Chronic exposure to elevated nitrate concentrations alters behaviour in the swordtail fish, Xiphophorus hellerii

Nitrate is a naturally occurring nutrient in aquatic ecosystems but can have a negative impact on organisms at elevated concentrations. The primary toxicological effects of nitrates on fish include changes in sex steroid concentration and decreased oxygen carrying capacity, responses which are often linked to lethargic behaviour. Using the green swordtail fish, Xiphophorus hellerii , we investigated the influence of nitrate nitrogen (NO 3 –N) on aggression, boldness and female mate preference. We first quantified the repeatability of multiple different measures of aggression and boldness behaviours. For aggression behaviours, total number of bites, time within proximity to the mirror and total number of lateral displays were repeatable (intraclass correlation coefficient, ICC >0.4). Latency to resume feeding under perceived risk was repeatable for a measure of boldness. Fish were then randomly assigned to one of three treatments: control (∼0 mg/litre NO 3 –N) and low (∼10 mg/litre NO 3 –N) or high (∼100 mg/litre NO 3 –N) concentrations of nitrate (NO 3 –N). After 30 days, we retested for aggression, boldness and female mate preference for large male size. High nitrate exposure increased total number of bites, indicating increased aggression, while both high and low concentrations decreased the number of lateral displays and time spent within proximity to the mirror. We also found that high exposure increased the time it took individuals to resume feeding after a perceived threat, indicating a decrease in boldness. And finally, nitrate exposure did not influence female mate preference. These results suggest that nitrate exposure can alter behaviours associated with aggression and boldness and that changes in these behaviours can occur even at lower concentrations. Future work should relate these changes in social interactions to fitness to better understand how nitrates can ultimately influence population persistence. • We examine behavioural responses of X. hellerii to elevated nitrate concentrations. • Elevated nitrate increased aggression and decreased boldness. • Elevated nitrate did not influence female mate preference. • Low levels of nitrate altered behaviours associated with survival/social structure.