Bengston, F. 1984. Studies of lead toxicity in bald eagles at the Lac Qui Parle Wildlife Refuge. M.S. Thesis, University of Minnesota. 95 pp.
Lead poisoning is a significant mortality factor in waterfowl populations and other aquatic related species. Recent evidence suggest that raptors, particularly bald eagles, also are affected from eating prey that contain embedded lead shot and/or tissue-bound lead. These 1981-1983 Lac qui Parle Wildlife Refuge (LQPWR) studies in western Minnesota monitored specific impacts of available lead shot on migrant bald eagles and Canada geese by various methods and evaluated effectiveness of current limited steel shot regulations as a management technique for these two species. Sampling for shot in the LQP traditional hunting areas, and observing occurrence, feeding behaviors, and determinating physiological presence of lead in bald eagles and Canada geese were undertaken to describe lead shot impacts on these resources.
Pre-hunting surface shot densities ranged from 13 - 1,939 pellets/ha for 1981-1983. Yearly significant decreases (p<0.05) in lead-steel shot ratios occurred from 1980-1983, although in 1983 lead shot constituted over 80% of sampled pre-hunting season surface shot. Yearly decreasing pre-hunting season surface lead-steel shot ratios follow very closely to a negative log-linear relationship (r=0.996). Private lands adjacent to the LQPWR had surface densities of lead and steel shot 2 - 150 times the amount found in state lands. Shot densities in LQP study plot soils ranged from 106,000 - 328,708 pellets/ha from 1981 - 1983. Seasonal shot deposition at state blinds ranged from 1,429 to 12,857 pellets/ha from 1981 - 1983. Lead shot used illegally accounted for 7%, 9%, and 7% of shot deposited for the three study years. Harvest of crops without cultivation reduced pre-hunting season lead shot densities at LQP by 59% and 32% in 1981 and 1982, respectively.
Yearly pre-hunting season surface lead-steel shot ratios are greater than ratios found in soil because geophysical processes of wind and water erosion in agricultural fields selectively expose a higher proportion of lead than steel shot. A 10 to 15 year period may be necessary before lead shot densities in and on LQP upland soils are reduced to acceptable levels.
Freshly dead waterfowl were preferred bald eagle food. Canada geese were scavenged over 60% of the time. Physical lead poisoning symptoms of collected Canada geese did not correlate with their liver tissue lead contents. Lead poisoning in Canada geese was substantially reduced at LQP from 1978 - 1982.
The proportion of shot positive castings from bald eagles varied between 9% and 20% from 1978 - 1983. Since 1980 the proportion of lead shot positive castings has varied between 68% and 78%. Two bald eagles with suspected lead poisoning symptoms were recovered at LQP in 1981 - 1983. Both died with death attributed to lead poisoning. Conservatively, 4 - 5% of the estimated 100-150 bald eagles that stop over at the LQPWR annually may eventually die of lead-related causes. Corrective management requires improved steel shot regulations for all waterfowl hunting nationally and internationally. Other management recommendations based on study results were listed.