Physiological consequences of chronic exposure of rainbow trout (Oncorhynchus mykiss) to suspended solid load in recirculating aquaculture systems

High levels of suspended solids, especially fines, are widely regarded as harmful to fish, particularly in recirculating aquaculture systems (RAS) where accumulation of particles is likely. However, little is known about the consequences of chronic exposure to system-related particles on the stress-levels, well-being and health of fish. In this study, the chronic effects of suspended solids on the physiology and performance of rainbow trout were investigated over the growout period, uncoupled from other potentially confounding water parameters. Compared with fish in a control system with a total suspended solid (TSS) load of 3.9 mg/L, fish in an otherwise comparable treatment system exposed to minimum particle concentrations of more than 30 mg/L exhibited no observable difference in hematological variables (differential leukocyte count, RBC and WBC counts, hematocrit), gill histology, fin condition, or heat shock protein 70 concentrations in gill, liver, skin and head kidney tissues. Slight alterations in feeding behavior and a slight increase in bacterial load on fish and in system water were observed in the treatment RAS, but without any apparent effect on fish performance or health. Furthermore, no significant difference in mortality occurred. The absence of expected effects across a wide range of physiological criteria after long-term exposure suggests that suspended solid levels over 30 mg/L are within the physiological tolerance of this species.