Gulf of Alaska | Fishes and Fish Habitats DOCTORATE POSTER PRESENTATION Can otolith stable isotope chemistry serve as a proxy for the metabolic rate of Pacific Cod ( Gadus macrocephalus )? Presenter: Hillary Thalmann , hillary.thalmann@oregonstate.edu, Oregon State University Ben Laurel , ben.laurel@noaa.gov, NOAA Alaska Fisheries Science Center Mary Beth Rew Hicks , marybeth.rew.hicks@noaa.gov, NOAA Alaska Fisheries Science Center Warming oceans and more frequent marine heatwave events will lead to increased metabolic rates in fish, resulting in changes to their foraging, growth, and eventual survival. The Gulf of Alaska Pacific Cod ( Gadus macrocephalus ) population declined dramatically in response to two recent marine heatwave events in the region. In response to these extreme temperatures, Pacific Cod either migrated to cooler and deeper water, or stayed behind to weather the warmer temperatures. If these fish remained in their historic Gulf of Alaska habitats, their metabolic rate would have increased in response to the warming. However, there is currently no well-established methodology for quantifying the metabolic rate of fish in the field. Several recent studies have suggested that metabolic rate may be estimated from the isotopic composition of carbon (del13C) in fish otoliths, but this potential proxy has not yet been validated for Pacific Cod. Our objectives for this study were to 1) assess the validity of otolith del13C as a proxy for metabolism for age-1 Pacific Cod, and 2) examine how Pacific Cod metabolic rate varies under different temperature conditions. We also examined otolith del18O, which has previously been used as a temperature proxy for the species, to evaluate how well otolith del18O reflected our experimental temperatures. We exposed Pacific Cod to 4 temperature treatments (2, 4, 6, and 8°C) for twelve months and determined their Standard Metabolic Rate (SMR) using swim tunnel respirometry. We then utilized the otoliths for del18O and del13C stable isotope chemistry and compared these values to the known temperature and SMR for each fish. SMR was only moderately explained by the temperature and mass of the Pacific Cod in the swim tunnel (R^2 = 0.30; P = 0.003). Otolith del18O values were inversely related to temperature, supporting their use as a temperature proxy for the species. However, otolith del13C did not vary among treatments, and more work is needed to determine the effectiveness of del13C as a metabolic proxy for Pacific Cod. A second experiment is currently underway to evaluate the relationships between Pacific Cod SMR and otolith del18O and del13C at higher temperatures (up to 14°C), consistent with those conditions observed in the marine heatwaves. A metabolic proxy for Pacific Cod would be helpful for evaluating changes in field metabolic rate under stressful conditions and for comparison with existing growth and condition metrics. L. Zoe Almeida , almeidle@oregonstate.edu, Oregon State University Jessica Miller , jessica.miller@oregonstate.edu, Oregon State University
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