Effects of diel-cycling hypoxia and pH on growth and behavior of juvenile weakfish (Cynoscion regalis)

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2015
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During summertime, dissolved oxygen (DO) and pH cycle daily in shallow estuarine waters as a result of organisms’ photosynthesis and respiration. Eutrophication exacerbates these physiochemical cycles, which can impact growth and behavior of young fishes which use estuaries as nursery habitat. Laboratory experiments were conducted on juvenile weakfish (Cynoscion regalis) to determine the individual and interactive impacts of natural levels of diel-cycling hypoxia and co-varying pH on growth rate and behavior. Computer-controlled, recirculating, aquarium systems were used to expose fish to three DO treatments [extreme (1-11 mg O2 l-1) and moderate (3-9 mg O2 l-1) cycling DO, and constant normoxia (7.5 mg O2 l-1)] and three pH treatments [extreme (6.8-8.1) and moderate (7.2-7.8) cycling pH, and constant pH (7.5)] in a 3X3 factorial design. Fish growth was not significantly impacted by either diel-cycling DO or pH conditions, nor was there any interactive effect, thus demonstrating that this species is physiologically tolerant of the harsh environments encountered by juvenile stages. However, all fish experienced mortality after exposure to a period of extreme diel-cycling DO lasting at least 10 days. Juvenile weakfish exhibited a variety of behavioral responses to diel-cycling DO, such as reduced swimming, that may have allowed fish to maintain high growth rates. By reducing swimming, fish were able to allocate more energy towards growth as opposed to movement when subjected to low DO levels. When fish were exposed to varying levels of diel-cycling pH, their behavior did not change significantly, indicating that juvenile weakfish are not stressed by fluctuating pH conditions.
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