Investigating the influence of nitrate nitrogen on post-smolt Atlantic salmon Salmo salar reproductive physiology in freshwater recirculation aquaculture systems

Authors: GOOD, CHRISTOPHER - Freshwater Institute; IWANOWICZ, LUKE - Us Geological Survey (USGS); MEYER, MICHAEL - Us Geological Survey (USGS); DAVIDSON, JOHN - Freshwater Institute; DIETZE, JULIE - Us Geological Survey (USGS); KOLPIN, DANA - Us Geological Survey (USGS); MARANCIK, DAVID - Fish Vet Group, Inc; Birkett, Jill; RUSSELL, CHRISTINA - Freshwater Institute; SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2016
Publication Date: 10/5/2016
Citation: Good, C., Iwanowicz, L., Meyer, M., Davidson, J., Dietze, J., Kolpin, D., Marancik, D., Birkett, J.E., Russell, C., Summerfelt, S. 2016. Investigating the influence of nitrate nitrogen on post-smolt Atlantic salmon Salmo salar reproductive physiology in freshwater recirculation aquaculture systems. Aquacultural Engineering. 78:2-8. https://doi.org/10.1016/j.aquaeng.2016.09.003.
DOI: https://doi.org/10.1016/j.aquaeng.2016.09.003

Interpretive Summary: Precocious male maturation in Atlantic salmon Salmo salar is a significant issue affecting land-based, closed containment operations utilizing water recirculation aquaculture system (RAS) technologies. Sexual maturation in Atlantic salmon is a highly variable, complex process that can be influenced by various environmental factors. Early maturing males’ exhibit reduced growth, feed conversion efficiency and product quality, and an increased susceptibility to opportunistic infections. A RAS can accumulate high levels of nitrate nitrogen (NO3-N). Elevated NO3-N levels have been shown to impact reproductive development and endogenous sex steroid production in a number of aquatic species, including Atlantic salmon. To investigate whether elevated NO3-N in RAS has an effect on early maturation in Atlantic salmon, we conducted an 8-month trial examining the reproductive physiology outcomes of high and low NO3-N systems. At the conclusion of the study, there was not an apparent effect of NO3-N level on male maturation as sexually mature males were highly prevalent in both treatment groups, and had similar blood levels of the major androgen in male fish, 11-ketotestosterone. No hormones, hormone conjugates or mycotoxins were detected in any of the water samples. While low levels of phytoestrogens and low-level estrogenicity was detected in the water samples, there was no apparent NO3-N treatment effect. We can conclude that at the NO3-N levels tested and at the 8-month exposure duration, there was not an apparent influence on Atlantic salmon maturation. The lack of detectable hormonally active compounds in the water suggests that their influence in sexual maturation is minor at most. Our results suggest manipulating NO3-N levels or removing hormones from the water in order to reduce precocious male maturation in RAS is not warranted.

Technical Abstract: An 8-month trial was carried out to assess the effects of NO3-N on a variety of performance and physiological outcomes in post-smolt Atlantic salmon Salmo salar (initial weight 102 plus or minus 1 g) reared in six replicated laboratory-scale water recirculation aquaculture systems (RAS). Three RAS randomly selected for dosing with high NO3-N (100 mg/L) and three RAS set for low NO3-N (10 mg/L). At 2-, 4-, 6-, and 8-months post-stocking, 5 fish were randomly sampled from each RAS, gonadosomatic index (GSI) data were collected, and plasma was sampled for 11-ketotestosterone (11-KT) quantification. Multiple tissues were collected and preserved in formalin for histopathology assessment. At 4- and 8-months post-stocking, samples of culture tank and spring makeup water were collected and tested for a range of hormonally active compounds using liquid chromatography / mass spectrometry, as well as for estrogenicity using the bioluminescent yeast estrogen screen (BLYES) reporter system. Finally, at 8-months post-stocking an additional 8-9 salmon were sampled from each RAS for blood gas and chemistry analyses using an i-Stat 1 portable blood analyzer. Overall, sexually mature males were highly prevalent in both NO3-N treatment groups by study's end, and there did not appear to be an effect of NO3-N on male maturation prevalence based on grilse identification, GSI, and 11-KT results, indicating that other culture parameters likely instigated early maturation. No important differences were noted between treatment groups for whole blood gas and chemistry parameters. No hormones, hormone conjugates, or mycotoxins were detected in any water samples; phytoestrogens were detected at low levels but appeared unrelated to NO3-N treatment. Finally, low-level estrogenicity was detected in RAS water, but again, there did not appear to be a NO3-N treatment effect.