Low-density silver carp Hypophthalmichthys molitrix (Valenciennes) polyculture does not prevent cyanobacterial off-flavours in channel catfish Ictalurus punctatus (Rafinesque).

摘要

Silver carp Hypophthalmichthys molitrix (Valenciennes) were co-cultured with channel catfish Ictalurus punctatus (Rafinesque) in 0.4 ha earthen ponds to determine the impacts of carp grazing on pond phytoplankton communities and cyanobacterial off-flavours in catfish. Carp were stocked at densities of 0, 75, or 250 fish ha-1 in seven replicate ponds per treatment. The mean chlorophyll a concentrations (a measure of phytoplankton standing crop) steadily increased in all treatments from about 100 micro g L-1 in April to more than 400 micro g L-1 by mid-October. Silver carp had no affect (P0.1) on chlorophyll a concentrations across all sampling dates (April though October) or for sampling dates late in the growing season (August-October) when the prevalence of cyanobacterial off-flavours among catfish populations is usually greatest. Silver carp did not eliminate odour-producing cyanobacteria from pond phytoplankton communities: on sampling dates in September and October, three to six ponds in all treatments contained populations of the odour-producing cyanobacteria Oscillatoria perornata, Anabaena spp., or both. Failure of silver carp to eliminate odour-producing cyanobacteria resulted in a relatively high incidence in all treatments of ponds with off-flavoured catfish. On sampling dates in September and October, catfish in three to five ponds in each treatment were tainted with either musty (2-methylisoborneol) or earthy (geosmin) off-flavours. The presence of silver carp had no obvious effect on off-flavour intensity: on each sampling date, at least three ponds in each treatment contained catfish described as distinctly to extremely off-flavored. Apparently, hypertrophic conditions in catfish ponds overwhelm the effect of silver carp grazing at the low carp densities used in this study..