Kinetics of the Chronic Stress Response to Stocking Density and Impacts on Infectious Hematopoietic Necrosis Virus Mortality in Rainbow Trout

Abstract

The goal of this study was to examine the effect of stocking density on the stress response and susceptibility to infectious hematopoietic necrosis virus (IHNV) induced mortality in fingerling rainbow trout (Oncorhynchus mykiss) housed in flow through systems at 15C. To characterize the stress response to holding conditions, fish (1-2 grams) were sorted into one of two selected stocking densities (high density “HD”, 20-40 kg/m3), or (low density, “LD”, 4-8 kg/m3). Stress indices including cortisol levels in serum, cortisol levels in water, and neutrophil:lymphocyte ratios from blood smears were then measured at multiple time points over 21 days. Serum cortisol was increased at 1-2 hours compared to baseline, and again at 14 days, although increases were only significant for one of the two density treatments at each peak. Water cortisol concentrations were significantly higher in LD tanks compared to HD tanks on day 14, and generally elevated in both groups on day 17 and 21. Neutrophil: lymphocyte ratios were significantly elevated in HD tanks on Day 14 compared to LD tanks and from baseline. In a virus challenge experiment, the effect of density on IHNV mortality with selected M genogroup isolate HG508 (4x102 pfu/ml) was compared between fish that were sorted into HD or LD conditions and acclimated for 14 days prior to virus challenge (HDa and LDa), and fish that were sorted into HD or LD conditions (HDna and LDna) without acclimation before being immediately challenged with virus at the same time as fish in the density acclimated tanks. Mortality was monitored for 21 days post challenge. No significant differences in cumulative percent mortality were found between HD and LD treatment groups, but overall fish held at higher density and/or fish not acclimated to holding conditions had slightly higher mortality than fish at low density and/or acclimated prior to challenge. Results indicate that: rainbow trout (1-2 grams) did not demonstrate significant differences in stress indices at the density conditions tested; were not significantly affected in the ability to adapt to densities of 4-26 kg/m3 over a 21 day period, (with the exception of day 14); and that stocking densities up to 26 kg/m3 did not have an effect on IHNV mortality.