Mercury Strategy for the Bay-Delta Ecosystem - CALFED Bay-Delta ...

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(7) To identify spatial and temporal patterns in mercury concentrations in bioindicator fishes.
Spatiotemporal patterns in mercury contamination of biosentinel fishes should be statistically
examined to assess their possible relation to ecosystem restoration activities or other potential
causal factors. The sampling design should, therefore, include monitoring sites in the vicinity of
wetland restoration projects.
Monitoring data would not – in the absence of other supporting information – conclusively
demonstrate cause-and-effect associations. The interpretation of data from a monitoring program
should be strengthened by linking monitoring efforts to investigations of ecological and
biogeochemical processes or factors that affect the abundance of methylmercury, as well as its
bioaccumulation and trophic transfer in aquatic food webs.
5. Assessment of Ecological Risk
Methylmercury is a potential threat to organisms in upper trophic levels of aquatic food webs in
mercury-contaminated ecosystems. In birds and mammals, methylmercury in reproducing
females readily passes to the developing egg or embryo, and the early developmental stages are
much more sensitive than the adult to methylmercury exposure (Scheuhammer 1991, Wiener et
al. 2003). Avian reproduction can be impaired in females fed diets with concentrations of
methylmercury that are one-fifth of the threshold dietary concentrations causing overt toxicity in
adult birds of the same species (Scheuhammer 1991).
A number of bird species that nest or feed in the Bay-Delta may be sensitive to methylmercury
exposure (Heinz 2002, Schwarzbach and Adelsbach 2002). Concentrations of total mercury
(probably present as methylmercury) in six failed eggs of the federally endangered clapper rail,
taken from the central San Francisco Bay, averaged 0.81 :g/g wet weight and ranged from 0.60
to 1.06 :g/g (Schwarzbach and Adelsbach 2002). Methylmercury concentrations of 0.8 :g/g
wet weight or greater in eggs adversely affected embryo survival in controlled, egg-injection
experiments with eggs of clapper rails (Heinz 2002). Diminished reproductive success could
have adverse population-level consequences for clapper rails and other species of wildlife and
fish exposed to high levels of methylmercury in the Bay-Delta ecosystem.
The decision process for adaptive restoration of the Bay-Delta System will require information
on methylmercury exposure and associated ecological effects in fish and wildlife. Estimates of
exposure thresholds associated with reproductive effects in species of concern, based on
methylmercury concentrations in tissues or the diet, are currently lacking but would provide
biologically relevant targets applicable to adaptive management and environmental decisions.
Information in exposure thresholds could also be used to identify those species that are most
vulnerable to methylmercury (in terms of sensitivity and exposure) and to assess whether
existing levels of methylmercury exposure in the ecosystem could impair recovery of at-risk
native species.
The overall science goal for this core component is to quantify methylmercury exposure and to
assess the likelihood that adverse ecological impacts are occurring or may occur in fish and
wildlife as a result of methylmercury exposure. The overall management goal is to protect fish
and wildlife from adverse effects of methylmercury exposure. Success in achieving this
mercury-specific management goal would directly support CALFED’s strategic restoration goals
concerning the recovery of at-risk native species and the rehabilitation of the Bay-Delta to
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