The effects of ozone and water exchange rates on water quality and rainbow trout Oncorhynchus mykiss performance in replicated water recirculating systems

Rainbow trout Oncorhynchus mykiss performance and water quality were evaluated and compared within six replicated 9.5 m(3) water recirculating aquaculture systems (WRAS) operated with and without ozone at various water exchange rates. Three separate studies were conducted: (1)low water exchange (0.26% of the total recycle flow) with and without ozone; (2) low water exchange with ozone versus high water exchange (2.6% of the total recycle flow) without ozone; and (3) near-zero water exchange (only backwash replacement) with and without ozone. Mean feed loading rates for WRAS operated at high, low, and near-zero exchange were 0.40, 3.98, and 55.9 kg feed/m(3) makeup water, respectively. Ozone significantly reduced total suspended solids, color, and biochemical oxygen demand and resulted in a significant increase in ultraviolet transmittance (%)(P<0.10). Ozone also created ambient water quality within low exchange WRAS that was comparable to that of WRAS operated at high water exchange (P>0.10). Additionally, dissolved copper and iron were significantly lower within WRAS operated with ozone (P<0.10). Dissolved zinc was also consistently lower in WRAS operated with ozone, but not significantly (P>0.10). In Studies 1 and 3, total ammonia nitrogen and nitrite nitrogen were slightly lower within the ozonated systems, but were not always significantly lower. In all studies, ozone did not prevent nitrate nitrogen accumulation. At the conclusion of Study 1, rainbow trout growth was significantly greater within low exchange WRAS operated with ozone (P=0.001). At the conclusion of Study 2, rainbow trout growth was similar between treatments (P=0.581), indicating that fish grew equally as well within ozonated WRAS operated at 1/10th the flushing rate as the non-ozonated and high flushing control systems. Overall, ozone created an improved water quality environment within low and near-zero exchange WRAS that generally resulted in enhanced rainbow trout growth rates, survival, feed conversion, and condition factor. (C) 2011 Elsevier B.V. All rights reserved.