Conserving critical ecosystem linkages: Native fish migrations in Great Lakes watersheds

Migratory animals have a unique ability to transport energy and nutrients across ecosystem boundaries.  Salmon are the most famous example of fish providing material subsidies through their breeding migrations.  Here, we evaluate whether native suckers serve a similar function in Great Lakes watersheds, and outline the major threats to such ecosystem linkages.  We recorded thousands of suckers breeding in small tributaries in northern Lake Michigan, and fishery records and other sources indicate hundreds of thousands of fish using larger rivers.  The sucker migration was closely associated with a pulse of dissolved nutrients, particularly in smaller streams.  These lake-derived nutrients enhanced leaf-litter breakdown and alleviated nitrogen limitation of periphyton production.  Fish migrations and the associated annual input of nutrients are frequently blocked by dams, and fish passage technologies are generally ineffective for suckers.  Road culverts are a common obstacle in smaller tributaries, and recreational and commercial fishing also affect migrations in some cases.  Our results suggest that native fish migrations play an essential role in tributary ecosystem functioning, and that maintaining these lake-stream linkages should be a priority for conservation planning and restoration efforts.