Ecological Implications of Lotic Fragmentation on Fish Communities in Alberta's Boreal Forest Watersheds

Throughout watersheds in Alberta’s forested landscape, thousands of hanging culverts are acting as barriers to upstream fish movement, thus fragmenting lotic habitat.  This landscape is driven by major environmental processes, such as annual freezing and periodic droughts and fires. Today, it is disturbed by high levels of industrial activity and agricultural expansion.  Fish species native to this dynamic landscape, notably Arctic Grayling and Athabasca Rainbow Trout, typically exist as subpopulations within watersheds, occupying favourable habitats separated by reaches of poor habitat conditions. Fish movement and mortality events occur in response to seasonal and often catastrophic changes in natural habitat conditions. This results in a mosaic of fluctuating subpopulations within larger drainage areas, with varying degrees of connectedness.  Though a dendritic system, this pattern is analogous to island biogeography. Building on this analogy, I apply island biogeography theory to develop a conceptual framework for considering the ecological implications and risks of man-caused watershed fragmentation.  In watersheds increasingly affected by human activities, fragmentation of lotic “islands” into smaller units may reduce the likelihood that sensitive fish populations will persist.  To mitigate this risk and increase population sustainability, triage of remediation efforts should place highest priority on increasing the size of occupied habitat units.  Secondarily, such efforts should aim to improve the connectedness of fish populations within larger watershed units.