Assessing the Efficiency of Herbivorous Fish as Nutrient Sinks in an Integrated Freshwater Cage Aquaculture System Using Phosphorus Mass Balance Model

Abstract

Integrated aquaculture systems simultaneously culture multiple species to reduce waste outputs by increasing trophic efficiency and nutrient retention. Using a phosphorus (P) mass balance model, this study assessed the efficacy of an integrated aquaculture system that used herbivorous fish to reduce waste loading into a freshwater reservoir in Guizhou Province, China. Six modified cages featured the addition of a 1 m wide outer cage surrounding a 10 m square inner cage stocked with intensively fed channel catfish (Ictalarus punctatus). Outer cages were stocked with filter-feeding herbivorous fish including, bighead carp (Hypophthalmichthys nobilis), common carp (Cyprinus carpio), and Nile tilapia (Oreochromis nilocticus). These species fed on phytoplankton, particulate waste from the inner cage, and periphyton, thereby retaining nutrients and possibly improving water quality around the cages. This experiment compared phosphorus balances of modified cages with three traditional cages also stocked with channel catfish, but which did not include an outer cage. Additional experiments compared growth rates of fish in outer cages to fish in a control cage distant from any impacts of intensively fed inner cages. Water samples were also taken to measure total phosphorus concentrations inside the cages, 1 m outside cages, and at reference sites 500-1000 m away from cages. Outer cage fish retained 0.08-0.1 kg P ton-1 harvested fish or <1% of total P inputs cage-1. Catfish in traditional cages grew slower (p<0.05) than fish in modified cages. However, there was no difference in retained phosphorus, total waste, soluble waste, or solid waste ton-1 of fish cultured between the cages. Total kg P input ton-1 fish harvested ranged from 14.1-18.8. According to the 2 model, catfish retained 24-43% of total P inputs; in addition, catfish particulate waste was 38.5% and soluble waste 18-37% of total P inputs, respectively. During grow-out, bighead carp and tilapia in outer cages increased body mass by 21% and 75%, respectively, while these same species in the control cage exhibited -3% and 0% growth. This supports the hypothesis that fish in outer cages had access to surplus energy drifting out of inner cages and successfully retained nutrients from inner cages. Contrary to expectations, total phosphorus concentrations in water samples showed no difference (p>0.05) between modified cages, traditional cages, and reference sites. Even though I concluded that outer cage fish retained nutrients from inner cages, retention was not substantial enough to improve water quality around modified cages. However, since phosphorus loading from cages had no impact on reservoir water quality, this suggests that phosphorus inputs from aquaculture are rapidly diluted and dispersed in the reservoir.