Cascading of habitat degradation: Oyster reefs invaded by refugee fishes escaping stress

Mobile consumers have potential to cause a cascading of habitat degradation beyond the region that is directly stressed, by concentrating in refuges w here they intensify biological interactions and can deplete prey resources. We tested this hypothesis on structurally complex, species-rich biogenic r eefs created by the eastern oyster, Crassostrea virginica, in the Neuse Riv er estuary, North Carolina, USA. We (1) sampled fishes and invertebrates on natural and restored reefs and on sand bottom to compare fish utilization of these different habitats and to characterize the trophic relations among large reef-associated fishes and benthic invertebrates, and (2) tested whe ther bottom-water hypoxia and fishery-caused degradation of reef habitat co mbine to induce mass emigration of fish that then modify community composit ion in refuges across an estuarine seascape. Experimentally restored oyster reefs of two heights (1 m tall "degraded" or 2 m tall "natural" reefs) wer e constructed at 3 and 6 m depths. We sampled hydrographic conditions withi n the estuary over the summer to monitor onset and duration of bottom-water hypoxia/anoxia, a disturbance resulting from density stratification and an thropogenic eutrophication. Reduction of reef height caused by oyster dredg ing exposed the reefs located in deep water to hypoxia/anoxia for >2 wk, ki lling reef-associated invertebrate prey and forcing mobile fishes into refu ge habitats. Refugee fishes accumulated at high densities on reefs in oxyge nated shallow water, where they depleted epibenthic crustacean prey populat ions. Thus, physical disturbances can impact remote, undisturbed refuge hab itats through the movement and abnormal concentration of refugee organisms that have subsequent strong trophic impacts. These results have implication s for the design of Marine Protected Areas (MPAs), showing that reserves pl aced in proximity to disturbed areas may be impacted indirectly but may ser ve a critical refuge function on a scale that matches the mobility of consu mers.