Mesohabitat Simulation Model (MesoHABSIM) addresses the requirements of watershed-based management of running waters. It builds upon pre-existing physical habitat simulation models (e.g. PHABSIM) to predict an aquatic community's response to habitat modification. The Physical Habitat Simulation System (PHABSIM) is a central part of the U.S. Government's method of setting minimum stream flow requirements. MesoHABSIM is an enhancement of this system developed during a restoration study on the Quinebaug River. The changing spatial distributions of physical attributes of a river as a result of variations in flow and the biological responses of aquatic species to these changes, provide the basis for simulating the consequences of ecosystem alteration, and consequently the justification of restoration measures. MesoHABSIM modifies the data acquisition technique and analytical approach of similar models by changing the scale of resolution from micro- to meso-scales. Earlier methods were limited by a few, short sampling sites that resulted in the simplification of hydraulic conditions. Due to this increase in scale, the model takes variations in stream morphology along the river into account and is more applicable to large-scale issues. We believe that habitat and fish measurements at larger spatial units are more practical, more relevant to river management, and more conducive to habitat modeling.

Mesohabitat types are defined by their hydromorphological units (HMUs), such as pools and rapids, geomorphology, land cover and other hydrological characteristics. Mesohabitats are mapped under multiple flow conditions at extensive sites along the river. Fish data is collected in randomly distributed mesohabitats where habitat surveys are also conducted. This allows modeling of available fish habitat at a range of flows (Figure 1). Rating curves represent the changes in relative area of suitable habitat in response to flow and allow for the determination of habitat quantity at any given flow within the range of surveys (Figure 2). These rating curves can be developed for river units of any size making them useful for drawing conclusions about the suitability of channel patterns or habitat structures for various species of fish for specific sections as well as for the entire river. Rating curves can also be used to evaluate the benefits of various restoration measures on the entire fish community. In combination with hydrologic time series, rating curves are used to create Continuous-Under-Threshold (CUT) curves for the analysis of frequency, magnitude and duration of significant habitat events. The CUT curve technique described by Capra et al. (1995) helps us define critical thresholds and determine what habitat variability and availability is necessary to support the target river fauna. CUT curves evaluate durations of unsuitable habitat under a specified threshold by comparing continuous durations in days under this threshold to the cumulative durations in the study period. A highly useful product of the CUT curves are reference tables that managers can use to determine how long a given species can tolerate unsuitable conditions depending on its life stage (Table 1).

To use physical habitat models to analyze and predict ecosystem potential, we must also determine the composition of the native fish community and select a subset of species for model development. Our development of a Reference Fish Community (RFC) is based on the Target Fish Community approach, described by Bain and Meixler (2000). A comprehensive list of species is generated from literature sources and available regional data collected on relatively intact river reaches. The species are ranked on the basis of abundance in long-term fish collection data from multiple rivers of similar character. Securing habitat for naturally occurring dominant species should preserve the most profound characteristics of the ecosystem, providing survival conditions for the majority of the aquatic community and therefore a reference for restoration efforts. The simplest way to create a river habitat model is therefore to select the five to ten highest ranking species for model development. We can assume that community structure reflects habitat structure; therefore, the most common species should indicate the most common habitat. Since habitat availability forms the structure of aquatic fauna, the affinity between the structure of the river habitat and the structure of the fish community can be used as a measure of habitat quality.

The results of MesoHABSIM create the framework for integrative analyses of many aspects of the ecosystem. It also allows managers to recreate reference conditions and evaluate possible instream and watershed restoration measures or alterations, such as dam removals or changes in water withdrawals. From the perspective of resource managers, it not only allows for quantitative measures of ecological integrity, but also creates a basis for making decisions where trade-offs between resource use and river restoration need to be considered. Click on the Evaluation link above for more figures.

Meixler. 2000. Defining a Target Fish Community for Planning and Evaluating Enhancement on the Quinebaug River in Massachusetts and Connecticut. Report for Quinebaug River Study Agencies. Cornell University, Ithaca, NY .

Capra, H., B. Pascal, and Y. Souchon. 1995. A new tool to interpret magnitude and duration of fish habitat variations. Regulated Rivers: Research and Management. 10: 281-289.