Preliminary Study On Fish Welfare Indicators In Marine Recirculating Aquaculture Systems

Since the pollution on the coastal area and the reduction of the undergroundseawater resource, the traditional flow-through or pond culture style did not go withthe tide of the aquaculture development. Recirculating aquaculture system is growingto the most important culture way in the future sea-farming in view of its advantageon the stability of the system, safety of the product, and environmental pollution-free.In pursuit of high quality and quantity of seafood, the research on fish welfare is moreand more crucial. In this study, two potential fish welfare indicators were selectedfrom the primary density stress study. Then the relation between fish health andindicators was investigated. It aims to promote RAS style in a healthy, sustainable,smooth way. The main results are as follows,1. Turbot (Scophthalmus maximus)(13.84±2.74g; average weight±SD) werereared at four different initial densities (low0.66, medium1.26, sub-high2.56,high4.00kg/m2) for10weeks in12independent recirculating systems (RAS) at23±1°C. Final densities were4.67,7.25,14.16, and17.47kg/m2, respectively,which translate to82,108,214, and282percent cover of the bottom of the tank.①Density had both negative and independent impacts on growth. The final meanweight, specific growth rate (SGR), and voluntary feed intake significantlydecreased and the coefficient of variation (CV) of final body weight increasedwith increased stocking density. While as a benthic fish, with enough supplyof feed and good water quality, stocking density had no influence on fishgrowth within a certain density range.②When density increased, feed intake decreased since the increased socialcompetition. Thus feed intake is an optimal indicator on density stressmonitoring. For culture at higher density, multi-level feeding devices aresuggested to ease feeding competition. ③The protease activity of the digestive tract at pH7,8.5,9, and10wassignificantly higher for the highest density group, whereas it had a tendency tobe lower (but not significantly) at pH4and8.5for the lowest density group.The intensity of protease activity was inversely related to feed intake at thedifferent densities possibly due to that fish at high density took compensatorystrategy to promote nutrition assimilations.④Catalase activity was higher (but not significantly) at the highest density,perhaps because high density induced an oxidative effect in turbot. It issuggested the density is lower than the high density.⑤Under different densities, fish body was obviously different visually. At thesame time, the regular water indexes were within an optimal range while ORP(oxidation-reduction potential) was different under different densities. Thus,body color and ORP would be two potential indicators for fish welfare.2. The relation between fish body color and welfare,①The experiment of9color backgrounds selection was conducted. Theappearance frequency of juvenile turbot in purple and pink background wassignificantly higher than that in black and red one.②Body hue change rate under black and red backgrounds was more drastic andintensive than under pink and purple backgrounds based on computer visiontechnology. Results indicated that fish need more physiological adjustment toadapt backgrounds of different colors and it is better to use lighter color as thebackground rather than black or red color.③Body brightness change rate under the black background decreasedsignificantly after acute ammonia stress. Thus turbot body brightness changerate was an optical indicator for welfare monitoring.3. The relation between ORP and fish welfare, water quality in marine RAS,①After several short-term ORP trials, ORP level in fish tank should be lowerthan320mV in marine RAS considering fish welfare and water quality inmarine RAS.②With the increase of ORP and dissolved oxygen in fish tanks, the blood Hctand Hb would decrease accompanied with the increased bloody oxygen evenafter two weeks recovery (normal ORP and DO).③When long-term ORP was kept between300and320mV, feed intake would decrease and growth was slowly. Thus for long-tem operation, ORP wassuggested around300mV.④It is the first time to found that with the long-term promoted ORP at300-320mV, fish were more against vibrio infection.