Effects of stocking density of the fishes fed formulated feed and EM product on phytoplankton community in a mussel-fish integrated system

Integrated culture of pearl mussel Hyriopsis cumingii and planktivorous fishes, such as Hypophthalmichthys molitrix and Aristichthys nobilis is widely used in commercial freshwater pearl mussel farming. Previous studies reported that stocking Ctenopharyngodon idella and Carassius auratus gibelio in an integrated system comprising H. cumingii, H. molitrix and A. nobilis and feeding C. idella and C. auratus gibelio with formulated feed could increase pearl production and mussel growth. At present, the integrated system of pearl mussel, C. idella, C. auratus gibelio, H. molitrix and A. nobilis has been widely used as a model for freshwater H. cumingii farming. However, the suitable stocking density of C. idella and C. auratus gibelio remains to be determined. Generally, increasing density of the fishes fed with formulated feed can increase either fish production or waste output, and the increased accumulation of toxic inorganic nitrogen (ammonia and nitrite) and organic matters can negatively affect health and growth of the farmed H. cumingii and fishes. Although microbial products have been recognized as a potential technique to alleviate the environmental impact and enhance the production performance in aquaculture practices, knowledge about application of microbial products in Hyriopsis cumingii farming is scarce. In the present study, a 93-day experiment was conducted in land-based enclosures to evaluate the effect of stocking density of the fishes (C. idella and C. auratus gibelio) fed with formulated feed and effective microorganisms (EM) on structure and function of phytoplankton community in an integrated system of H. cumingii, C. idella, C. auratus gibelio, H. molitrix and A. nobilis. Four treatments abbreviated as LF0 (20 C. idella + 10 C. auratus gibelio + 40 mussel + 8 H. molitrix + 2 A. nobilis), LFA (20 C. idella + 10 C. auratus gibelio + 40 mussel + 8 H. molitrix + 2 A. nobilis + EM supplementation), HF0 (40 C. idella + 20 C. auratus gibelio + 40 mussel + 8 H. molitrix + 2 A. nobilis), and HFA (40 C. idella + 20 C. auratus gibelio + 40 mussel + 8 H. molitrix + 2 A. nobilis + EM supplementation) were established. The pearl mussel included 20 grafted mussel and 20 non-grafted mussel. Each treatment was three replicated, and total 12 enclosures were used. During the experiment, species and biomass of phytoplankton were monitored at the intervals of four weeks, and chlorophyll a (Chl.a) and primary productivity were measured at the intervals of two weeks. Results showed that more than 81 phytoplankton species that belonged to 7 phyla, 32 families and 73 genera were identified. Microcyslis sp. and Scenedesmus sp. dominated in phytoplankton community at the early stage of the experiment, and Microcyslis sp., Merismopedia sp. and Coelosphaerium sp. dominated at the later stage. The phytoplankton biomass ranged from 3.2×10⁸ to 38.3×10⁸ cells/L. No significant differences were found in the species composition of phytoplankton among treatments LF0, LFA, HF0 and HFA. The increase of stocking density of C. idella and C. auratus gibelio resulted in the increase of phytoplankton biomass and community respiration and the decline of the primary productivity. EM supplementation did not significantly affect species composition and diversity of phytoplankton, but resulted in the decrease of the ratio of blue-green algae to phytoplankton biomass and the increase of the primary productivity. This study reveals that the fishes (C. idella and C. auratus gibelio) fed with formulated feed should not be stocked at high densities in the mussel-fish integrated system.