Impact of urea nitrogen fertilizer on the physiology, behavior, and histology of juvenile rainbow trout.

Excessive use of nitrogen (N) fertilizers can induce metabolic alterations in fish by disrupting nitrogen metabolism and depleting energy reserves needed to maintain nitrogen homeostasis. This study evaluated the acute (96-h LC50) and prolonged (14-day) effects of urea nitrogen fertilizer (5 g L⁻¹ was determined to be the lowest tested concentration with no observable effects) on the physiological, histological, and behavioral responses of rainbow trout (Oncorhynchus mykiss) juveniles. Significant increases in internal urea levels were observed in the blood, gills, and kidneys of exposed fish, alongside decreased glucose levels in blood and renal tissues. The kidneys showed time-dependent changes in urea levels, while circulating glucose levels varied across tissues, with the kidneys exhibiting the most pronounced response. These variations reflect physiological compensatory mechanisms in response to urea-N exposure. A 14-day exposure to urea caused both functional and structural changes in the gills, liver, and kidneys. Increased swimming activity was also noted, persisting throughout the exposure period. Water urea measurements throughout 14-days period were significant but less informative for identifying patterns associated with urea level changes. Further research is needed to investigate the effects of other nitrogen compounds, particularly those associated with eutrophication, and to optimize the use of these biomarkers for indicating metabolic consequences across various biological matrices.