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Open-File Report 03-85

Nearshore Benthic Habitat GIS for the Channel Islands National Marine Sanctuary and Southern California State Fisheries Reserves Volume 1

By Guy R. Cochrane, Nicole M. Nasby, Jane A. Reid, Ben Waltenberger, Kristen M. Lee

GIS Description
  Abstract
  Introduction
  Updates
  File formats
  Acknowledgments
  References
  Contacts

Regional Data
  Basemaps
  Bathymetry
  Navigation
  Samples

Mapped areas
  S. Vandenberg
  N. Anacapa
  S. San Miguel
  Big Sycamore

Data Catalog

Revision History

GIS DATA FOR STUDY AREAS

Sidescan Sonar Images

Sidescan sonar imaging provides high-resolution images of the sea floor by recording the intensity of sound reflected off the sea floor (backscatter). The sidescan data have been processed and mosaicked into continuous images of four study areas. The sidescan GIS data layer is an image built from data with a horizontal resolution of 1 m with grayscale values proportional to the sonar backscatter of the sea floor. Sonar images of the sea floor were produced from 100 KHz sidescan sonar records obtained with a Klein 2000 sonar. The sidescan sonar data were processed following the methodology of Chavez (1984), using the USGS Mini Image Processing System (MIPS).

Diver and Video Observation Data

Point based visual observations of benthic habitat from a variety of scuba and rov dives. Unknown postional accuracy. Some video data mantained by Russ Vetter, national Marine Fisheries Service, Southwest Fisheries Science Center.

Benthic Habitat Polygon Data

In this project, we use textural image analysis to generate a habitat classification of the sea floor, a method which has been used successfully in other sidescan sonar mapping studies (Blondel, 1996). Textural image analysis involves the calculation of image homogeneity and entropy (as defined by Shokr, 1991). Entropy measures the roughness of ascoustic texture, and homogeneity measures the degree of organization of the texture (Blondel and Murton, 1997). These values were computed for 10x10 pixel sized training regions for areas identified in the point coverage of visual observations (Cochrane and Lafferty, 2002). These homogeneity and entropy index values from the training regions were then applied to the entire sidescan image to classify the pixels. The classified image was then converted to an Arcinfo grid and then a polygon coverage with a bottom type attribute as defined by Greene and others (1999) which consists of either h for hard bottom, m for mixed low relief rocky bottom and thin sediment, or s for thick sediment bottom. The polygon coverage was edited to remove areas where noise or other sonar data degradation produced erroneous classification. Polygons were added or edited by hand for areas where the sonar data quality was insufficient to allow textural analysis classification but other information was available. A Megahabitat attribute was added to all the polygons at this point, most polygons are located on the Shelf (S) while a few are located on basin or canyon flanks (F).

The scuba and video observations were combined in an Arcinfo point coverage, which could be overlain on the sonar images to aid in interpretation. The bottom observations were used to select areas of the image that represented rocky sea floor for textural image analysis (discussed above). The observations were also used to identify macrohabitat, the third polygon habitat attribute of Greene and others (1999). The macrohabitat classification could be improved significantly with additional diving. Macrohabitats shown in this report include rock rills (hr), rock pinnacles (hp), rock scarps (hs), rock with kelp (hk), mixed sand and rock areas with sandwaves (mw), and mixed areas forming a scarp (ms).

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South Vandenberg

Habitat in only the southern part of the Vandenberg reserve was mapped with sidescan sonar (Table 1, svanhab.html). Combined the sidescan data and diver observations suggest that the habitat within the reserve is predominantly sandy, with rocky areas restricted to extensions of onshore rock into the subtidal water. Outside the reserve, the habitat consists of some areas of rock in subtidal waters. We also observed carbonate mounds formed during petroleum seepage (code Shm).

Table 1. Distribution of habitat in southern part of Vandenberg reserve. Each polygon code is listed with the type of habitat, area in square meters, and area as a percentage of total mapped area.

CODE HABITAT Sq m %
Sh Shelf hard bottom 750,748 1.079
Shm gas or oil seeps 110,444 0.159
Shr Rock rills 19,857 0.029
Sm Shelf mixed hard and soft bottom 1,349,267 1.939
Ss Shelf soft bottom 67,482,705 96.982
Ssw Sand waves 77,523 0.111

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South San Miguel

Habitat in the South San Miguel study area is rocky (Table 2, smighab.html). Sidescan imaging was impeded in water depths less than 20 m by extensive kelp, which infers large areas of rock in those water depths. Rocky area persists out to the shelf break south of San Miguel Island. Extensive areas of sand waves suggest strong bottom currents are common in this area.

Table 2. Distribution of habitat in southern part of San Miguel. Each polygon code is listed with the type of habitat, area in square meters, and area as a percentage of total mapped area.

CODE HABITAT Sq m %
Sh Shelf hard bottom 3,251,039 6.843
Shk kelp 1,680,022 3.536
Sm Shelf mixed hard and soft bottom 2,114,957 4.452
Ss Self soft bottom 42,144,196 88.706
Ssw Sand waves 3,259,749 6.861

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North Anacapa

Habitat in the North Anacapa study area can be summarized as rocky from the shoreline to approximately 80 m water depth and sandy from 80 to 100 m water depths (Table 3, nanahab.html).

Table 3. Distribution of habitat in northern part of Anacapa. Each polygon code is listed with the type of habitat, area in square meters, and area as a percentage of total mapped area.

CODE HABITAT Sq m %
Sh Shelf hard bottom 1,679,169 3.131
Shr rock rills 32,907 0.061
Shp pinnacle 1,903 0.004
Shs scarps 1,767 0.003
Shk kelp 59,167 0.110
Sm Shelf mixed hard and soft bottom 8,514,794 15.878
Smw Sand waves 626,529 1.168
Sms scarp 187,434 0.350
Ss Shelf soft bottom 43,433,732 80.991
Fm Basin flank mixed hard and soft bottom 991,029 1.848

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Big Sycamore

The Big Sycamore Reserve is composed entirely of sandy habitat (Shk). Expansion of the reserve seaward will not protect any rocky habitat, but expansion to the southeast would (Table 4, bsychab.html).

Table 4. Distribution of habitat in Big Sycamore Reserve. Each polygon code is listed with the type of habitat, area in square meters, and area as a percentage of total mapped area.

CODE HABITAT Sq m %
Sh Shelf hard bottom 540,558 0.973
Shk kelp 409,856 0.738
Sm Shelf mixed hard and soft bottom 635,875 1.145
Ss Shelf soft bottom 51,042,692 91.876
Ssw Sand waves 80,431 0.145
Fm Basin flank mixed hard and soft bottom 99,546 0.179
Fs Basin flank soft bottom 32,377,359 5.827

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