The expert habitat mapping approach is another relatively new approach that is very similar to the habitat criteria mapping approach with one key distinction. Rather than mapping polygons of preferred habitat based on habitat criteria values from the literature, the expert habitat mapping approach relies on a combination of field experience, insight, and quantitative habitat criteria. Although the output, and ability to extrapolate to long river reaches is similar to habitat criteria mapping, this one distinction results in different advantages and disadvantages, as described below.
4.1.5.1 Advantages and disadvantages of approach
Use of expert habitat mapping to assess instream flows has advantages and disadvantages relative to the other approaches evaluated, as discussed below:
Advantages
- Examines the entire reach for habitat, rather than potentially biased cross sections.
- Criteria for mapping are not based on potentially biased suitability criteria, although professional judgment is used during the mapping of polygons.
- Because mapping criteria are not limited by specific measured physical criteria, it allows the simultaneous consideration of an array of qualitative and quantitative factors influencing habitat, including shear zones, vertical velocities, cover, and professional judgment.
- Does not average water velocities, and can account for substrate oriented organisms (e.g., bull trout) separately from mid-column oriented ones (e.g., cutthroat trout).
- By estimating total area available at each flow (rather than an index), the approach allows the results of the habitat mapping to be used in population modeling efforts to determine habitat-based production within a limiting factors framework by quantifying estimates of habitat area for key life stages under different flows. This provides the ability to evaluate potential population responses to changes in instream flows (e.g., effects of doubling spawning habitat while decreasing juvenile rearing habitat).
- Compatible with calibration and validation by use of direct observation snorkel dives in areas mapped as preferred or not preferred.
- Effective at building a consensus to accept results of analysis by including stakeholders in data collection.
- Relatively fast, and thus allows long reaches to be surveyed.
Disadvantages
- Possible bias, due to differences in interpretation of habitat suitability and preference among experts.
- Can only analyze flows that are studied empirically, and cannot simulate habitat flow relationship beyond measured flows.
- Cannot determine statistical error bounds on predicted habitat-flow relationship.
- Does not allow for adjustment after data collection if new information regarding selected habitat criteria becomes available.
- The approach is not repeatable, although it may be possible to determine measurement error, and variance of habitat area estimates.
- Difficult to test and calibrate; requires experts who may be difficult or expensive to include in the process.
4.1.5.2 Site-specific considerations and applicability
Many of the advantages and disadvantages of this approach will depend on site-specific considerations, as discussed below:
- Can be applied in hydraulically simple or complex streams. Therefore it works well in high-gradient, boulder-dominated systems, where other methods do not work as well. However, it does not work well in areas that are too deep to wade safely.
- By using mesohabitats as sampling units, the approach is able to estimate the relationship between flow and habitat for long river reaches.
- Works best when a process is established to resolve potential disagreements among experts, and when a collaborative "expert" report is produced to avoid disputed conclusions.
- Has been applied on the Oak Grove Fork of the Clackamas River at the Clackamas River Hydroelectric Project (FERC Project No. 2195), Oregon; and at the Blue Lake Hydroelectric Project (FERC Project No. 2230), on Sawmill Creek, Alaska.
4.1.5.3 Selected references
McBain and Trush. 2003. Estimating salmonid habitat availability in the lower Oak Grove Fork using expert habitat mapping: summary of methods and preliminary results. Prepared for Clackamas Instream Flow/Geomorphology Subgroup, Portland General Electric, Portland, Oregon by McBain and Trush, Arcata, California.
Morhardt, J. E., D. F. Hansen, and P. J. Coulston. 1983. Instream flow: increased accuracy using habitat mapping. Ecological Analysts, Inc., Lafayette, California.
Stanfield, L. W., and M. L. Jones. 1998. A comparison of full-station visual and transect-based methods of conducting habitat surveys in support of habitat suitability index models for southern Ontario. North American Journal of Fisheries Management 18: 657-675.
