WWW.WA S .ORG • WORLD AQUACULTURE • DECEMBER 2022 37 FIGURE 4. Concentration of (a) total ammonia nitrogen (TAN) (b) nitrite and (c) nitrate during the 15 days after the goldfish challenge. and only in two cases was 0.25 mg/L and in one case 0.5 mg/L measured. These are safe values for fish at the pH levels measured, between 6.8 and 7.8. For nitrite, only in two cases was anything measured above the sensitivity of the assay (1 mg/L), both in group A with neither fish nor biofilter. For nitrate, values up to 100 mg/L were measured (group F, no fish, “high” Biokos) and an outlier at 250 mg/L (group B, no fish, biofilter, no Biokos). For all groups with fish, values did not exceed 50 mg/L. This is a value in the high end for fish safety but is to be expected in the absence of growing plants in the tanks and absence of water exchange. Summarizing these data, there was no apparent difference in biofilter performance with or without Biokos treatment. Degradation Study Presumed to be a defense system, some bacteria have developed enzymatic systems for their cleavage and thereby functional inactivation (Nielsen and Sørensen 2003, D’Costa et al. 2020). While this means one must carefully avoid contamination with foreign microorganisms during production of Biokos, it is convenient when it comes to using this product in the field. Such inactivation systems are widely distributed in nature, so all applied Biokos will eventually be cleaved and degraded further to harmless constituents. This study was designed to obtain an understanding of the degradation kinetics of Biokos in different types of “natural water” and at different concentrations. We tested four types of water: 1) Fresh water from zebrafish tanks at a research facility at the University of Copenhagen, 2) Fresh water from a mature private ornamental fish tank, 3) Fresh water from three different rainbow trout farms in Denmark, and 4) Salt water from a public aquarium, National Aquarium Denmark. We chose different temperature settings for the different types of water, in all cases corresponding to the temperature of the type of water at sampling. Concentrations of Biokos used were therapeutic (“low”) and double (“high”) dosing. The remaining Biokos was measured using HPLC analysis at least every 24 hr and until all (or the majority) had been degraded (Fig. 5). Water from a zebrafish laboratory was expected to be the cleanest and thus show the slowest Biokos degradation. All the other types were from “mature” culture systems, i.e., long-established fish tanks or trout ponds. In zebrafish water at 27 C, half of the Biokos was degraded at 70 and 95 hr, respectively, depending on concentration, whereas in mature water from a private fish tank at 27 C, this happened faster, after 50 hr at both concentrations and in line with expectations. The water from Danish trout farms was collected during winter and therefore we performed this degradation study at 4 C. In the first sample, half of the Biokos was gone after 185 hr and in the second sample this happened after 215 and 162 hr. In the third, things happened more slowly: 50 percent of Biokos added at the “low” concentration was gone after 215 hr while this point was not reached for the “high” concentration within the timeframe of the experiment, although some degradation was measured. Finally, for saltwater in a public aquarium at 25 C, half the Biokos was gone after 113 and 137 hr. Two things should be emphasized: 1) sampling during the experiment was done at 24-hr intervals, meaning that the 50 percent degradation point in many cases would be significantly earlier than the time points mentioned above; and 2) Biokos would most likely have been degraded much faster in the trout pond water samples at a higher temperature, which is when trout farmers most often encounter Ich infestations. These data indicate Biokos will be degraded within a few days in any relevant type of water, which is ideal because Biokos kills the Ich parasites within hours. Conclusion and Summary Efficacious and inexpensive solutions are needed to control pests and diseases in an environmentally sustainable manner. Current remedies are either somewhat ineffective and/or difficult to use FIGURE 5. The time required (in hours) for more than 50 percent of Biokos to be degraded in different types of water. ( C O N T I N U E D O N P A G E 3 8 )
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