Assessing the Effects of Hypoxia on Fish Population Ecology Using Elements and Isotopes

EPA Grant Number: FP917487
Title: Assessing the Effects of Hypoxia on Fish Population Ecology Using Elements and Isotopes
Investigators: Mohan, John Austin
Institution: The University of Texas at Austin
EPA Project Officer: Jones, Brandon
Project Period: August 29, 2012 through August 28, 2015
Project Amount: $126,000
RFA: STAR Graduate Fellowships (2012) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Fellowship - Marine Sciences


The objectives of this research are to use fish otolith (earstone) chemistry as a natural chronological indicator of hypoxia exposure, and investigate links between exposure history, fish performance and trophic dynamics in the seasonally hypoxic northern Gulf of Mexico. Specific research questions include: Does the redox-sensitive element manganese (Mn) get released from the sediment during hypoxic conditions and become incorporated into fish otoliths, offering a proxy of exposure? Does hypoxic exposure alter fish growth rates, conditions and trophic interactions?


For preliminary studies, Atlantic croakers were exposed for 4 weeks to constant hypoxia that mimicked field values. Hypoxic stress altered growth rates but not otolith chemistry, suggesting that endogenous physiological mechanisms exert minor influence on otolith composition. Atlantic croakers were collected from the northern Gulf of Mexico at both hypoxic and normoxic sites, in October 2010. Otolith Mn concentrations were orders of magnitude (up to 100x) different between sites, suggesting strong environmental exogenous influence on otolith chemistry, perhaps related to redox release of Mn from the sediments. This study will use two controlled experiments to (1) examine relationships between dissolved oxygen (DO) level and food type, on somatic growth, otolith growth and tissue carbon and nitrogen stable isotopes; and (2) investigate relationships of ambient water Mn to otolith Mn and the effects of food ration and growth on elemental uptake using a spiking study. For the first experiment, treatments include hypoxic (DO = 1.7 mg/L) or normoxic (DO > 5 mg/L) water and benthic (clam based) or pelagic (fish-based) diet. Prior to the experiment, otoliths will be chemically marked and fish individually tagged to track individual somatic and otolith growth throughout the 8-week study. At 2-week intervals, length and weight measurements and scale samples will be collected to determine growth and characterize the isotope signatures and estimate the incorporation rates of d13C and d15N retained in the scales. The spiking experiment will involve three dissolved Mn treatments of low, medium, or high and low or high food ration for 6 weeks. Otoliths will be chemically marked prior to spiking, and water samples will be collected weekly to monitor dissolved Mn. This will allow precise temporal matching of otolith Mn to water Mn and comparisons to growth rate. Validating and calibrating these relationships in the laboratory is essential to interpret patterns observed in wild croakers collected in the northern Gulf of Mexico.

Expected Results:

Linking hypoxic exposure to trophic dynamics of Atlantic croaker, an abundant fish and integral component in Gulf of Mexico food webs, will provide information on ecosystem structure and functioning in response to seasonal hypoxia. Additionally, validating a natural permanent chronological recorder of hypoxia in fish will allow new hypotheses to be tested regarding historic hypoxic episodes by examining archived otoliths of other species and in other ecosystems across the world.

Potential to Further Environmental/Human Health Protection
Hypoxia is one of many stressors that threaten the sustainability of valuable ecosystem services provided by estuarine and coastal regions across the United States and world. Understanding the sub-lethal populationwide effects of hypoxia, such as reduced individual fitness and altered trophic structure of ecologically and economically important fish species, is necessary to improve objectives of environmental policy and enhance resource protection measures.

Supplemental Keywords:

hypoxia, otolith chemistry, trophic ecology