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

This study determined effects of nitrate (NO(3)-)-induced stress on faecal cortisol levels in koi (Cyprinus carpio). NO(3)- 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 NO(3)- (as NaNO(3)) 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. NO(3)- increased plasma and faecal cortisol 4.4-fold and 3.9-fold respectively. Plasma cortisol was not measured further. After a 74% NO(3)- decrease by dilution, faecal cortisol decreased to baseline within 24 h, and restimulation by NO(3)- (1.0 g L-1) elevated faecal cortisol to maximal study levels. In a separate experiment, exposure to 0.4 g L-1 NO(3)- increased faecal cortisol 6.9-fold. However, three additional 0.4 g L-1 NO(3)- 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 NO(3)--induced stress in the aquatic environment.