We have been running several closed loop recirculating aquaponics systems (RAS) since 2019. Routine water chemistry parameters were monitored such as pH, alkalinity, ammonia, nitrate and nitrite. After some unexplainable mortalities, we were encouraged to run samples using ICPMS, and found chronic toxicity levels of Aluminum (Al), Zinc (Zn) and Copper (Cu) (Hebert et al., 2023). We immediately moved the fish to another location, cleaned the system and started over. We were encouraged to monitor the metals weekly and found pH levels were a good indicator of when metals might spike, so used pH levels to trigger major water changes (Evans et al., 2024). Although we were able to control Al and Zn levels, our Cu levels were still high, even if we flushed the water lines before filling our system, suggesting high Cu levels in our source water, likely from Cu plumbing (Overly et al., 2025).
To assess the effects of the chronic metal toxicity on the fish tissues, we removed the two remaining fish from our Atlantic salmon (ATS) aquaponics system, and performed necropsy for light and scanning electron microscopy (SEM), ICPMS and uXRF analysis. These fish were 3 years old (3 yr) and had been raised in the system their whole life. Our control was a 1 year old (1 yr) that had been raised in the LSSU CFRE hatchery.
Samples for histology were fixed in Bouin’s fluid, dehydrated through an ethanol series, embedded in JB-4, thin sectioned at 3 microns and stained with cresyl violet for light microscopy and 5 microns left unstained for the SEM. Tissue samples were also frozen for ICPMS analysis (These results are still pending).
Our histology results showed a striking difference between the spleen samples. While the 1 yr tissue (figure a) appeared normal, the 3 yr whole samples exuded a dark substance (Figure b and c). After sectioning the 1 yr histology the spleen revealed melanomacrophage aggregates (Figure d), but the 3 yr samples were covered in a dark substance (Figure e). The liver tissue of the 1 yr sample looked normal, while the 3 yr sample had crystal deposits around the hepatocytes. Other differences in the 3 yr samples included an increase in goblet cell density on the gill filaments, lesions and possible parasites in the brain. Analysis of head and trunk kidney is pending.
Overall, we see evidence that the fish maintained in our system for 3 years show evidence of stress and possible tissue damage from a combination of heavy metal toxicity, pH changes and possibly parasites. Although most production aquaculture will not hold fish for 3 years, there may be situations where brood stock may be held for longer periods and heavy metal accumulation could be a factor.