Behavioral response of Dungeness crab (Cancer magister) to dredged sediment deposition events assessed with acoustic positional telemetry

Maintenance of river mouths and harbors, coupled with environmental impacts of rising sea levels on ocean beaches, has led to management strategies that direct sediment supply to impacted areas. Judicious "beneficial" placement of sediment requires limiting negative impacts to critical species. The effects of one such strategy, "thin-layer deposition", was investigated on its impact on Dungeness crab (Cancer [Metacarcinus] magister), a prized fishery species, at the mouth of the Columbia River, USA. These deposition events include an energetic "lateral surge" of sediment that was hypothesized to stress or injure animals, and would be manifested as reduced activity (quiescence). The study details a novel approach: using acoustic positional telemetry, derived behavioral metrics, and a before-after control-impact (BACI) statistical design to measure activity of tagged Dungeness crab immediately following deposition events. Crab movement metrics (average velocity, linearity, and duration in arrays) were first compared to reference and loose tags to evaluate possible tag loss or mortality. Few crabs had activity patterns similar to these more quiescent tags. Crab positions (tracks) in control and impact treatment areas (receiver arrays) were then compared using velocity and linearity as response variables. No statistically significant differences in crab activity metrics were detected between control and impact treatments. Crabs were active with few quiescent periods both before and after sediment deposition, and residence times within arrays was generally short (<2 d). For a subset of crabs, movements recorded days to weeks post-release were similar to those observed at the initial release. Compared to previous acoustic studies in estuaries and the Salish Sea, there was high motility in coastal crabs. Thin-layer deposition was deemed effective at distributing sediment while minimizing adverse effects on biota. Acoustic positional telemetry provided a means to measure crab behaviors at meter-scale accuracy and offers a methodology relevant to a number of other epifaunal species and sediment deposition scenarios.