The potential of fish production based on periphyton

Periphyton is composed of attached plant and animal organisms embedded in a mucopolysaccharide matrix. This review summarizes research on periphyton-based fish production and on periphyton productivity and ingestion by fish, and explores the potential of developing periphyton-based aquaculture. Important systems with periphyton are brush-parks in lagoon areas and freshwater ponds with maximum extrapolated fish production of 8 t ha(-1) y(-1) and 7 t ha(-1) y(-1), respectively. Experiments with a variety of substrates and fish species have been done, sometimes with supplemental feeding. In most experiments, fish production was greater with additional substrates compared to controls without substrates. Colonization of substrates starts with the deposition of organic substances and attraction of bacteria, followed by algae and invertebrates. After initial colonization, biomass density increases to a maximum when competition for light and nutrients prevents a further increase. Often, more than 50% of the periphyton ash-free dry matter is of non-algal origin. Highest biomass (dm) in natural systems ranges from 0 to 700 g m(-2) and in aquaculture experiments was around 100 g m(-2). Highest productivity was found on bamboo in brush-parks (7.9 g C m(-2) d(-1)) and on coral reefs (3 g C m(-2) d(-1)). Inorganic and organic nutrients stimulate periphyton production. Grazing is the main factor determining periphyton density, while substrate type also affects productivity and biomass. Better growth was observed on natural (tree branches and bamboo) than on artifical materials (plastic and PVC). Many herbivorous and omnivorous fish can utilize periphyton. Estimates of periphyton ingestion by fish range from 0.24 to 112 mg dm (g fish)(-1) d(-1). Ingestion rates are influenced by temperature, fish size, fish species and the nutritional quality of the periphyton. Periphyton composition is generally similar to that of natural feeds in fishponds, with a higher ash content due to the entrapment of sand particles and formation of carbonates. Protein/Metabolizable Energy (P/ME) ratios of periphyton vary from 10 to 40 kJ g(-1). Overall assimilation efficiency of fish growing on periphyton was 20-50%. The limited work on feed conversion ratios resulted in values between 2 and 3. A simple simulation model of periphyton-based fish production estimates fish production at approximately 2.8 t ha(-1) y(-1). Together with other food resources in fishponds, total fish production with the current technology level is estimated at about 5 t ha(-1) y(-1). Because grazing pressure is determined by fish stocking rates, productivity of periphyton is currently the main factor limiting fish production. We conclude that periphyton can increase the productivity and efficiency of aquaculture systems, but more research is needed for optimization. Areas for attention include the implementation and control of periphyton production (nutrient levels, substate types and conformations), the ratio of fish to periphyton biomass, options for utilizing periphyton in intensive aquaculture systems and with marine fish, and possibilities for periphyton-based shrimp culture.