Effects of hypoxia-induced habitat compression on growth of juvenile fish in the Neuse River Estuary, North Carolina, USA

Hypoxia is thought to have negative effects on fish in coastal ecosystems, but quantifying those effects can be difficult. Direct exposure to hypoxia can reduce fish growth or survival, but fish can also rapidly detect and avoid low dissolved oxygen levels. However, avoidance behavior may result in indirect effects that reduce fish growth. For example, when hypoxic conditions expand, fish densities may increase in nearshore oxygenated refuges, potentially causing density-dependent reductions in growth. We evaluated this hypothesis for juvenile demersal fish species (primarily spot Leiostomus xanthurus and Atlantic croaker Micropogonias undulatus). By monitoring water quality and fish density across the Neuse River Estuary, North Carolina, USA, under varying water quality conditions during summer 2007, we showed that fish effectively avoided hypoxia despite rapidly changing conditions (minutes to hours), moving away from incursions of hypoxic water and then rapidly redistributing into affected areas after these events passed. Fish densities in nearshore oxygenated refuges increased nearly 2-fold when habitat was compressed by hypoxic waters. Spot in compressed refuges also had significantly less food in their stomachs during June. Based on published estimates of density-dependent spot growth, we estimated that average spot growth rate was reduced 17% during habitat compression events, which occurred 21.5% of the time, translating into an average reduction in growth rate of 4% over the summer. This likely is a conservative estimate of indirect hypoxia effects on growth, as hypoxia was relatively mild in 2007, and density dependence is only one indirect mechanism by which hypoxia may potentially reduce growth.