Analysis of Patterns in Diadromous Fish Distributions Using GIS

Understanding the factors limiting migratory behaviour is fundamental to conservation of diadromous fish. Applications of indices of habitat suitability are problematic for diadromous fish because fish presence and abundance in relation to habitat quality are confounded by barriers to fish migration. An alternative approach is to assess diadromous fish distributions in proportion to distance inland and altitude above mean sea level, and subsequently generate trajectories for the various species. This approach, however, may be problematic. We show that river distance inland and elevation are only weakly correlated in our study area. Thus, in areas where steep slopes are not encountered, fish migrations to significant elevations and inland distances can be expected. In other areas, coastal cliffs and geologic fault lines provide for steep stream gradients close to the sea, and fish do not migrate far inland. To solve this issue, we developed methods for improving species trajectory approaches to explain the distribution of diadromous fish using a GIS. We adjusted distance and altitude categories so that each stratum was represented by the same number of site records, with flexible intervals for each stratum. For species capable of forming land-locked populations we manipulated input values for elevation and river distance inland to account for migrations from lakes, rather than sea. Additionally, a new GIS derived variable was introduced to better explain the distribution of diadromous fish; the maximum stream slope a fish would encounter during upstream migration. This new slope variable, independent of distance inland and elevation, is likely to be a better predictor of migratory fish occurrences than elevation above mean sea level, as the different species will have different slope-thresholds that they can overcome.