Feature ArticleDeploying Dissolved Oxygen Sensors On Crab Pots for Ocean Observations
By R. Kipp Shearman
College of Oceanic and Atmospheric Sciences
Oregon State University
Jeremy L. Childress
The Sexton Co.
A major challenge facing any observing system is achieving cost-effective spatial and temporal coverage for resolving the physical, biological and chemical processes that occur in the coastal ocean, with timescales ranging from less than a day to decades and spatial scales from a kilometer to coastwide. Observational buoys are too few, and ship-based towed platforms can only cover a small area of the ocean for a limited amount of time. AUVs, such as gliders, combine some aspects of ship-based and moored platforms; however, in the coastal ocean the operational costs remain relatively high.
One strategy for increasing coastal ocean observations is collaborating with fishermen to use their vessels and their gear as sensor platforms. A few programs have developed to take advantage of this natural collaboration to achieve a greater density of observations. Since 2001, the eMOLT (Environmental Monitors on Lobster Traps) project has been collaborating with lobster fishermen in the Gulf of Maine, deploying temperature sensors and recently adding surface drifters and bottom current measurements using a new inclinometer device.
The Dungeness crab fishery along the U.S. West Coast offers a similar opportunity for collaboration. Off Oregon for example, the Dungeness crab season runs from December 1 through August 14 (although the fishing tapers off rapidly over the first few months). During the crab season, more than 350 vessels deploy up to 500 crab pots, steel traps about 1 meter in diameter that rest on the seafloor and attract crab, with bait. Each pot is a potential platform for scientific observations.
Oregon Fishermen in Observational Research
In 2005, the authors began collaborating with commercial Dungeness crab fishermen on the central Oregon coast and deployed temperature sensors on fishermen's gear. The project began with one fisherman out of Newport, Oregon, deploying 20 crab pots with sensors. The project has expanded each year and now has 15 participating fishermen from Port Orford, Oregon, to Quinault, Washington, deploying 80 temperature sensors in their crab pots and on their surface floats (the surface to bottom temperature difference can be used as a proxy for bulk stratification).
Pots are deployed in water depths ranging from 2 to 50 meters, and the location is typically repeated year to year. Temperature is measured using Onset Computer Corp.'s (Bourne, Massachusetts) TidBit temperature loggers, and measurements are recorded every 10 minutes and averaged into hourly values. The fishermen log the GPS time and position and water depth from the ship's echosounder at each deployment and recovery. Fishermen also record catch data per recovery.
In 2009, the authors expanded their collaboration with commercial crab fishermen to include observations of near-bottom dissolved oxygen, using a sensor designed in-house. The dissolved oxygen instrument packages were designed with the specific goals of making high-quality measurements of benthic dissolved oxygen and having a low overall cost per unit so that an array of instruments can be deployed for concurrent monitoring of dissolved oxygen at higher spatial resolution than present.
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R. Kipp Shearman is an associate professor of oceanography at Oregon State University. His research focuses on the physical dynamics of the coastal ocean and physical impacts on coastal ecosystems. His approach is observational, using sampling platforms such as commercial crab pots and AUV gliders.
Jeremy L. Childress, designer and project manager for the Sexton Co., studied marine biology at Millersville University. For graduate school, he studied marine resource management at Oregon State University and researched crab pots as platforms of opportunity for ocean observation. Childress also serves as chair for the Marine Technology Society's Oregon section.