Geology topics
D. H. Rusch
Publications and source records attributed to D. H. Rusch.
Prevalence of lead exposure among age and sex cohorts of Canada geese
We examined the prevalence of lead exposure from ingestion of waste lead shot among age and sex cohorts of Canada geese ( Branta canadensis ) on the breeding, migration, and wintering grounds of the Eastern Prairie Population. Blood samples from 6963 geese were assayed for lead concentration by atomic absorption spectrophotometry. On the breeding grounds, no goslings and <1% of adults showed evidence of recent exposure to lead shot (i.e., concentrations in the blood elevated above the threshold value of 0.18 ppm lead). However, median background blood lead concentrations (i.e., blood samples with < 0.18 ppm lead) were higher in adults than goslings, indicating that exposure of adults to lead had occurred during previous seasons. Waste lead shot was available on the migration and wintering grounds, where a larger proportion of the blood samples from immatures (< 1 year old) than adults (> 1 year old) had lead concentrations ≥ 0.18 ppm. Median background lead levels remained higher in adults than in immatures throughout fall and winter. We also found that more immature males than immature females had elevated lead concentrations. Higher rates of intake of food and grit (including shot) probably partially account for the higher prevalence of elevated lead concentrations in immature Canada geese.
Lead exposure in Canada geese of the eastern prairie population
We monitored lead exposure in Eastern Prairie Population Canada geese during summer-winter, 1986-1987 and 1987-1988 at 5 areas. Blood lead concentrations in geese trapped during summer at Cape Churchill Manitoba were below levels indicative of recent lead exposure (0.18 ppm). Geese exposed to lead (≥0.18 ppm blood lead) increased to 7.6% at Oak Hammock Wildlife Management Area (WMA), southern Manitoba, where lead shot was still in use, and to 10.0% at Roseau River WMA, northern Minnesota, when fall-staging geese were close to a source of lead shot in Manitoba. Proportion of birds exposed to lead dropped to <2% at Lac Qui Parle WMA, Minnesota, a steel shot zone since 1980. On the wintering grounds at Swan Lake National Wildlife Refuge in Missouri, 4.9% of all geese showed exposure to lead before the hunting season. Lead exposure rose to 10.0% after hunting ended and then decreased to 5.2% in late winter. Incidence of lead shot in gizzards and concentrations of lead in livers supported blood assay data. Soil samples indicated that lead shot continues to be available to geese at Swan Lake, even though the area was established as a non-toxic shot zone in 1978. Steel shot zones have reduced lead exposure in the Eastern Prairie Population, but lead shot persists in the environment and continues to account for lead exposure and mortality in Eastern Prairie Population Canada geese.
Effects of resighting errors on capture-resight estimates for neck-banded Canada geese
Biologists who study neck-banded Canada Geese (Branta canadensis) have used capture and resighting histories to estimate annual resighting rates, survival rates and the number of marked birds in the population. Resighting errors were associated with 9.4% ( n = 155) of the birds from a sample of Canada Geese neckbanded in the Mississippi flyway, 1974-1987, and constituted 3.0% ( n = 208) of the resightings. Resighting errors significantly reduced estimated resighting rates and significantly increased estimated numbers of marked geese in the sample. Estimates of survival rates were not significantly affected by resighting errors. Recommendations are offered for using neck-band characters that may reduce resighting errors.
Estimating population parameters for geese from band-recovery and mark-recapture data
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Population ecology and harvest of the American black duck: a review
1. The purpose of our review was to examine available data on population trends and current status of black ducks and trends in natality and survival and to relate these, where possible, to changes in habitat, predation, disease, contaminants, harvest, and hybridization with mallards. 2. The number of black ducks tallied in the winter survey has declined steadily over the past 30 years at an average rate of about 3%/ year. Reliability and precision of the survey are uncertain; it may not provide an adequate index to the continental population of black ducks. Breeding surveys are incomplete and sporadic, but black ducks have decreased in Ontario and increased in the Maritime Provinces and Quebec. 3. Recent declines in numbers of black ducks tallied in the winter survey are not unusual in magnitude or much different from those that have occurred among several other species of waterfowl. 4. At present, black ducks are not especially scarce relative to numbers of several other ducks in eastern North America. 5. There is no solid evidence of major decreases in quality or quantity of breeding habitat for black ducks in recent years; in some areas, habitat has improved. 6. Natural mortality of black ducks has not been well studied, but does not seem unusually high compared to other dabbling ducks. 7. Harvest rates of black ducks are similar to those of sympatric mallards as determined by banding analyses. 8. There is no strong evidence for direct effects of contaminants on black ducks, but some indirect effects through invertebrate food resources have been detected. 9. Age ratios in black ducks show no trend in the past 18 years. 10. The quality and quantity of wintering habitat for black ducks have decreased substantially in some areas. 11. Disease and other natural mortality that affect black ducks do .not occur in unusually high frequency. 12. A decline in harvest of black ducks has occurred; most of the decline has been in the United States, especially since restrictive regulations were implemented in 1983. 13. Recovery rates of black ducks have declined recently in the U.S., but not in Canada. 14. Survival rates of black ducks are 56-63% for adults and 43% for young. These rates of survival are similar to comparable estimates in sympatric mallards. 15. Long hunting seasons may depress survival in some sex-age classes of black ducks, buteffects of small reductions in survival on population trends are unknown. 16. Available evidence does not support the contention that hunting is either the sole or most important cause of the decline in the winter tally of black duck numbers. 17. Surveys and banding of black ducks should be thoroughly reviewed and maintained or improved as warranted. Obtaining or maintaining a reasonable index to numbers of black ducks is the top priority among survey needs. 18. Experimental manipulation of hunting seasons should be considered to elucidate relationships among regulations, harvest, survival, and population trends. 19. Black ducks and mallards are genetically similar; there is as much genetic differentiation within the 2 species as there is between them. 20. Black duck x mallard hybrids are fertile. Hybrids are difficult to detect by plumage and thus published frequencies (0-13%) of hybrids may be low. 21. Hybridization could be a result of concomitant mallard increases and black duck decreases, or changes in black duck-to-mallard ratios could be from hybridization and genetic swamping of black ducks.
Workshop summary: Hunting vulnerability and mortality
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