Physiological consequences for rainbow trout (Oncorhynchus mykiss) of short-term exposure to increased suspended solid load

Abstract

Suspended solids are an unavoidable component of fish farming, and especially so in recirculating aquaculture systems (RAS) where fine particles accumulate over time. High levels of fine particles are widely regarded as harmful to fish health and welfare. However, little is known about the direct impact of these accumulating particles on stress, performance, health or other welfare parameters of fish. In this study, the effects of solids on rainbow trout were investigated specifically, uncoupled from other potentially confounding water parameters. To this end, the suspended solid load in a replicate RAS was artificially increased by a factor of 7 to over 30mg total suspended solids (TSS)/L, alongside a sister system operating normally, with a maximum suspended solid concentration of 5mg/L. At these levels, the concentrations of particles smaller than 32μm amounted to 0.3±0.3mg/L in the control system and to 8.0±2.7mg/L in the treatment system. With the exception of turbidity, all further water parameters generally considered important to salmonid welfare were kept comparable and well below harmful levels. In consequence, fish performed well in both RAS, indicating good husbandry conditions. Feeding behavior was observed to differ slightly between control and treatment RAS, but without any apparent effect on performance.The impact of the accumulating particles on fish was examined using a wide range of physiological assays. No significant differences in stress markers (heat shock protein 70, plasma cortisol) were detected between fish in the control and treatment RAS. The same was found for hematological assays (differential leukocyte count, hematocrit, RBC indices, etc.). Fin condition was also unaffected by increased suspended solid load and most surprisingly, histological examination not only revealed no detrimental effects of particle accumulation, but showed the gill status of fish in the solid treatment to be better than that of control fish.Overall, this study shows that by itself, short-term exposure to suspended solids at concentrations of 30mg/L has no detrimental effect on rainbow trout.