Noninvasive assessment of nitrate-induced stress in koi Cyprinus carpio L. by faecal cortisol measurement

Abstract

This study determined effects of nitrate (NO3−)-induced stress on faecal cortisol levels in koi (Cyprinus carpio). NO3− was presented in two formats: (1) bulk increase followed by dilution to assess cortisol response and recovery and (2) bulk increase followed by incremental increases to assess cortisol response limit. Fish were maintained group-wise in treatment and control aquaria (n=6 per group), and 0.5 g L−1 NO3− (as NaNO3) was added to the water. Faecal samples were collected daily and blood samples were taken pretreatment and 72-h posttreatment and were assayed for cortisol via ELISA. NO3− increased plasma and faecal cortisol 4.4-fold and 3.9-fold respectively. Plasma cortisol was not measured further. After a 74% NO3− decrease by dilution, faecal cortisol decreased to baseline within 24 h, and restimulation by NO3− (1.0 g L−1) elevated faecal cortisol to maximal study levels. In a separate experiment, exposure to 0.4 g L−1 NO3− increased faecal cortisol 6.9-fold. However, three additional 0.4 g L−1 NO3− increments across 9 days did not further increase cortisol. This study demonstrates that faecal cortisol measurement in fish via an ELISA can be useful as an indicator of NO3−-induced stress in the aquatic environment.