Several fish mortality events in our closed loop aquaponics system appeared to be related to chronic toxicity from heavy metals. We had been operating parallel Koi and Tilapia aquaponics systems for many years . In two separate events we had sudden deaths of a number of fish. In both events, we did not see any evidence of fish stress, such as gasping at the surface or erratic swimming prior to the mortality. Water quality had been monitored on a regular basis using the API freshwater test kits and drinking water test strips. However, most of these kits do not give readings below a pH of 6.0, so any values reported as pH=6 were likely much lower.
Following the mortality events, we analyzed water samples on LSSU’s Inductively C oupled P lasma M ass Spectrometry (ICP-MS; Agilent 7800) , and found highly elevated levels of some metal ions. In particular, Aluminum, zinc and copper were elevated in the range of both chronic and acute toxicity based on NOAA Screening Quick Referenc e Tables (SQuiRTs ). As we regularly remove water from the system with the solid waste and replace with aged water, we wer e surprised to observe increased concentrations of these metal ions.
We use aged municipal water sourced from Lake Superior, which has low concentrations of most dissolved ions, and equipment within the aqua systems has few metal components. Further investigation suggested that sources of elevated metals were the commercial Tilapia and Koi food. Additionally, copper pipes may also contribute to elevated Cu and the concentration though Cu concentration varies with residence time of the water in copper pipes . We analyzed all feed samples for metal content and found high levels of aluminum, zinc, and copper. Typically, these metals are not mobilized, so should not pose a risk. However low pH and low oxygen conditions could be a factor in the high levels of these metals. Our results suggest that caution should be taken with closed loop systems and recommend testing water for metals on a regular basis. Although not all aquaculture facilities have access to an ICP-MS, many academic and commercial labs will analyze samples.
We recently restarted the systems with yellow perch, and Atlantic salmon and take regular water samples. We plan to thoroughly flush the source water plumbing to minimize copper leaching from the pipes prior to filling our aging tanks.
We were encouraged to add affordable digital meters to both systems; allowing a real time measure of pH, oxidation reduction potential , temperature, alerts to low pH/oxygen events. We highly encourage constant monitoring of water chemistry in closed loop systems to avoid low pH events and excessive metal accumulation that could lead to metal toxicity.