A low concentration of dissolved oxygen (DO) is commonly the limiting factor in fish culture systems. Hypoxia tolerance in Rainbow Trout Oncorhynchus mykiss can be affected by both history of domestication and growth rate. As such, selecting strains for specific characteristics such as growth rate or disease resistance could potentially affect DO tolerance, making culture difficult. Here we used two experiments to examine the differences in tolerance to lower DO concentrations among four Rainbow Trout strains and crosses selected for resistance to whirling disease Myxobolus cerebralis. The first experiment examined differences in critical DO concentrations of fry (≥73 mm total length [TL]) when exposed to rapid decreases in DO at 30, 60, 90, and 120 d postswim‐up. In addition, since formalin is a common chemical used in aquaculture to treat for external parasites, the effect of exposure to formalin on tolerance to low DO was evaluated. The second experiment evaluated critical DO concentrations among four strains and crosses exposed to a prolonged decrease in DO at age 7 months (averaging 178 mm TL). Formalin exposure had an effect on low‐DO tolerance, with DO concentrations that resulted in a loss of equilibrium decreasing with an increase in formalin concentration. In addition, low‐DO tolerances were diminished with an increase in fish size, with larger fish losing equilibrium at higher DO concentrations. Differences in DO concentrations resulting in loss of equilibrium and mortality were evident among the strains and crosses in the second experiment. This experiment demonstrated that DO concentrations must be below 2.0 mg/L before loss of equilibrium is observed. However, if fish are soon returned to well‐oxygenated water, losses can be minimized. Additionally, other hatchery practices that compromise hypoxia tolerance may increase mortality more quickly following low‐DO exposure, and care should be taken to correct low‐DO issues shortly after loss of equilibrium is observed.
The US Environmental Protection Agency's short-term freshwater effluent test methods include a fish (Pimephales promelas), a cladoceran (Ceriodaphnia dubia), and a green alga (Raphidocelis subcapitata). There is a recognized need for additional taxa to accompany the three standard species for effluent testing. An appropriate additional taxon is unionid mussels because mussels are widely distributed, live burrowed in sediment and filter particles from the water column for food, and exhibit high sensitivity to a variety of contaminants. Multiple studies were conducted to develop a relevant and robust short-term test method for mussels. We first evaluated the comparative sensitivity of two mussel species (Villosa constricta and Lampsilis siliquoidea) and two standard species (P. promelas and C. dubia) using two mock effluents prepared by mixing ammonia and five metals (cadmium, copper, nickel, lead, and zinc) or a field-collected effluent in 7-day exposures. Both mussel species were equally or more sensitive (more than two-fold) to effluents compared with the standard species. Next, we refined the mussel test method by first determining the best feeding rate of a commercial algal mixture for three age groups (1, 2, and 3 weeks old) of L. siliquoidea in a 7-day feeding experiment, and then used the derived optimal feeding rates to assess the sensitivity of the three ages of juveniles in a 7-day reference toxicant (sodium chloride [NaCl]) test. Juvenile mussels grew substantially (30%-52% length increase) when the 1-or 2-week-old mussels were fed 2 ml twice daily and the 3-week-old mussels were fed 3 ml twice daily. The 25% inhibition concentrations (IC25s) for NaCl were similar (314-520 mg Cl/L) among the three age groups, indicating that an age range of 1-to 3-week-old mussels can be used for a 7-day test. Finally, using the refined test method, we conducted an interlaboratory study among 13 laboratories to evaluate the performance of a 7-day NaCl test with L. siliquoidea. Eleven laboratories successfully completed the test, with more than 80% control survival and reliable growth data. The IC25s ranged from 296 to 1076 mg Cl/L, with a low (34%) coefficient of This article includes online-only Supporting Information.
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