Cascading of Habitat Degradation: Oyster Reefs Invaded by Refugee Fishes Escaping Stress

Abstract

Mobile consumers have potential to cause a cascading of habitat degradation beyond the region that is directly stressed, by concentrating in refuges where they intensify biological interactions and can deplete prey resources. We tested this hypothesis on structurally complex, species-rich biogenic reefs created by the eastern oyster, Crassostrea virginica, in the Neuse River 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 whether bottom-water hypoxia and fishery-caused degradation of reef habitat combine to induce mass emigration of fish that then modify community composition in refuges across an estuarine seascape. Experimentally restored oyster reefs of two heights (1 m tall ''degraded'' or 2 m tall ''natural'' reefs) were constructed at 3 and 6 m depths. We sampled hydrographic conditions within the estuary over the summer to monitor onset and duration of bottom-water hypoxia/anoxia, a disturbance resulting from density stratification and anthropogenic eutrophication. Reduction of reef height caused by oyster dredging exposed the reefs located in deep water to hypoxia/anoxia for Ͼ2 wk, killing reef-associated invertebrate prey and forcing mobile fishes into refuge habitats. Refugee fishes accumulated at high densities on reefs in oxygenated shallow water, where they depleted epibenthic crustacean prey populations. Thus, physical disturbances can impact remote, undisturbed refuge habitats through the movement and abnormal concentration of refugee organisms that have subsequent strong trophic impacts. These results have implications for the design of Marine Protected Areas (MPAs), showing that reserves placed in proximity to disturbed areas may be impacted indirectly but may serve a critical refuge function on a scale that matches the mobility of consumers.

Seasonal Estimates of Fish Biomass and Length Distributions Using Acoustics and Traditional Nets to Identify Estuarine Habitat Preferences in Barataria Bay, Louisiana

“…The literature suggests that oyster reef communities of fish and macroinvertebrates are often highly diverse and include many species that are never or only rarely found in adjacent soft-bottom habitats (see Coen et al 1999). Although an association between fish and oyster habitat has been demonstrated (Coen et al 1999;Lenihan et al 2001;Lehnert and Allen 2002), the relative value of such habitat to fish is uncertain. Given the presumed ecosystem function of oyster reefs and their importance to fish (as areas for feeding, reproduction, and recruitment, as well as refugia; Coen et al 1999;Harding and Mann 2001;Lehnert and Allen 2002), it is not surprising that the overall fish biomass estimates were generally higher at oyster habitat.…”

Movements of Marine Fish and Decapod Crustaceans: Process, Theory and Application

“…Direct human activity such as physical manipulations to the benthos from dredging can also result in some resident animals abandoning or temporarily avoiding an area and opportunistic species moving in to use the disturbed area (Jennings and Kaiser, 1998). Lenihan et al (2001) found that eutrophication, density stratification and oyster dredging combined to deplete dissolved oxygen in bottom waters in the Neuse River estuary, North Carolina, USA. In response, some fishes (termed 'refugees' by Lenihan et al, 2001) abandoned the area and moved to nearby oyster reefs in well-oxygenated waters, where they accumulated in high densities, which resulted in a rapid depletion of prey populations.…”

“…Lenihan et al (2001) found that eutrophication, density stratification and oyster dredging combined to deplete dissolved oxygen in bottom waters in the Neuse River estuary, North Carolina, USA. In response, some fishes (termed 'refugees' by Lenihan et al, 2001) abandoned the area and moved to nearby oyster reefs in well-oxygenated waters, where they accumulated in high densities, which resulted in a rapid depletion of prey populations. Changes in animal density and assemblage composition, therefore, can also result in changes to the availability of resources and the dynamics of competition, which may evoke a relocation response.…”

Behavioral responses of free-ranging blue crabs to episodic hypoxia. I. Movement

“…Hypoxia is generally a seasonal phenomenon in temperate estuaries, and is usually restricted to deeper water basins, which remain hypoxic for most of the summer. Hypoxic 'dead zones' often form in deep channels (Officer et al 1984); thus, reducing the amount of usable habitat for mobile organisms and concentrating them in shallower zones where oxygen concentrations are relatively high (Pihl et al 1991, Breitburg 1992, Lenihan et al 2001. The spatial and temporal extent of hypoxic water can be dynamic because winds and tides periodically cause hypoxic bottom water to upwell onto shallow, nearshore habitats (Sanford et al 1990, Breitburg 1992, Luettich et al 2000.…”