Flora Bank Eelgrass Survey


Skeena River Estuary

The Skeena River originates high in the coastal mountains of northwestern British Columbia, at the edge of the Spatsizi Plateau, and flows 610 km to reach the Pacific Ocean. Draining a total area of 54,400 km2, the Skeena is the second largest river in the province, and one of the longest un-dammed rivers in the world.

The Skeena River estuary is a unique system in that it does not have a single distinct intertidal delta typical of most estuary systems. Instead, deposited sediments form a region of extensive mudflats and shallow, intertidal passages around DeHorsey Island, through Inverness Passage, and between Kitson Island and Lelu Island.

These mudflats and intertidal areas have been identified by the North Coast Wetlands Program as important migratory/wintering waterfowl habitat. Several rare species, including the red-listed western grebe and the blue-listed trumpeter swan, brant, oldsquaw and great blue heron have all been recorded in the wetlands. A Department of Fisheries and Oceans fisheries habitat study identified Inverness Passage, Flora Bank, and DeHorsey Passage, in that order, as critical habitats for Skeena River juvenile salmon, as well as important eulachon habitat.

While Flora Bank is recognized as one of the largest eelgrass beds in British Columbia and a region of high habitat value, relatively few studies have been done on the nature and extent of the eelgrass in this area. During August, 1997, a Compact Airborne Spectrographic Imager (CASI) survey of Prince Rupert Harbour and vicinity was carried out (see link). Mapped habitats included kelp and eelgrass beds, sandflats, and intertidal vegetation. The amount of eelgrass present on Flora Bank during 1997 as estimated from the CASI study was approximately 0.80 km2. Note that almost all of the reported eelgrass was located in the intertidal zone.

Flora Bank Eelgrass Study

In 2008, WWF had discussions with Ocean Ecology regarding the use of a towed video camera system to observe eelgrass on Flora Bank. In particular, since subtidal eelgrass occurs commonly in the North Coast area, and since the CASI study was limited to eelgrass in the intertidal region, there was a keen interest to see if the Flora Bank eelgrass bed extended any significant distance subtidally. After several attempts in 2008 to collect video from the site, a successful set of video data was collected during May, 2009.

Click here to view the Flora Bank GIS project (to view the project in Internet Explorer, download the Adobe SVG Viewer 3.03 and add "oceanecology.ca" to Internet Explorer's "Compatability View").

See the link for the report describing the survey methodology and the results of the Flora Bank eelgrass survey.

The following conclusions and recommendations were made based on the results of the survey:

  1. Some general observations about the eelgrass survey:
    • Approximately 97% of the observed eelgrass was intertidal, and appeared to be Zostera marina typica.
    • Approximately 96% of the observed eelgrass was either within, or in very close proximity to, those areas where the 1997 Borstad CASI survey indicated eelgrass to be present.
    • The fact that there was very little eelgrass observed in areas at a distance from the previously identified beds seems to suggest that the eelgrass has not been actively expanding since 1997.
  2. Navigational hazards:
    • Conclusion: shallow water depths, strong tidal currents, high turbidity, and large woody debris made conditions for towed video work difficult.
    • Recommendation: based on the intertidal nature of the Flora Bank eelgrass bed, and the significant navigational hazards associated with Flora Bank, it is suggested that future surveys of the eelgrass bed be undertaken at low tide using light-weight, highly mobile craft, such as kayaks, which can be carried along the bed as the survey progresses, thus reducing the risk of stranding. Utilizing experienced paddlers would also be highly recommended.
  3. Distribution of Eelgrass on Flora Bank:
    • Conclusion: given the high turbidity of the site, eelgrass growing in the subtidal environment is probably light limited. Thus, the Flora Bank eelgrass bed is most likely limited to only those regions where the depth is shallow enough to allow good light penetration.
    • Recommendation: since the Skeena River plume plays an important role in controlling the growth of eelgrass on Flora Bank through changes in turbidity, further studies on the relationship between the volume, timing, and sediment load of the Skeena River freshet and the growth of eelgrass on Flora Bank should be undertaken, particularly in light of possible changes in the river's seasonal patterns as a result of global climate change.
  4. New Approaches for Monitoring Eelgrass:
    • Conclusion: eelgrass was successfully visualized using the Humminbird sidescan sonar. The image quality of the sidescan data produced by this unit was comparable with that of images produced by more expensive systems. This may allow small organizations with limited funding to be able to collect high quality sidescan data.
    • Conclusion: the use of the downward-looking sonar to quantify eelgrass height may prove valuable in deeper water habitats.
    • Recommendation: based on the results obtained so far with the Humminbird sidescan unit, it is recommended that further experimental trials be carried out on subtidal eelgrass beds.

Shown below is a screen capture of raw sidescan data using the HumViewer software.

Several directions for future research on eelgrass in the Northcoast area were suggested during a presentation of the Flora Bank study to WWF (see link):

  1. Further studies on the relationship between the volume, timing, and sediment load of the Skeena River flows and the growth of eelgrass on Flora Bank
    • see our link on hydrological modelling
    • see our link on ocean circulation modelling
  2. Further studies on the use of side scan sonar for mapping of subtidal eelgrass beds
    • see the link for a report on the use of side scan sonar during an eelgrass inventory in Snohomish County, Washington
  3. New studies on eelgrass beds with less "stress" as baseline examples of what a "healthy" Northcoast eelgrass bed should look like
  4. New studies on spatial modeling of eelgrass habitat
  5. New studies on conservation strategies for eelgrass beds using modeling software
    • see our link on conservation planning