| Summary: | Integrated recirculating aquaculture systems (IRASs) were developed to overcome the problems of nutrient overloading and to address the issues of animal welfare in recirculating aquaculture systems. Barramundi (Lates calcarifer Bloch) (initialweight of 13.34±0.01 g)were reared for 84 days in 12 (4× 3 design) independent IRASs at four densities: 10.10, 12.98, 15.86 and 18.75 kgm-3. Throughout the trial, the airwas the only source of dissolved oxygen to the fish-rearing tank, and duckweed (Lemna minor Linnaeus) as a biofilter mediumwas used as an additional macrophytic crop in order to qualify as an IRAS. Therefore, each IRAS consisted of a fish-rearing tank, a waste-collection tank and a biofilter tank that housed the L. minor. At the end of the trial, blood samples were taken to determine levels of stress-related parameters (cortisol, glucose, lactate and whole-blood haemoglobin [Hb]) and thyroid hormones (thyroxine [T4] and triiodothyronine [T3]). In order to evaluate the acute stress response, the remaining fish were further exposed to elevated total ammonia, of which a portion was in the more toxic unionised form followed by blood analysis so as to determine the levels of stress-related parameters and thyroid hormones. The results demonstrated that a high stocking density of 18.75 kgm-3 of barramundi significantly decreased the specific growth rate, increased the feed conversion ratio and altered plasma levels of total T4, total T3, cortisol, glucose, lactate and whole-blood Hb. Total plasma concentrations of T3 were inversely correlated with stocking densities, whereas total plasma T4 levels of juvenile barramundi were significantly higher when the juvenile barramundi were stocked at a higher density of 18.75 kg m-3. These results also suggest that the growth suppression of the juvenile barramundi caused by a high stocking density might be associated with a decrease in circulating levels of thyroid hormones as a consequence of small but significant increased cortisol level. Therefore, a stocking density of lower than 18.75 kgm-3 of barramundi is recommended for culturing in IRASs. Statement of relevance: 1. An IRAS is developed to overcome the problem of nutrient overloading and animal welfare problems in recirculating aquaculture systems (RASs). 2. The study is required to establish an optimumstocking density requirement for juvenile barramundi in IRASs. 3. No studies have investigated the effects of stocking density on growth and physiological responses of juvenile barramundi species reared in integrated recirculating aquaculture systems (IRASs).
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