ACR releases new research about egret foraging behavior in Tomales Bay

ACR researchers recently published results on December 31, 2021, from our study of GPS-tagged Great Egrets foraging on Tomales Bay. “Great egret (Ardea alba) habitat selection and foraging behavior in a temperate estuary: comparing natural wetlands to areas with shellfish aquaculture,” co-authored by Scott Jennings, David Lumpkin, Nils Warnock, T. Emiko Condeso, and John P. Kelly, appears in “PLOS One.”

Do shellfish aquaculture and natural tidal wetlands both provide foraging habitat?

As the tide flows in and out of the bay, Great Egrets can choose to get their lunch from a diverse array of natural tidal wetlands (including eelgrass, tidal marsh, and unvegetated mudflats) and areas where shellfish are commercially grown.

Shellfish aquaculture on Tomales Bay is important to the local economy and provides a valuable connection to locally-produced food. However, the agencies that determine how much aquaculture should be allowed on the bay, and where it should be placed, lack information about how wildlife might interact with these areas.

By tracking egrets with small GPS backpacks, we found that foraging egrets preferred natural tidal wetlands

Between 2017 and 2019, we placed small GPS backpacks on egrets to measure whether they chose natural wetlands more often than shellfish areas to forage for their diet of fish and aquatic invertebrates.

We tagged 10 Great Egrets and trackedtheir foraging on Tomales Bay for 73–360 days per bird. Three tagged egrets essentially avoided foraging in shellfish areas. The remaining seven egrets used shellfish areas, but only when they were right at the tidal water level. The egrets avoided wading into deep shellfish areas or using those areas when they were exposed by low tides. In contrast, these egrets foraged in natural wetlands in all water depths that they could access. Although eelgrass beds are known to be important habitat for a wide range of species, we found that all types of natural wetlands we evaluated (eelgrass, tidal marsh, and mudflat) appear to be equally important to foraging Great Egrets throughout the tidal cycle.

Summarized results will be invaluable for future habitat and conservation management

Our results suggest that although egrets appear to perceive shellfish areas as foraging habitat, those areas provide less foraging opportunity throughout the tidal cycle than natural wetlands.

The takeaway for natural resources and aquaculture managers: Replacing more natural wetlands with shellfish aquaculture could reduce egret foraging opportunities, and possibly alter ecological roles of egrets as top predators in these natural wetlands.

Next steps: We are sharing these results with local management agencies and organizations (CA Coastal Commission, West Marin Environmental Action Committee, NPS) to help them better manage and plan human actions on Tomales Bay. We are also studying how human actions on Tomales Bay (including shellfish aquaculture, recreation, and more) might affect how well the bay provides habitat for other bird species as well.