Using the common carp Cyprinus carpio L. in polyculture ponds to increase both phyto- and zooplankton availability has become a popular practice for the cultivation of planktivorous fish, such as rohu (Labeo rohita Hamilton).
Most of the nutrients in ponds are stored in bottom sediments in both organic and inorganic forms. According to previous research, sediment can store 100 to 1000 times more nutrients than water.
Transference of nutrients back into the water column by the resuspension of sediment can have an important influence on the limnology of ponds.
Several fish species often resuspend sediment; the best known is the common carp Cyprinus carpio.
The common carp feeds on benthic organisms, and it often affects aerobic decomposition of organic matter and nutrient availability in the water column via bioturbation of benthic sediment.
An increase in nutrient availability may enhance photosynthesis and the re lated phyto- and zooplankton production. Therefore, common carp is commonly used in polyculture ponds to increase both phyto- and zooplankton availability for planktivorous fish.
Planktivorous fish reduce algal biomass and water turbidity, which in turn has an important im pact on photosynthesis. Therefore, the combined effect of benthivorous and planktivorous fish on pond limnology is highly complex and has not been well quantified.
If resuspension of sediment by common carp is excessive, soluble phosphorus availability and water clarity typically decrease, thereby reducing photosynthesis and phytoplankton production.
Therefore, the effect of common carp on pond limnology is largely dependent on its density.
Considering both the positive and negative effects of common carp on nutrient and food web dynamics, stocking density of common carp is important for understanding pond ecology.
Almost nothing is known about the effects of different densities of common carp on pond ecology in the presence of planktivorous fish.
In a prior study, the effects of different densities of common carp on pond environments, plankton and benthic macroinvertebrate availability, and fish production in rohu ponds were examined.
The prior study did not reveal the underlying mechanism affecting how the limnological condition of ponds changes over time. Such information is important to ensure adequate limnological conditions for the production of fish.
Surplus natural food, along with good environmental conditions, lead to the stocking of an additional species or increasing the stocking density. Both activities further increase fish yield.
In the current study, the effects of different densities of common carp on pond ecology with respect to the effects on nutrients, phytoplankton, zooplankton and benthic macroinvertebrate dynamics, and fish growth over time are described.
The main objectives were: (1) to better understand the nature of nutrients, phytoplankton, zooplankton and benthic macroinvertebrate dynamics over time in rohu ponds with different densities of common carp and (2) to elucidate the possibility of further increasing fish production in these types of ponds.
The results showed that all environmental parameters and all groups of phytoplankton, zooplankton and benthic macroinvertebrates significantly changed over time, although trends in these changes were inconsistent at different common carp densities.
The correlation between phosphate-phosphorus (PO4-P) and total phytoplankton biomass indicated that the phytoplankton biomass was limited by low PO4-P concentrations in ponds without common carp.
Common carp-driven resuspension increased nitrogen and phosphate fluxes from the sediment to the water column and subsequently increased primary and secondary production.
A stocking density of 0.5 common carp m−2 had strong effects on nutrients and both phyto- and zooplankton availability, with an increasing trend over time.
These effects were partially lost in ponds with 1 common carp m−2, which can be considered as overstocking.
This study suggests that an optimal density of common carp can be used as a management tool to manipulate the aquaculture environment for better growth and production of fish.
May 2015
