appendix b final 2008 biological surveys of los angeles and long ...

8.0 Eelgrass
8.2.2 Side-Scan Sonar
The acoustic surveys involved integration of dGPS and side-scan sonar, with the surveys
focused on areas that were less than -20 feet (ft) MLLW in depth and which had previously been
documented as supporting eelgrass (MEC 2002). TDI analyzed digitally collected data using an
Edge Tech 272-TD side-scan sonar deployed from a 13-ft Boston Whaler. Side-scan sonar
data were collected at 500 kHz with approximately 60% overlap between transects (~20 m
apart) to capture all notable features. Survey operations were performed as close to shore as
possible. Navigation was maintained through an onboard sonar computer as well as a
redundant navigation laptop on board the survey vessel. Edge Tech Discover 560 A/D software
was used to log the raw data directly from the side-scan sonar to a lap top computer for postprocessing
rectification and referencing. Concurrently, sonar signal imagery was collected and
processed using Chesapeake SonarWiz Map software in conjunction with a Trimble AG 122
DGPS to provide a real-time, geo-referenced mosaic of the bathymetric topography. Differential
correction was provided using the Coast Guard COORS DGPS signal.
Relative densities (high or low) for eelgrass communities were determined from side scan sonar
imagery in conjunction with diver observations (see below) made during turion density counts.
Density classifications were determined based on similar side scan sonar imagery within
individual areas. Since eelgrass communities are not distributed homogeneously throughout
their range, diver transects and turion counts provided the most accurate evaluation of density.
Diver transects and turion counts were performed three times more frequently during the 2008
evaluation than in the previous evaluation and provided in depth insight to eelgrass
communities.
8.2.3 Diver Surveys
Divers verified side-scan sonar acoustic data records
of identified eelgrass beds at multiple locations. They
made observations to characterize the health and
vigor, epiphytic load, and associated invertebrate and
fish species within individual eelgrass patches.
Methods included the use of 1/16 m 2 quadrats to
estimate eelgrass turion density at multiple locations
and to determine the extent and consistency of various
eelgrass beds.
After reviewing the side-scan sonar acoustic data
records from subsequent mapping efforts, divers
selected several locations within the most prominent
eelgrass beds as targets to collect density data using linear transects along specific bathymetric
contours. They deployed a small weighted buoy at each selected location and recorded dGPS
coordinates and then conducted the survey using a 50 m tape and 1/16 m 2 quadrats, collecting
density data within unique quadrats every 2 meters along the tape. A total of twenty-five 1/16
m 2 quadrat density counts were performed along each transect at each location.
During the spring surveys, divers completed 200 quadrats at eight separate locations. During
the fall, they expanded surveys to 300 quadrats at twelve separate, adding four new locations to
the eight from the spring surveys. Diver transects were increased during the spring survey
effort to increase the sample size and ensure that observed turions densities were
representative of identified eelgrass beds. Diver transects performed in the fall used the same
methodology as during the spring, but expanded sampling along a bathymetric gradient in both
directions from a single point rather than just one direction. Divers performed observations
2008 Biological Surveys of Los Angeles and Long Beach Harbors 8–3
April 2010