Effects of elevated nitrate on endocrine function in Atlantic salmon, Salmo salar

Recirculating aquaculture systems (RAS) have recently emerged as a sustainable alternative to traditional net pen or flow-through aquaculture. These systems reduce the environmental impact of fish production by treating and recycling culture water, and could be used in the large-scale commercial production of a variety of fish species including Atlantic salmon (Salmo salar). One major concern surrounding RAS is the natural accumulation of nitrate in the culture water. An experiment was conducted to investigate the sublethal effects of elevated nitrate on juvenile Atlantic salmon, with emphasis on thyroidal and steroidogenic endpoints. Animals were exposed to 5.2, 10.3 or 101.8 mg/L nitrate-N for 27 days. Upon completion of the trial, the animals were euthanized and bled by puncture of the caudal vein. Mean plasma nitrate/nitrite concentrations increased significantly with increasing ambient nitrate-N concentration. Plasma testosterone concentrations displayed a highly significant non-monotonic dose response to increasing nitrate-N concentration, and were elevated at 10.3 mg/L nitrate-N. Plasma 11-ketotestosterone, total thyroxine and total triiodothyronine concentrations did not differ significantly between treatments. These results suggest that elevated nitrate can interfere with the synthesis or metabolism of sex steroids, but that Atlantic salmon may be relatively insensitive, in terms of growth and most endocrine endpoints examined, to nitrate-N concentrations up to 101.8 mg/L, and are a promising candidate for production in RAS.