Filter Design. Filters determine the relationship between the risk components (sources, habitats, and impacts to assessment endpoints). A filter consists of the weighting factors, 0 or 1, which indicate either a low or a high probability. We have incorporated two types of filters: an exposure filter and an effects filter. The exposure filter screens the source and habitat types for the combinations which are more likely to result in exposures (i.e., receptors in the habitat will come into contact with stressors generated by the source). The effects filter screens the source and habitat combinations for those that are more likely to affect a specific assessment endpoint. An example describes the design of both an exposure and an effects filter (see Example 1). The first step in designing an exposure filter is to determine which stressors are produced by the sources. Professional knowledge is then used to answer two sequential questions about each stressor in relation to specific source-habitat combinations • Will the source release or cause the stressor? • Will the stressor then occur and persist in the habitat? If the answer to both questions is yes, then a 1 is assigned to the source-habitat combination. If the answer to either question is no, then a 0 is assigned. The design of an effects filter is similar, but a separate filter is made for each assessment endpoint. The first step in this process is to determine what type of effects are important to the specific endpoint. For instance, if maintaining crab populations is an assessment endpoint, some of the important effects to consider are toxicity, predation, and food availability. The questions asked to develop the effects filters are • Will the source release stressors that are known to cause this particular effect to the endpoint? • Are receptors associated with the endpoint sensitive to the stressor in this habitat? If the answer to both questions is yes, then a 1 is assigned to the source-habitat combination. If the answer to either question is no, then a 0 is assigned.
Filter Design. This example designs an exposure filter for three sources (i.e., effluent discharges, road runoff, and seafood processing xxxxx) and three habitats (i.e., mudflats, rocky intertidal, and deep benthic). The stressors examined include hydrocarbons and solids. Hydrocarbons are more likely to result from effluents and runoff than from seafood, while solids are more likely to result from seafood wastes and runoff than effluents. Combinations where the stressor is likely to be related to the source are assigned a value of 1. Hydrocarbons Solids effluents 1 runoff 1 seafood 0 effluents 0 runoff 1 seafood 1 Not all of these sources will release stressors into the same habitats. The baseline established in the former tables is expanded to include different habitat types. Hydrocarbons Solids mudflats benthic rocky runoff 1 1 1 seafood 0 0 0 mudflats benthic rocky seafood 1 1 1 The assigned values of 1 are then re-evaluated and changed to 0 if the habitat is not likely to be exposed by that source. For instance hydrocarbons from liquid effluents discharged year-round could conceivably occur in any habitat. However, hydrocarbons from runoff are more likely to affect shoreline habitats than deep benthic habitats. Likewise, seafood processing plants are allowed to release solids offshore, but not to mudflats or rocky intertidal areas. Hydrocarbons Solids mudflats benthic rocky seafood 0 0 0 mudflats benthic rocky The completed exposure filter is made by merging each individual filter. During merging, a 1 over-rides a 0 so that all stressors are represented in the final filter. Exposure Filter mudflats benthic rocky seafood 0 1 0 Example 1, cont’d Effects Filter Design for Fisheries This example designs an effects filter associated with an impact to the commercial fishery: decreased survival of the hatchery pink salmon fry released by the Solomon Gulch Hatchery. Effects that could influence this endpoint include: (1) acute toxicity to fry causing death during their migration out of the Port, and (2) increased predation on the migrating fry. Sources that can directly or indirectly cause these effects are assigned a 1. For instance, effluents and runoff can carry contaminants into the Port and directly cause toxicity. Seafood accumulations can result in anoxia and production of hydrogen sulfide by benthic bacteria, which indirectly results in toxicity. Seafood can also attract more scavengers and predators to an area. Acute Toxicity Predation effluents 1 seafood 1 effluent...
