The effects of introduction of common carp, Cyprinus carpio (L,), in poiycuiture with major Indian carps and silver carp were studied in fertilized and fed earthen ponds in Mymensingh. Bangladesh. Bottom-dwelling common carp were found to exert an influence on the quality and quantity of planktonic food organisms. The presence of common carp significantly increased (P < 0.05) the growth of other carps including the bottom-feeder mrigal, Cirrhinus mrigala (Hamilton). A significantly higher fish yield (P<0.05) was observed in the fish ponds containing common carp when the combined yields of all species in three treatments were compared. 619

The tilapia Oreochromis niloticus and the silver carp Hypophthalmichthys molitrix were exposed to toxic and non‐toxic strains of the cyanobacterium Microcystis aeruginosa in order to determine if cells of the toxic strain were ingested and, if not, by what mechanism they were excluded. Enumeration of cyanobacterial particles before and after exposure to fish showed that there were no significant differences (P<0.05) at the end of the trial between the toxic treatment and the control consisting of toxic M. aeruginosa with no fish. Fish exposed to the non‐toxic strain increased opercular beat rate, elevating the volumes of water and food material passed over the gills whereas those that were held in the toxic strain did not. Of the cyanobacterial toxins (microcystins) presented to the fish, most were in the cyanobacterial cells, toxin levels in the water being below the level of detectability (<250 ng l−1), The ability of the fish to differentiate between toxic and non‐toxic cyanobacterial strains may thus be determined by very low levels of extracellular microcystins or/and other features which distinguish toxic from non‐toxic M. aeruginosa strains, such as cell surface components.

Abstract:In order to understand the present status of hatcheries, questionnaire interview and participatory rural appraisal tools were used in North-west Fisheries Extension Project (NFEP), Parbatipur during the period from July to December 2015. There are two hatcheries in NFEP, Parbatipur-a carp hatchery and a prawn hatchery. The production of spawn was around 611 kg in the year 2015. According to the target hatchery manager collected brood fish from rivers and other sources which he maintained in their brood stock ponds with proper management. The hatchery manager maintained proper age and weight of broods for spawning and followed the guideline of hatchery operation so that the quality of spawn and fry were good. It was found that the hatcheries produced fish seed of a variety of species such as Indian major carps, carpio, sarpunti, silver carp, grass carp, magur, shing as well as freshwater prawn. The hatchery production activities took place starting from February and continued up to September. The quantity of fish seed produce varied from 25 to 75 kg per hatching cycle depending on size and facilities of hatcheries. The price of fry in NFEP was being fixed by the hatchery manager. However, the price variation was seen due to seasonal variation, species and demand. The average price of fish seed was highest in monsoon and lowest in winter. A number of middlemen were involved in the fry marketing channel of NFEP at Parbatipur. They were responsible for increasing the price of fry at each stage of marketing channel.

Single-species groups of fry of the planktivorous, filter-feeding carps (silver carp Hypophthalmichthys molitrix, bighead carp Aristichthys nobilis, rohu Labeo rohita and catla Catla catla) were exposed to the bacterium Chromobacterium violaceum to assess if they could detect or ingest unattached bacteria. The results indicate that planktivorous carp fry are able to ingest unattached bacteria and that the numbers ingested increase with time and with bacterial concentration in the media, although probably by passive means. It is also suggested that unattached bacteria are of little nutritional significance to carps.

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