Effects of rearing density and water current on the respiratory physiology and haematology in rainbow trout, Oncorhynchus mykiss at high temperature

Rearing fish at high densities is considered stressful. For many species this leads to poor feed conversion, decreased feed intake and reduced growth, possibly caused by a change in energy partitioning or expenditure. To test this theory, we examined how rearing density and water current affected the respiratory physiology during rest and exercise in rainbow trout. Trout were also subjected to acute stress to examine whether rearing conditions affected the haematological response. Fish were acclimated at low (L) 25kgm⁻³ and high (H) 100kgm⁻³ densities, in either still water (O) at <5cms⁻¹ or in a water current (C) at a speed of 0.9bodylengths⁻¹, in 600litercircular tanks for a period of 63days. As recirculation aquaculture facilities aim to reduce water use, this is frequently associated with increasing water temperatures, particularly during summer months. For this reason experiments were performed at 19°C. Water flow rate to each tank was maintained at 60lmin⁻¹, to eliminate water quality and feed availability as influencing parameters. Standard metabolic rate of fish reared in a current was significantly lower (118.4±2.7mg O₂kg⁻¹h⁻¹) than for those reared in still water (133.4±3.7mg O₂kg⁻¹h⁻¹) with no effect of density. The absence of any effects of density on standard metabolic rate showed that rearing at high densities in still water did not affect energy required for maintenance in inactive, post-absorptive rainbow trout. Swimming at high speeds incurred a greater cost of transport when reared in a current. The haematological profile of fishes at rest was not affected by rearing conditions, suggesting that changes in metabolism are not related to the oxygen carrying capacity of the blood, and all treatment groups showed a haematological response to stress of similar magnitude. The absence of any effects of rearing density on the respiratory physiology in the present study emphasises the importance of water quality and food availability in farming of rainbow trout.