Acute and Chronic Toxicity of Nitrate to Early Life Stages of Zebrafish--Setting Nitrate Safety Levels for Zebrafish Rearing.

Abstract

Recirculating aquaculture systems (RAS) have been widely used for zebrafish rearing, allowing holding of many thousands of fish at high densities. Water quality in RAS largely depends on biofilters that ultimately convert the extremely toxic ammonia excreted by fish into the much less toxic nitrate. However, when water renewal is minimal in RAS, nitrate can accumulate to high enough levels to negatively impact fish welfare and performance. Therefore, the setting of safety levels of nitrate for zebrafish should be a priority to avoid unwanted effects in both the intensive production of this species and research outputs. The present study aimed to define nitrate safety levels for zebrafish based on acute and chronic toxicity bioassays in early life stages of this species. Acute bioassays revealed ontogenetic changes in response to high nitrate levels. Based on NOEC (no observed effect concentration) values, safety levels should be set at 1450, 1855, and 1075 mg/L NO3(-)-N to prevent acute lethal effects in embryos, newly-hatched larvae, and swim-up larvae, respectively. In the chronic bioassay, larvae were exposed to nitrate concentrations of 50, 100, 200, and 400 mg/L NO3(-)-N during the entire larval period (23 days). No negative effects were observed either on larval performance or condition at concentrations up to 200 mg/L NO3(-)-N. However, at 400 mg/L NO3(-)-N, survival drastically decreased and fish showed reduced growth and evidence of morphological abnormalities. Accordingly, a safety level of 200 mg/L NO3(-)-N is recommended during the larval rearing of zebrafish to prevent negative impacts on juvenile production.