Research Spotlight: High Salinity and Limited Wetlands Reduce Duckling Habitat for Waterfowl

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During the spring and summer, the availability of wetlands with sufficiently low salinities near upland nesting habitat can influence waterfowl reproductive success and population recruitment. Although adult ducks are able to tolerate some salinity in their drinking water, salinity levels as low as 2 parts per thousand (ppt) can impair duckling growth and influence behavior, and mortality can occur above 9 ppt. In a brackish marsh, this low salinity tolerance can mean that suitable brooding habitat is limited. Brooding habitat can be further limited in May through mid-July, when ducklings are hatching, because many of the seasonal wetlands flooded for the duck hunting season (October-January) are dry by this time.

USGS researchers used satellite imagery and surface water samples to map the available wetland habitat for breeding waterfowl and measure salinity concentrations in available water on public and private land on Grizzly Island, Suisun Marsh, during 3 time periods (April, May, and July) over a four year period from 2016 to 2019. 

They found that the amount of flooded wetland habitats decreased from April to July by up to 86% each year. By July, few wetlands remained flooded and most had water salinity concentrations high enough to potentially impair growth and survival of young ducklings. The majority (64% to 100% each year) of wetlands were above levels known to impair ducklings (>2 ppt) and up to 42% of wetland area was above levels that were associated with duckling mortality in other studies (>9 ppt). In drier years, there was less water availability and higher salinity than in wetter years. In May of 2016, the driest year, only 0.5 km² of low salinity water (<2 ppt) was available to ducklings on Grizzly Island, compared to 2.6 km² in May of 2017, the wettest year. Private duck hunting clubs were important providers of summer water, and consistently had a greater percentage of flooded habitat than publicly owned wetlands. However, although there was more water available to ducklings on private land, private land also had higher salinity levels.

The results can inform waterfowl management practices that prioritize breeding duck populations, such as providing fresher water during peak duckling production in May and retaining more water (regardless of salinity) in wetlands through July for older ducklings with higher salt tolerance.

The study was conducted in partnership with California Department of Water Resources, California Department of Fish and Wildlife, Suisun Resource Conservation District, and many private duck hunting clubs.

Management Implications

• California waterfowl populations may benefit from the addition of fresher water in May, during peak duckling production
• Breeding waterfowl may benefit from management actions that retain water through July to maintain wetland habitat, even if that water has higher salinities, for older ducklings that can tolerate higher salinity levels.
• Coordination between public entities and private landowners is important to maintain wetland habitats for breeding waterfowl

This research spotlight refers to:

Schacter, C, SH Peterson, MP Herzog, CA Hartman, ML Casazza, and JT Ackerman. 2021. Wetland availability and salinity concentrations for breeding waterfowl in Suisun Marsh, California. San Francisco Estuary and Watershed Science 19(3):5:1-25. https://doi.org/10.15447/sfews.2021v19iss3art5

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