Abstract
This chapter describes the application and optimization of the recirculating aquaculture systems (RAS) technology in the marine sector, in particular the development of urban recirculating mariculture for high-value marine fish. The system's performance and economic feasibility were tested in a pilot urban mariculture programme in the city of Baltimore (Maryland, USA), studying the Mediterranean gilthead seabream (Sparus aurata [Pagrus aurata]) as its candidate species. This fish, a non-native species in North America, commands a local retail price of up to US$20/kg. The Baltimore Urban Recirculating Mariculture System was designed to produce high-value marine fishes that cannot be farmed in net-pens or ponds, to use municipal pre-existing infrastructure and services, to have the ability to locate anywhere and to maximize the re-use of water. The life support system consisted of a particle removal microscreen drum filter, a moving bed nitrifying reactor, an ozone-based protein skimmer and a low head oxygenation unit. Conditioned artificial seawater was automatically delivered to provide the desired salinity and temperature. pH, ozone levels and photoperiod were continuously monitored and adjusted. Strict biosecurity was achieved by disinfecting all waste effluents before their discharge to the municipal sanitary sewer. Using this system, gilthead seabream of two strains were grown from 0.5 to 400 g commercial size in 268 days (first strain) and to 410 g in 232 days (second strain). Survival rates exceeded 90% and food conversion rates varied from 0.87 to 1.89, depending on fish growth. Growing densities ranged from 44 to 47 kg/m3 at 7-10% daily water exchange rates. Total ammonia and nitrite levels remained significantly below stressful concentrations. To increase the economic feasibility of the system, microbial communities associated with biofiltration were studied in an effort to improve nitrogen removal and thus maximize re-use of the saltwater. New bacterial-mediated nitrogen removal processes are described herein and addition of an anaerobic denitrification unit was also studied, both of which enhanced the ability to minimize saltwater discharge. The environmentally compatible recirculating mariculture pilot system described here can be scaled up to cost-effectively produce high-value marine fish in an urban setting.