Long-term spatial-temporal eelgrass (Zostera marina) habitat change in the Salish Sea (1932-2016)

Eelgrass (Zostera marina) is a critical nearshore marine habitat for juvenile Pacific salmon (Oncorhynchus spp.) as they depart from their natal streams. Given the poor marine survival of Coho (O. kisutch) and Chinook (O. tshawytscha) salmon juveniles in recent decades, it is hypothesized that deteriorating eelgrass habitats could contribute to their low survival. For three small estuaries in the Southern Gulf Islands of British Columbia, changes in eelgrass area coverage and shape index over the period of 1932-2016 were assessed using historic aerial photographs and Unmanned Aerial Vehicle (UAV) imagery. In addition, changes in eelgrass area and shape index were evaluated in relation to landscape level coastal environmental indicators, namely shoreline activities and alterations and residential housing density. All three eelgrass meadows showed a deteriorating trend in eelgrass condition; on average, eelgrass area coverage decreases by 41% while meadow complexity as indicated by the shape index increases by 76%. Shoreline activities (number of boats, docks, log booms, bulkhead) and residential housing density increase dramatically at all sites over the study period, which are moderately to very strongly correlated to eelgrass area coverage and shape index. Changes in these landscape level indicators over this time period corroborate the observed decline in eelgrass habitat condition as they suggest an overall deterioration of coastal environmental health in the Salish Sea due to increased use of the coastal zone as well as declines in water quality due to urbanization