Behavioral responses of common juvenile estuarine fishes to diel-cycling hypoxia and corresponding pH fluctuations: a comparative approach

Dixon, Rachel L.
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University of Delaware
Diel-cycling hypoxia and co-varying pH fluctuations reduces the quality of shallow estuarine nursery habitat for juvenile fishes. Avoidance or compensatory behaviors allow fish to mitigate stress associated with low dissolved oxygen (DO). Aquatic surface respiration (ASR), respiring in the thin, air-saturated, surface layer, is widespread among teleosts and provides an advantage to fish routinely exposed to hypoxia. ASR begins when oxygen becomes physiologically limiting, and serves as an indicator for hypoxia tolerance in a species. 24-hour video observations were conducted under controlled laboratory conditions using computer-monitored recirculating aquarium systems. Behavioral responses of juvenile striped bass ( Morone saxatilis ), Atlantic silversides (Menidia menidia ), striped killifish (Fundulus majalis ), and mummichog (Fundulus heteroclitus ) were observed during exposure to different severities and combinations of diel-cycling DO (3-9 mg O2 l-1 or 1-11 mg O2 l -1 ) and corresponding pH (7.2-7.8 or 6.8-8.1), compared with responses at static normoxia (7.5 mg O2 l-1 ) and pH (7.5). Trials ran from 11:00 - 11:00 each day; DO decreased overnight (6 hours) to the lowest DO level in both diel-cycles, with the hypoxic period lasting through the morning (4.5 hours) before trials ended and oxygen increased again. The incidence of ASR, activity level, and position of each individual was recorded at pre-determined time intervals. A second set of observations was conducted under conditions where lowest DO levels (1.0 - 1.5 mg O 2 l-1 ) were prolonged throughout the day; DO was decreased overnight from super-saturation (~10 mg O2 l-1 ) to the oxygen minima (6 hours), low levels were extended (18 hours) then DO was increased back to saturation (6 hours) during the second night. Based on results of the first set of trials, another set of video observations was conducted on F. heteroclitus that were acclimated to moderate levels of diel-cycling hypoxia and pH (3-9 mg O2 l-1 ; 7.2-7.8) for ≥10 days. ASR was initiated in M. menidia and F. majalis only when diel-cycling DO fell to (1.40 - 1.60 mg O2 l-1 ) and (1.31 - 1.46 mg O2 l-1 ), respectively, with both cycling and static pH. In M. menidia , under extended low DO, all individuals exhibited ASR and mortality occurred in all fish shortly after onset, suggesting that ASR is a last-ditch effort to prolong survival. Onset of ASR among F. majalis under diel-cycling conditions varied among individuals; once initiated the behavior was intermittent with variable duration until the end of the trial, with no mortality observed. At corresponding DO levels, M. saxatilis reduced swimming activity and exhibited positive rheotaxis, but no ASR was observed. ASR was also not observed in F. heteroclitus in diel-cycling treatments, or in acclimated fish, but when low DO (1.0 mg O 2 l-1 ) was prolonged, fish performed ASR after approximately 5 hours exposure. This suggests that in F. heteroclitus , the incidence of ASR is a combination of dissolved oxygen concentration and duration of exposure. Given trade-offs associated with repeated surfacing and skimming, mummichogs appear to delay engaging in ASR under short term low DO exposure occurring during diel-cycles. Utilization of ASR as an adaptive strategy is dependent upon species-specific hypoxia tolerance, and the temporal scale over which low oxygen occurs.