Direct and Indirect Effects of Hypoxia on Juvenile Fish in the Neuse River Estuary, North Carolina

In North Carolina and elsewhere, there is concern that excessive nutrient loading and resulting hypoxic conditions in coastal ecosystems are adversely affecting living resources, but quantifying the effects on fish can be difficult. Direct exposure to hypoxia can reduce fish growth or survival, but fish can also rapidly detect and avoid low dissolved oxygen (DO) levels. In the wild, hypoxia may reduce fish growth via direct exposure, or indirectly (e.g., costs of avoidance, reduced food availability, density-dependent effects in oxygenated refuges). I evaluated this hypothesis for juvenile spot Leiostomus xanthurus, a representative estuary-dependent species. By monitoring water quality and fish density across the Neuse River Estuary (NRE) during varying water quality conditions I showed that fish effectively avoided hypoxia despite rapidly changing conditions, moving away from incursions of hypoxic water and then rapidly redistributing into affected areas after these events passed. Densities of fish in nearshore oxygenated refuges increased nearly two-fold when habitat was compressed by hypoxic waters. Fish in compressed refuges also had significantly less food in their stomachs. Based on published estimates of density-dependent spot growth I estimated that habitat compression reduced average spot growth rate over summer 2007 by at least 4%. Hypoxic events during my study proved to be more spatially and temporally dynamic than anticipated. Given the fine spatial and temporal scale of oxygen dynamics in the NRE, evaluating the impacts of hypoxia on fish growth required novel, short-term growth indicators that integrate the effects of rapidly changing environmental conditions. My overall goal was to directly quantify how the frequency, duration and severity of hypoxia events affects growth of juvenile estuary-dependent fishes in nursery habitats. To do this, I experimentally determined the sensitivity and response time of a suite of bioindicators of recent growth (RNA:DNA ratio and RNA concentration in muscle tissue, circulating plasma levels of insulin-like growth factor-I (IGF-I), IGF-I mRNA expression in the liver, Hepatosomatic Index, and Fulton's K) to changes in spot specific growth rate in laboratory experiments. Results indicated that using multiple bioindicators in a predictive growth rate model was more informative than models based on individual indicators. The best model, identified using an Information Theoretic Approach, estimated specific growth rate over the previous week based on HSI, RNA concentration, DNA concentration, Fulton's K and temperature, and accounted for 80% of the variability in specific growth rates among fish in laboratory trials. I used this model to estimate recent growth rates of spot collected from the NRE and other estuarine locations and related them to DO conditions over the week prior to collection. 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 NRE in 2007-2010, I estimated that hypoxia dynamics reduced growth of spot over the summer 6-18% in these years relative to growth under constant Good DO conditions. Many studies have employed growth rate relationships with single bioindicators, but I demonstrated the value of combining multiple physiological and morphological bioindicators, along with environmental data, to produce a more robust predictive growth model that can be applied to fish in the field. This approach can be used to evaluate impacts of observed or modeled scenarios of water quality dynamics on growth of juvenile spot and serve as a template for development of predictive growth models for other species. The findings presented here could only be achieved by combining biotechnology techniques with traditional field ecology methods. Such interdisciplinary approaches are becoming essential to address many of the increasingly complex questions and issues facing society.