Effects of unionized ammonia and suspended solids on rainbow trout (Oncorhynchus mykiss) in recirculating aquaculture systems

Abstract

This study investigates the individual and combined effects of chronic exposure of rainbow trout to unionized ammonia and suspended solids in a farm-scale recirculating aquaculture system (RAS) over 13 weeks. Unionized ammonia nitrogen concentration was four times (0.05 mg/L) the generally accepted ‘safe’ threshold while total suspended solids (TSS) exceeded the ‘safe’ threshold of 25 mg/L by a factor of >2.5. Still, rainbow trout revealed high survival rates of >99% and no observable detrimental effects of TSS. Bacterial activity showed a close positive linear correlation with solid load and was almost exclusively explained by solid load for TSS concentration > 10 mg/L. However, bacterial activity had no apparent detrimental effect on fish health or performance. Increased unionized ammonia nitrogen concentrations had no relevant detrimental effect on rainbow trout physiology and performance at concentrations of up to 0.05 mg/L. Furthermore, the absent to minor solid-related effects across a wide range of physiological criteria combined with chronic exposure to unionized ammonia demonstrates that chemical or physical irritants are not problematic in RAS if other water and holding parameters are optimal. These findings suggest a greater than expected tolerance of rainbow trout to chronic TSS-related effects which should result in a revision of water quality threshold criteria for RAS.