Regional Variability in Atlantic Salmon Riverscapes: a Simple Landscape Ecology Model Explaining the Large Variability in Size of Salmon Runs Across Gaspé Watersheds, Canada

Regional variability in Atlantic salmon (Salmo salar) riverscapes: a simple landscape ecology model explaining the large variability in size of salmon runs across Gaspé watersheds, Canada

Michel Lapointe  Department of Geography McGill University,

Atlantic salmon (Salmo salar) rivers in the Gaspé Peninsula of Quebec present a 20 to 1 variability in the average numbers of returning adult salmon per km² of watershed area (here the ‘specific run size’). We hypothesize that, for any given watershed size or total stream length, the relative spatial distribution of key, complementary life stage related habitats along  these riverscapes (adult holding pools, spawning beds, rearing habitats for older parr) is an  important factor controlling the production of salmon  and by extension in the numbers of adults returning each year to spawn. Based on observed spatial patterns in  density  of juveniles and in redd distributions along Gaspe rivers, we developed a simple riverine landscape ecology model that uses easily accessible map and airphoto data sources to identify optimally productive river segments based  on large scale river and valley features,  and inferred spatial organization of complementary salmon habitats associated with these distinct valley types. The ability of this model to predict salmon run sizes was tested for 14 watersheds in the Gaspé Peninsula, Quebec. The aggregate length of optimally productive habitat, as defined by our model, is a strong predictor of the average size of the annual salmon runs for these watersheds (R² = 0.922, p < 0.0005). Furthermore, salmon run sizes were accurately predicted even when the effects of scale (watershed area) were removed (R² = 0.750, p < 0.0005). Our model also much more accurately predicted salmon run size than the current reach scale, Atlantic salmon habitat assessment method used by state wildlife agencies; a model that ignores the spatial organization of  these key, complementary habitats.