The Study Plan

Effects of contaminants and nonindigenous invasive species on surf scoters in the San Francisco Bay estuary

Each year as much as 78% of the surf scoter population wintering in the lower Pacific Flyway is found in the SFB estuary (Accurso 1992, Trost 2002, U.S. Fish and Wildlife 2002). The breeding population of this sea duck species has been declining since at least 1978, however the exact cause of decline is unknown (Hodges et al. 1996, Savard et al. 1998). Sea ducks as a group and surf scoter in particular are not well studied, and there is a great need to improve our information on this species to better understand its changing demography (Bellrose 1980, Savard et al. 1998). In 1998 the Sea Duck Joint Venture (SDJV), a partnership among state, federal, and provincial governments and conservation organizations, was formed under the auspices of the North American Waterfowl Management Plan to address sea duck declines. Among the areas identified by the SDJV and previous researchers as needing further study are bioaccumulation of contaminants that may impact over winter survival and reproductive capability, habitat change in wintering areas, winter movements and use of winter habitat, and affiliations between wintering and breeding sites (Savard et al. 1998, SDJV Management Board 2001).

Trace elements such as selenium (Se), mercury (Hg) and cadmium (Cd), are among the major contaminants in SFB. Sources of such inorganic contaminants include Se and Hg from oil refineries in the North Bay, Hg discharged from abandoned mines and disturbed geologic deposits in Bay watersheds, Hg, Cd, and Se from water treatment facilities, and Se from irrigation drain water and the erosion of Cretaceous marine shales from the west side of the San Joaquin Valley. Studies conducted in the 1980s indicate that surf scoter accumulate higher concentrations of Hg and Se while wintering in SFB than any other waterfowl species. Such concentrations are beyond those that negatively affect reproduction in several other species of waterbirds, including dabbling ducks (Anas spp.) (Ohlendorf et al. 1986, Skorupa and Ohlendorf 1991, Hoffman et al. 1998). In addition to reproductive effects, hepatic Hg concentrations have been associated with decreased body, liver, and heart weights, and oxidative stress was correlated with Se and Hg concentrations in SFB estuary surf scoter (Hoffman et al. 1998, Takekawa et al. 2002). Elevated Cd concentrations (24.6 ppm, dry wt) have also been found in kidneys of surf scoter wintering in the Bay (Ohlendorf et al. 1986). Henny et al. (1991) found that increased Cd concentrations (range 19.3 – 108 ppm, dry wt) were negatively correlated with body and liver weights in surf scoter from the Pacific Northwest. Cadmium is not transferred to eggs like Hg or Se; however, high concentrations of Cd can cause testicular atrophy that may result in sterility or reduced fertility (Furness 1996).

While evidence of winter contaminant uptake that could potentially influence surf scoter reproductive success exists, no studies have been undertaken to link wintering and breeding areas of this species. Current population data suggest that surf scoter wintering in SFB may nest in the Northwest Territories of Canada (Bellrose 1980, Savard et al. 1998), but no direct affiliations have been established. Most sea ducks exhibit a high degree of winter site fidelity and pair formation occurs during winter before migration to breeding grounds, increasing the likelihood that both the male and female of a mated pair are exposed to SFB contaminants year after year. Understanding the annual cycle of these populations is essential to determining the reproductive effects of contaminants accumulated during winter. Satellite telemetry is a uniquely suited approach for determining linkages between wintering and breeding locations. Recent advancements in size and packaging of satellite platform terminal transmitters (PTTs) have made it possible to use them on large migratory birds, including surf scoter (Rosenburg and Petrula 2000). Use of satellite technology to find broad-scale migration patterns can be coupled with use of less expensive VHF transmitters that help pinpoint winter habitats and summer nesting sites.

Diving ducks accumulate contaminants mainly from their invertebrate prey, and thus knowledge of a species’ foraging ecology is imperative for understanding contaminant uptake. Benthic invertebrates are known to have elevated contaminant concentrations in the SFB ecosystem (Luoma and Cain 1979, Linville et al. 2002). In addition, introductions of nonindigenous invasive species (NIS) such as the Asian clam (Potamocorbula amurensis; Cohen and Carlton 1995) have drastically altered benthic food resources for avian benthivores in the estuary. Recent studies indicate that the Asian clam has increased in the North Bay area (Carlton et al. 1990, J. Thompson, pers. comm.), and that this mollusk can concentrate contaminants like Se as much as three times more than other clam species in the estuary (Linville et al. 2002). Benthic-foraging ducks seem to take advantage of increasing abundances of NIS by exploiting them as a prey source (Custer and Custer 1996). In an ongoing study of Aythya diving duck species in SFB, Takekawa et al. (unpub. data) found that lesser scaup (Aythya affinis) consume the Asian clam almost exclusively, and greater scaup (Aythya marila) and canvasback (Aythya valisineria) also incorporated this invasive clam into their diet. Limited work on surf scoter diets indicates that they consume Asian clam (White et al. 1989) in Suisun Bay, and may exploit different resources such as Macoma balthica, and unidentified species of mussels, barnacles and algae in San Pablo Bay and the south Bay (Lovvorn, unpub. data). Elevated contaminant concentrations were reported (Miles and Ohlendorf 1993, Ohlendorf et al. 1986) in benthic-foraging ducks collected from the estuary including lesser and greater scaup, canvasback, and surf scoter in studies conducted before the Asian clam invasion. Despite increased regulations on oil refineries and subsequent decreases in their Se outputs, Se concentrations in Asian clams remain high (Luoma and Presser 2000). Preliminary data suggest that Se and Hg concentrations in lesser and greater scaup and canvasback collected between 1998 and 2000 have remained the same or increased slightly since the 1980s (Miles et al. in prep).

Carrying capacity is frequently used to measure a wintering area’s value to waterfowl. While there are variable definitions of the term, carrying capacity is based on the idea that there is a limit to the number of birds the food supply of an area can support (Goss-Custard et al. 2002). Energy- based models that take into account the energy requirements of a forager and the metabolizable energy in prey, are frequently employed to determine carrying capacity (i.e. Guthery 1999). However in systems like the SFBE, interactions of such factors as predation, disturbance, contaminants, NIS, and abiotic conditions have the potential to affect prey quality, quantity and accessibility, and thus may influence carrying capacity. Furthermore, even if carrying capacity of an area has not been reached, poor habitat conditions or disturbance may prevent some individuals from meeting their energy needs (Goss-Custard et al. 2002) . Recent work on clam populations in San Pablo Bay showed that Asian clam populations decline steeply across winter at some sites (Poulton et al. 2002), a trend that seems to coincide with the dwindling numbers of benthic foraging ducks in those areas during late winter (Takekawa et al., unpubl. data). Because the SFBE is the wintering site for such a large percent of declining Pacific flyway diving ducks, it is important that future work establishes the potential number of benthic-foraging waterfowl the SFBE can support and the factors that most influence waterfowl use of the estuary.

We designed this study to address several major issues identified as possible causes for surf scoter declines. First, we will study surf scoter winter foraging ecology and determine relationships between foraging patterns and the high concentrations of contaminants they are known to accumulate in SFB. Carrying capacity of the Bay may influence population dynamics of surf scoter. We will map habitat use and prey distributions and combine this with metabolic data to determine the winter SFB carrying capacity. Lastly, we will pioneer cross-seasonal research to establish linkages between wintering and breeding areas, and determine if contaminants accumulate in SFB are found in elevated concentrations in surf scoter eggs. We will use satellite and conventional telemetry to identify migratory routes and breeding sites of SFB scoter. Once nest sites of the marked birds are found we will collect eggs for contaminant and stable isotope analyses. This will help us determine if contaminants occur at concentrations that may cause reproductive harm, as well as begin to unravel the prey sources of those contaminants.

OBJECTIVES:

1. Examine foraging ecology and contaminant uptake of surf scoter wintering in the SFB estuary.
2. Determine surf scoter and greater scaup carrying capacity in SFB.
3. Determine migratory pathways, ultimate breeding grounds, cross-seasonal persistence of contaminants in SFB surf scoter.

LITERATURE CITED

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Carlton, J. T., J. K. Thompson, L. E. Schemel, and F. H. Nichols. 1990. Remarkable invasion of San Francisco Bay (California, USA) by the Asian clam Potamocorbula amurensis. I. Introduction and dispersal. Marine Ecol. Prog. Ser. 66:81-94.

Cohen, A. N., and J. T. Carlton. 1995. Nonindigenous aquatic species in a United States estuary: a case study of the biological invasions of the San Francisco Bay and Delta. Rep. To U. S. Fish and Wildlife Service, and National Sea Grant College Program, Connecticut Sea Grant. 246pp.

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