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Declines of Greater and Lesser Scaup Populations: Issues, Hypotheses, and Research Directions

Review Of Current Knowledge


Contaminants have caused population declines in a number of avian species, such as brown pelicans (Pelecanus occidentalis), peregrine falcons (Falco peregrinus), and double-crested cormorants (see Keith 1996, Blus 1996, Peakall 1996 for reviews). Contaminants can affect populations directly, by causing mortality or impaired reproduction, or indirectly, by reducing food availability or quality available to a population. Organochlorine compounds, such as DDT and its metabolites, and dieldrin (Blus 1996, Peakall 1996) have adversely affected avian populations. Trace elements, such as mercury, selenium, and lead, also are detrimental to birds (Paine 1996, Thompson 1996, Heinz 1996). Finally, newer organophosphate and carbamate insecticides have caused avian mortalities, but effects are more often local.

The Great Lakes area is known for environmental contamination problems (Government of Canada 1991). Because of migration shifts by scaup to and through the Great Lakes and longer stopovers there since the zebra mussel invasion (Wormington and Leach 1992), migrant and wintering scaup may be more exposed to contaminants (cf. Austin et al. 1998, Afton and Anderson in review). Potential contaminant exposure in the Great Lakes may be exacerbated by the filtering capabilities of zebra mussels, which may bioaccumulate more contaminants than do other traditional scaup food items. Most persistent organochlorine chemicals have declined in the Great Lakes since the 1970s (Government of Canada 1991). This trend also is evident in tissues of lesser scaup from western Lake Erie/Detroit River area (Smith et al. 1985; C. Custer and T. Custer, unpubl. data). Persistent organochlorines in tissues of lesser scaup have dropped approximately 10-fold in the Detroit River area.

Long Island Sound and adjacent waters also are heavily polluted, based on contaminant levels in sediments, shellfish, and finfish (Grieg et al. 1977, Grieg and Sennefelder 1985). This region has historically been an important wintering area for scaup (Bellrose 1980), but scaup numbers there have declined (Merola and Chasko 1989, Barclay and Zingo 1993). Studies on organic and heavy metal contaminants in scaup indicate they are accumulating contaminants through the food base (J. S. Barclay, unpubl. data).

Selenium is an essential element, and normal concentrations in birds living in fresh water are 4-10 ug/g (ppm, dry weight; Heinz 1996). Lesser scaup collected from western Lake Erie and lower Lake Michigan in 1991-92 contained exceptionally high selenium concentrations. The lowest mean levels of selenium in lesser scaup livers were 2 to 4 times background levels (C. Custer and T. Custer, unpubl. data). Only 1 of 34 livers was in the normal range for selenium. Selenium can impair reproduction via embryo abnormalities and mortality, reduce growth and survival of young, cause pathological changes in tissues, and can cause adult mortality. Ten ppm (dry wt) or greater in the liver of laying females is associated with reproductive impairment in some avian species, and 33 ppm in livers is considered harmful to birds (Heinz 1996). Fourteen of 34 liver samples in the Custers' study were above 33 ppm. Selenium concentrations in these scaup were equivalent to those reported in diving ducks from San Francisco Bay, California, an area known to be contaminated with selenium (Hothem et al. 1998).

Concentrations of other trace elements (Cd, Hg, and Mn) in lesser scaup collected from Lake Erie and Lake Michigan (C. Custer and T. Custer, unpubl. data) were elevated but not at levels indicative of problems. These 3 trace elements also were elevated in liver samples of greater scaup collected in Florida (1990-92; T. Michot, W. Benson, and J. O'Neil, unpubl. data) and in Long Island Sound (J. S. Barclay, unpubl. data); selenium concentrations in those samples, however, were within normal levels. Greater scaup from Florida also showed elevated levels of cadmium (kidney) and arsenic relative to redheads and buffleheads (Bucephala albeola) from Florida or Louisiana/Texas. Differences among species appeared to be related to diet differences (T. Michot and L. Reynolds, unpubl. data), or contaminants accumulated away from that wintering site.

Whether selenium or other contaminants are affecting scaup health, behavior, or reproductive success is unknown. No studies have examined contaminant levels in migrant scaup once they have left the Great Lakes or on the breeding grounds. Such data are needed to assess whether high selenium concentrations, accumulated in the Great Lakes, affect scaup reproduction. This could be determined from a collection of eggs that are analyzed for appropriate chemicals. Additionally, the effects of high selenium concentrations on survival and migration patterns of scaup in the Great Lakes are unknown. This question is less tractable, but could be addressed with a combination of techniques such as satellite telemetry and/or banding.

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