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

A new publication by USGS scientists examines wetland availability and salinity in Suisun Marsh, with particular focus on the implications for young ducklings with low salinity tolerance. The results indicated that a majority of wetland area in Suisun Marsh has salinity concentrations high enough to have detrimental effects on duckling growth and survival.

Set of maps showing seasonal extent of water in wetlands and ditches and mean salinity concentration of sampled bodies of water
Seasonal extent of wetted areas in wetlands and canals, and mean salinity concentration of sampled bodies of water at Grizzly Island within Suisun Marsh, California during 2018. Salinity concentrations were divided into categories based on studies that tested the effects of salinity on duckling growth and survival: ranging from no detectable negative effects (< 2 ppt) to rapidly lethal effects (> 21 ppt), with the severity of effects increasing with each colored bin. Areas in black were wet earlier in the breeding season (April or May) but dried up later in the season (May or July), thus providing no habitat for ducklings. Background imagery source: ESRI DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User CommunityFrom: Schacter et al. 2021

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.

Click here to download a PDF version of this research spotlight.



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