| Abstract | 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 landlocked
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.
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