Investigation of Dredged Sediment Deposition Events on Dungeness Crab at the Mouth of the Columbia River

This research is an experiment to investigate dredged sediment deposition events on Dungeness crab, Cancer magister. Our main site is in the nearshore ocean just south of the mouth of the Columbia River, with other receivers located north of the river mouth and inside the estuary. We have three years of data (2014-2016) conducted in late August through October, which is when sediment deposition is allowed.

The first two years we used a similar design. There were 2 arrays (control and impact, separated by ~1.8 km) of 4 receivers each arranged in a square formation. Receiver moorings in an array were ~300 meters apart.

Each mooring had a V2RW or VR2Tx receiver located 7 m above the bottom and a synch tag 8 m above the bottom (for V2RW units). A subsurface float kept the mooring line near-vertical. We used this arrangement with the Vemco VPS to compare trajectories of crabs at impact and control treatments.

In 2015 we also had 2 receivers along the South Jetty for a portion of the study (SJ.1 and SJ.2) to act as a “gateway”. In 2016 we expanded and re-arranged the receivers into 13 receivers in a 3 x 5 node rectangle formation (minus 2 interior nodes). The array was divided into impact (north) and control (south) sub-arrays.

We also deployed 2 moorings as a gateway near the South Jetty, as in 2015. Synch tag signals from these moorings (which were in contact with each other) may be detected by some of the moorings in the main array. A third mooring was positioned several kilometers away as a lone sentinel.

It recorded presence of some our tagged crabs but was outside the range of the other receivers. Two additional nodes were deployed inside the Columbia River Estuary (ETI and TS), also out of range of other receivers. During an experiment, we released batches of 10 crabs in the center of each subarray.

In the impact array, a large hopper dredge deposited a plume of sediment on the crabs\; the control array received no sediment. We repeated the experiment three time in 2014 and 2015 and twice in 2016. In 2016, we also released 20 tagged crabs in the estuary.

There were reference tags deployed in each array as well. Tags on the Dungeness crab had > 200 d battery life, and the crabs are capable of significant movements in that period.