A Modified Clod Card Design for Measuring Mass Transfer as a Proxy for Water Motion Inside Oyster Cages

Physical conditions within shellfish cages are often not well characterized in situ, largely due to difficulties finding inexpensive tools that can withstand turbulent environments with a small footprint. A novel clod card design was developed and tested for use in oyster cages for 4–12 h deployments. This clod card is smaller than previous designs and is fitted inside an aluminum bait cage for protection. Our objective was to produce an inexpensive tool capable of monitoring relative water movement in fine, spatially limited locations, such as the inside of oyster aquaculture cages. We tested the dissolution rate of plaster of Paris clod cards across a range of temperatures (15°C–30°C) and salinities (0–30 ppt) in a steady laboratory flow field to develop a predictive model used to convert units of dissolution rate (% mass lost/ h) into units of velocity (m/s). Model predictions of clod cards deployed in the field were then compared to measurements of a nearby acoustic doppler current profiler (ADCP) to determine the accuracy of the predictive model. We also tested the clod cards in a series of small laboratory and field trials to challenge the tool and determine needs for future development, and to further optimize the tool for aquaculture settings. Unfortunately, poor model fits limit the ability to convert plaster dissolution to velocity using known values of dissolution rate, water temperature, and salinity. This suggests that clod card measurements more accurately characterize mass transfer than they do velocity. While clod cards can be useful tools for measuring relative mass transfer inside oyster aquaculture cages, more research is needed to produce accurate conversions to water velocity.