Natural Salmonid Colonization of Streams Following Glacial Retreat; Drivers and Implications for Natural Recovery

Glacial recession in Glacier Bay in south east Alaska has created streams of different ages (35 to 220 years) and stages of development which permits unique insights into the natural colonization of stream habitat by salmonids.  Due to the rapidity of the ice recession, some of the youngest streams were far distant from source populations and the stream with the longest record of development (Wolf Point Creek) was first colonized by pink salmon some 30 years after its formation.  However within a few generations numbers had increased from a few hundred fish to over 10,000.  This first colonization also coincided with a massive run of pinks in southeast Alaska. On the other hand a new stream system formed more recently and closer to source populations was colonized within 10 years of its formation. The first colonizers with resident juvenile stages are typically Dolly Varden as they are more adapted to lack of pool habitat typical of younger streams but coho and sockeye salmon follow soon after, particularly where side channels or kettle lakes remain.  However many of the sockeye populations have been lost with stream development as lakes become detached from main stream channels.  Habitat heterogeneity is highest in the middle aged streams (100 to 150 years) but juvenile coho populations are related to the size, complexity, and orientation of coarse woody debris recruited into the stream system.  Adjacent riparian vegetation must be of a sufficient stage of development (in terms of size and maturity) for the recruitment of woody debris and its subsequent complexity to occur and increases with watershed age and in more dynamic streams where channels shift.  These findings have implications for natural recovery of salmon populations and illustrate that it may take a long time where source populations are distant and where coarse woody debris is not being recruited.  A recent 1 in a 100 year flood in Wolf Point Creek reset stream development and reduced pink spawners to a few hundred fish but numbers have rapidly recovered to pre-disturbance levels.