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Common carp (Cyprinus carpio) is considered to be a very important aquaculture species in many Asian and some European countries. It affects the aerobic decomposition of organic matter and nutrient availability in the water column via bioturbation of benthic sediment during feeding on benthic organisms. If the density of common carp is not excessive, an increase in nutrient availability may enhance photosynthesis and plankton production, whereas if it is excessive, it causes dramatic ecological disruption at both the community and ecosystem levels by changing the abiotic properties of the water. Therefore, the density of common carp is a very important factor that has a great effect on the aquatic ecosystem. The critical density of common carp largely depends on its habitat. In polyculture ponds, water quality, natural food resources and fish growth are strongly affected when the density of common carp approaches more than about 1000 kg ha −1. The critical density can be doubled if artificial feed is supplied to the carp. When its preferred food is not sufficient, the common carp switches to less preferred food and changes its behaviour and feeding niche. These factors make common carp a potential candidate not only for monoculture but also for polyculture ponds. This article reviews the role of common carp on the aquatic ecosystem, and the production and behaviour of fish in aquaculture production systems.

SummaryThis study describes the length-weight (LWR) and lengthlength (LLR) relationships for ten small indigenous fish species from the lower part of the Ganges, Bangladesh, namely Ailia coila, Amblypharyngodon mola, Aspidoparia morar, Clupisoma atherinoides, Eutropiichthyes vacha, Glossogobius giuris, Gudusia chapra, Lepidocephalus guntea, Mystus vittatus, and Puntius ticto. A total of 2142 specimens, representing 10 species of 5 families used for this study were caught by traditional fishing gear from March 2006 to February 2007. Standard length (SL), total length (TL) and fork length (FL) for each specimen were measured by digital slide calipers and each body weight (BW) was taken by a digital balance. The allometric coefficient b of the LWR was close to the isometric value (b = 3.001) in G. giuris, although it suggested negative allometric growth in A. coila, A. morar, C. atherinoides, E. vacha, and P. ticto, whilst positive allometric growth in rest of the species. The results further indicated that the LLRs were highly correlated (r 2 > 0.890; P < 0.01). To the best our knowledge, this study presented the first reference on LWR and LLR for most of the species in Bangladesh.

The e¡ects of introducing common carp (CC) and of adding arti¢cial feed to fertilized rohu ponds on water quality and nutrient accumulation e⁄ciency were studied. All ponds were stocked with 15000 rohu ha À1 . Treatments included ponds with rohu alone, rohu plus 5000 common carp ha À1 and rohu plus 10 000 CC ha À1 . A comparison was also made between supplementally fed and non-fed ponds. The overall highest nitrogen (N) and phosphorus (P) concentrations were observed in ponds with 5000 CC ha À1 , followed by ponds with 10 000 and 0 CC ha À1 . The largest fractions of N and P inputs accumulating in ¢sh, phytoplankton and zooplankton were observed in ponds with 5000 CC ha À1 , followed by ponds with 10 000 CC ha À1 and subsequently ponds without CC. Relatively more nutrients accumulated in benthic organisms in ponds without than in ponds with CC. A smaller fraction of the nutrient input was retained in ¢sh, plankton and benthic organisms in ponds without CC compared with ponds with CC. Compared with 5000 CC ha À1 , stocking10 000 CC ha À1 can be considered as overstocking, because this leads to lower ¢sh production and relatively less nutrients retained in plankton and benthic organisms.

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The embryonic and larval development of local Guchibaim, Mastacembelus pancalus (Hamilton) was studied during May to October 2007. This study presents preliminary observations on the embryonic and larval development of Mastacembelus pancalus under laboratory conditions. The parents stock was collected from different places of Mymensingh district. The eggs were obtained through induction of spawning by use of hormones. At fertilization; the eggs were 0.50 mm in diameter. Samples were taken every 10 minutes interval till completion of morula and then every 1 hour interval up to hatching. After hatching, daily observations took place until the attainment of the fingerling stage. The eggs presented coloration varying from yellow to brownish-green. They were spherical, demersal and adhesive. The stages of embryonic development observed with cleavage, followed by blastula, morula, early gastrula, middle gastrula, late gastrula and until hatching of non-pigmented larvae which displayed total average length of 1.3 mm ± 0.22, 35 hours after fertilization. First cleavage was recorded within 1.05 hrs after fertilization and the embryonic rudiments of developing eggs appeared at 24.30 hrs at 27.0-31.0°C. The yolk sac was completely absorbed at 67 hrs during embryonic development on attainment of 5.50 mm total length. At the same time the digestive system became fully developed and the larvae searched for feeding.