus, Wallago), Channidae (murrels), Perciformes (tilapia, Anabas) and Clupeiformes (Hilsa, a clupeid, not a carp).
Anatomy that answers marks
- Fins: dorsal, paired pectoral and pelvic, anal and caudal; the caudal fin is homocercal in most bony fishes.
- Scales: cycloid in carps, ctenoid in perch-like fishes; scale rings are read for age.
- Gills: four pairs of arches bearing filaments and gill rakers; rakers are long and close-set in plankton feeders.
- Swim bladder: hydrostatic organ; in cyprinids it connects to the inner ear through the Weberian apparatus.
- Accessory air-breathing organs allow survival in low-oxygen wetlands: arborescent organs in Clarias, a suprabranchial chamber in Anabas and Heteropneustes.
- Mouth position shows the feeding zone: upturned and superior in catla (surface planktivore), terminal in rohu (column omnivore), inferior in mrigal (bottom detritivore). Catla has no barbels; rohu has one pair of small maxillary barbels.
- Gut length: short in carnivores, long and coiled in herbivores.
Worked example
A rohu of 30 cm total length weighs 450 g. Fulton condition factor K = 100 W / L-cubed = (100 x 450) / 27,000 = 1.67. A value near 1.0-1.5 indicates a normal well-fed carp; 1.67 suggests good condition, while a falling K across sampling dates signals poor feeding or stress. The length-weight relation W = aL^b with b near 3 shows isometric growth.
Traps that cost marks
- Placing catla at the bottom because of its large size, or giving rohu the surface zone.
- Writing Hilsa under Cyprinidae.
- Mixing up ctenoid and cycloid scale types between carps and perches.