Development and Field Application of a Model Predicting Effects of Episodic Hypoxia on Short-Term Growth of Spot

Abstract

In North Carolina and elsewhere, there is concern that excessive nutrient loading and resulting hypoxic conditions in coastal ecosystems are adversely affecting the native fauna, but quantifying the effects on fish can be difficult. Hypoxia may reduce fish growth via direct exposure or indirectly (e.g., cost of low-oxygen avoidance, reduced food availability, and density-dependent effects in oxygenated refuges). Given the fine spatial and temporal scale of oxygen dynamics in estuarine habitats, evaluating the impacts of hypoxia on fish growth requires short-term growth indicators that integrate the effects of rapidly changing environmental conditions. To address this need, we experimentally determined the sensitivity and response time of a suite of bioindicators of recent growth (RNA:DNA ratio and RNA concentration in muscle tissue; insulin-like growth factor-I messenger RNA expression in the liver; hepatosomatic index; and Fulton’s condition factor K) to changes in the specific growth rate of juvenile Spot Leiostomus xanthurus. A model based on multiple bioindicators was better at estimating growth rate than models based on single indicators. We used this model to estimate recent growth rates of juvenile Spot collected from the Neuse River estuary and related them to recent dissolved oxygen (DO) conditions. Estimated growth rates of Spot collected after a week of good DO conditions were almost twice those of Spot collected after a week of poor DO conditions. Using these results and DO data from the Neuse River estuary in 2007–2010, we estimated that hypoxia dynamics reduced Spot growth over the summer by 6–18% in these years relative to growth under constant good DO conditions. This approach can be used to evaluate impacts of observed or modeled scenarios of water quality dynamics on growth of juvenile Spot and serves as a template for development of predictive growth models for other species.

Received July 18, 2016; accepted July 27, 2017

ACKNOWLEDGMENTS

These experiments were conducted at the wet laboratory facilities of the Institute of Marine Sciences, University of North Carolina–Chapel Hill, and the aquaculture facilities of North Carolina State University (NCSU). Fish were cared for in accordance with Institutional Animal Care and Use Committee Protocol 08-070-O. We thank T. Averett, R. Rassmusen, R. Faison, S. Petre, and S. Polland for assistance with these experiments and in the field; D. Baltzeger for assistance and guidance in qPCR; and K. Gross for statistical consultation. This project was supported by Grant NA06OAR4170104 (Project R/MER-55) from the National Oceanic and Atmospheric Administration (NOAA) National Sea Grant College Program to the North Carolina Sea Grant Program; by a U.S. Department of Education Graduate Assistance in Areas of National Need Biotechnology Fellowship (Grant P200A070582) to L. Campbell; and by an NCSU Doctoral Dissertation Completion Grant to L. Campbell. The views expressed herein are those of the authors and do not necessarily reflect the views of the NOAA or any of its subagencies.