Date

Fall 2023

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Global climate change is expected to increase the frequency and severity of upwelling events in the California Current Ecosystem, yielding concurrent reductions in pH and dissolved oxygen (DO) in coastal marine environments. Juvenile copper (Sebastes caurinus) and gopher (Sebastes carnatus) rockfish may be particularly vulnerable to low pH and DO because they settle nearshore during the upwelling season. Previous studies often use static (i.e., fixed) pH and DO treatments in the laboratory, but ocean chemistry is dynamic and these conditions fluctuate naturally in upwelling zones. To determine how fluctuations in ocean chemistry will impact rockfish behavioral and physiological performance at this critical life history stage, I exposed juveniles of both species to one of the following pH/DO treatments: static control (8.0 pH, 8.3 mg/L DO), static moderate (7.5 pH, 4.0 mg/L DO), static extreme (7.3 pH, 2.0 mg/L DO), and two fluctuating treatments, upwelling and relaxation (recurring cycles of 8 days of 7.3 pH, 2.0 mg/L DO and 8 days of recovery at control conditions). Responses to sublethal stress were evaluated after 1-13 weeks of exposure (or up to six 16-day fluctuating cycles) through behavioral and physiological metrics including lateralization tests, escape time trials, startle response, critical swimming speed (Ucrit), metabolic performance (standard and maximum metabolic rates, capacity for aerobic activity), hypoxia tolerance (critical oxygen tension [Pcrit]), growth rates, body condition, and mortality. I did not observe any significant effects of upwelling conditions on the behavioral metrics of lateralization, escape time, or startle response. Rockfish exhibited impairment in aerobic scope (due to a reduced maximum metabolic rate), swimming speed, and increased ventilation rates and hypoxia tolerance, under simulated upwelling conditions, performing equally to fish in the extreme treatment. In the fluctuating upwelling treatment, both species appeared to recover fully for most physiological metrics when returned to control seawater for 6-7 days (i.e., a simulated oceanographic relaxation event). Both species exhibited the lowest growth rate in the extreme low pH/DO treatment and intermediate growth in the two fluctuating treatments, suggesting lasting cumulative negative impacts of exposure to future upwelling conditions. Mortality was highest in the extreme low treatment and body condition tended to decrease as treatments became more extreme. While juvenile rockfish are susceptible to physiological impairment under extreme climate change scenarios, the severity and duration of future hypoxic, acidic events will ultimately set the consequences for survivorship and physiological fitness, influencing the outcome of the population replenishment process and the long-term sustainability of economically and ecologically important nearshore rockfish species.

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