Effects Of Lead Shot
Photo: Carmen Thomas
Effects of Tillage on Lead Shot Distribution in Wetland Sediments
Poisoning of birds by lead shot has been a recognized problem for more than 100 years. Poisoning occurs when birds feed in hunted areas and inadvertently swallow spent lead pellets.
In wetlands underlain by hardpan clay layers, pellets are prevented from settling beyond the level of availability to ducks and swans, and may result in persistent lead poisoning in these areas.
One such area appears to be the Sacramento National Wildlife Refuge in northern California. Although non-toxic shot has been required for waterfowl hunting on the Sacramento National Wildlife Refuge since 1986, lead poisoning of waterfowl continued to be diagnosed annually.
At Sacramento National Wildlife Refuge, California, we examined 2 types of deep tillage (disking and plowing) as possible management options for reducing lead pellet densities in wetlands. In addition, we examined the vegetation changes that resulted from tilling. Read the full report.
Both disking and plowing moved lead pellets below the zone of availability for dabbling ducks (>10 cm). However, plowing moved a higher percentage of pellets into the 15-20-cm layer of sediment. Similarly, plowing was more effective than disking or controls in redistributing pellets below the deeper zone of availability for tundra swans (Cygnus columbianus).
Maximum height of vegetation increased on tilled plots during the first and second year after treatment. Tillage initially reduced percent cover and density of swamp timothy (Crypsis schoenoides), but resulted in increased swamp timothy cover and stem density by the second year post-treatment.
Percent cover by California loosestrife (Lythrum hyssopifolium) showed the opposite trend, with an initial increase, followed by a decrease to levels similar to control plots in the second year.
In certain managed wetlands, disking and plowing can be effective management tools for redistributing residual lead shot deeper into wetland sediments and potentially reducing waterbird mortality due to lead poisoning.