Measuring the effects of ammonia and dissolved oxygen on juvenile burbot (Lota lota) growth and survival

2021 Summer.Includes bibliographical references.Burbot, Lota lota are a candidate species for commercial aquaculture because of their palatability and optimal growth at temperatures similar to those used in freshwater trout aquaculture. However, data on burbot environmental tolerances and requirements are sparse, especially with reference to water quality parameters relevant to aquaculture, such as un-ionized ammonia (UIA) and dissolved oxygen concentrations. First, we used a two-phased approach to evaluate the effects of un-ionized ammonia on the growth and survival of burbot. We measured the acute toxicity of ammonia to juvenile burbot (mean SL: 144 ± 6 mm; mean wet weight: 27.3 ± 3.4 g) and calculated a 96-hr LC50 of 0.58 mg·L-1 UIA. We then measured the 60-d growth, food consumption rate, and performance of burbot (mean initial SL: 190 ± 6.9 mm; mean initial weight: 67.0 ± 4.5 g) reared in 0.00, 0.03, 0.06, 0.12, or 0.19 mg·L-1 UIA using a 20-tank flow-through system under optimal temperature (14.7°C) and dissolved oxygen (DO > 80% saturation) conditions. Elevated ammonia concentration significantly reduced daily food consumption and subsequent growth. Fish exposed to 0.03 and 0.06 mg·L-1 UIA showed temporal acclimation to UIA, achieving food consumption and growth rates on par with control fish after 30 days of exposure. The estimated effective UIA concentrations for 10 and 20 percent reductions in growth (EC10 and EC20) based on our data are: EC10 = 0.03 ± 0.006 mg·L-1 and EC20 = 0.050 ± 0.004 mg·L-1. We recommend rearing burbot under conditions that keep UIA levels ≤ 0.03 mg·L-1 based on our finding that above 0.03 mg·L-1 cause measurable reductions in growth rate. Following the ammonia studies, we exposed juvenile burbot (19.5 ± 2.2 g) to five dissolved oxygen concentrations (5.0, 5.8, 6.6, 7.4, and 8.3 (control) mg·L-1) for 9 weeks at 15°C. Variability was high in all treatments, and food consumption and growth rates did not differ among DO levels, although fish at 8.3 mg·L-1 grew ca. 29% larger than those at 5.0 mg·L-1. We also measured short-term hypoxia tolerance and resting routine oxygen consumption rates (MO2) of burbot that had been chronically acclimated to the same DO concentrations. Burbot acclimated to 8.3 mg·L-1 lost equilibrium (LOEcrit) at a significantly higher concentration (1.85 ± 0.33 mg·L-1) than that of the 7.4 and 6.6 mg·L-1 acclimated fish (1.50 ± 0.37 and 1.49 ± 0.27; respectively), while all other groups were intermediate (1.67 ± 0.28). The MO2s were not statistically distinguishable among acclimation groups (p-value = 0.25), but MO2 trended lower with decreasing DO concentrations typifying an oxyconforming species. In summary, juvenile burbot are quite tolerant to DO concentrations down to 5.0 mg·L-1. We recommend rearing burbot at DO concentrations > 7.0 mg·L-1 and that minimum short-term DO concentrations be kept > 4.0 mg·L-1in culture environments where multiple stressors may be present