Abstract:
Re-establishing connectivity is a primary restoration activity for enhancing the
recovery of migratory fishes, but actions are often limited by lack of funds and
understanding of the benefits of individual projects. The objective of this study was to
develop a Bayesian Network (BN) to assess priorities for restoration of aquatic
connectivity as accomplished by replacement of culverts at road stream crossings that
may act as passage barriers to winter run Steelhead (Oncorhynchus mykiss) in the
North and South Santiam Rivers (state of Oregon). The model predicted the
probability of biological benefit obtained by removal or replacement of a culvert. The
degree of passage impairment, habitat suitability and probability of habitat use
influenced the predicted biological benefit. This model structure was populated with
conditional probability table values derived from expert opinion and a Bayesian
learning algorithm to produce outcomes based on different model inputs. Both models
were then used to assess 141 data scenarios land and fishery managers would likely
encounter. Results of the BN indicate that culverts that 1) are barriers to adult and
juvenile steelhead, 2) are located in Oregon Department of Environmental Quality
(DEQ) designated cold core water habitat, 3) have a high capacity for rearing juvenile
fish, and 4) have a high probability of habitat use will provide the highest overall
benefit. As anticipated, culverts that are not barriers to upstream migrating fish
provided the lowest benefit, regardless of habitat suitability or habitat use. In addition
to specific results for the Santiam basin, comparison between the two models and
across information scenarios illustrated the sensitivity of such models to various
conditions likely to be encountered by decision makers; in general, the two models
agreed when all input nodes were engaged by having a state value entered, yet
disagreed as fewer input nodes were engaged. The passage impairment of a culvert
and the probability of habitat use exerted a strong influence on model output. Finally,
this model may serve as a template for providing a coarse evaluation of culverts in
other basins or may be a foundation upon which additional nodes may be added.
