February 15-18, 2009 Washington State Convention Center Seattle ...

12
ASSESSING THE HEALTH OF RAINBOW TROUT Oncorhynchus mykiss IN LOW-
EXCHANGE RECIRCULATING SYSTEMS OPERATED WITH OR WITHOUT OZONATION
Christopher Good*, John Davidson, Carla Welsh and Steven Summerfelt
The Conservation Fund’s Freshwater Institute
1098 Turner Road
Shepherdstown, WV 25443 USA
c.good@freshwaterinstitute.org
Previous observations by the authors have indicated that rainbow trout Oncorhynchus mykiss mortality increases sharply in
recirculating aquaculture systems (RAS) when water exchange rates are lowered to the point where feed loading rates are approximately
1.3-2.0 kg/d per m 3 /d of make-up water flow. All measured water quality parameters were within safe limits at the
time, and no specific cause of mortality was ascertained. Ozonation of the water appeared to remediate the fish health decline.
Subsequent research at The Freshwater Institute has focused on examining environmental conditions and fish health in low
exchange RAS with high feeding rates, as the previously observed mortality phenomenon could represent an important barrier
for economic sustainability in commercial recirculating aquaculture.
The objective of the present study was to evaluate the effects of water ozonation on fish health and performance in RAS with
low exchange and high feeding rates. Six replicated RAS, all operated at low exchange rates (0.26% of the total recirculating
flow), were each stocked with 1000 rainbow trout, and densities were maintained between 40 and 80 kg/m 3 for the entire study
period. Additionally, three small tanks in a flow-through system each received 100 rainbow trout, and were fed and maintained
under similar conditions as a physiological comparison group. Three of the six RAS were randomly selected to receive water
ozonation (to an oxidation-reduction potential of 250mV), while the remaining three RAS did not receive ozonation. Average
fish weight across all systems was approximately 300 grams at the time of treatment initiation. Fish were subsequently raised
for four months under these conditions; during the first month after ozone addition, tanks were fed equal portions to investigate
water quality and unit process treatment efficiency differences, after which each tank was fed to satiation.
Fish health and performance were assessed with daily mortality and monthly length and weight data collection, as well as fin
index evaluation, gill, liver and ovary minerals and metals content screening, blood gas and blood chemistry analyses, and multiple
tissue sampling for histopathological assessment, all carried out during the final week of the study period. By study’s end,
there was no significant (p