Coarse woody debris as a refuge from predation in aquatic communities

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Abstract
This study demonstrates experimentally that coarse woody debris (CWD) can provide refuge from predation in aquatic habitats. In the Rhode River subestuary of Chesapeake Bay, Maryland, (USA), we (1) measured the abundance of CWD, (2) examined the utilization of CWD by mobile epibenthic fish and crustaceans, and (3) tested experimentally the value of CWD as a refuge from predation. CWD was the dominant above-bottom physical structure in shallow water, ranging in size from small branches (50 cm diameter). In response to experimental additions of CWD, densities of common epibenthic cpecies (Callinectes sapidus, Fundulus heteroclitus, Fundulus majalis, Gobiosoma bosc, Gobiesox strumosus, Palaemonetes pugio, and Rithropanopeus harrisii) increased significantly compared to control sites without CWD. In laboratory experiments, grass shrimp (P. pugio) responded to predatory fish (F. heteroclitus and Micropogonias undulatus) by utilizing shelter at CWD more frequently than in absence of fish. Access to CWD increased survivorship of grass shrimp in laboratory and field experiments. These experimental results (1) support the hypothesis, commonly proposed but untested for freshwater habitats, that CWD can provide a refuge from predation for epibenthic fish and invertebrates and (2) extend the recognized functional importance of CWD in freshwater to estuarine and marine communities. We hypothesize that CWD is an especially important refuge habitat in the many estuarine and freshwater systems for which alternative physical structure (e.g., vegetation or oyster reefs) are absent or in low abundance.