Effect of rearing density on nervous necrosis virus infection in Senegalese sole (Solea senegalensis)

Intensive fish farming at high densities results in a wide range of adverse consequences on fish welfare, including pathogen spreading, stress and increased mortality rates. In this work, we have assessed whether the survival of Senegalese sole infected with the nervous necrosis virus (NNV), a pathogen responsible for severe disease outbreaks, is affected by rearing density. Based on the different fish ratios per surface area (g cm−2) and water volume (g L−1), our research showed an earlier mortality onset in the tanks containing NNV‐infected fish reared at medium density (MD: 0.071 g cm−2/5 g L−1) and high density (HD: 0.142 g cm−2/10 g L−1), as well as higher cumulative mortality values. However, transcription analysis of hsp70, gr1 and pepck genes, well‐known stress biomarkers, seems to indicate that none of the challenged fish were under high stress conditions. NNV load was slightly higher both in dead and in sampled fish from MD and HD groups, and especially in the rearing water from these groups, where peaks in mortality seemed to correlate with increasing NNV load in the water. In conclusion, our results suggest that rearing NNV‐infected Senegalese sole at high densities resulted in an earlier mortality onset and higher cumulative values and viral load.