The second data set consisted of still images collected at marked
index sites. For each site, overlapping geographically referenced
images were stitched together into tiled-image mosaics using
Autopano Giga 3.0.8 (Kolor) with a combination of automatic and
manual selection of control points for pattern recognition. Mosaics
were sometimes considerably distorted by small differences in
camera angle, target direction, and sponge growth patterns. The
quality of mosaics representation of index sites was assessed
visually by watching corresponding video; the degree of image
distortion was assessed by measuring objects of known size and
shape (e.g., cable and trawl float markers) in source image and inmosaics. As a result, full series of mosaics of acceptable quality were
obtained for two cable and two control index sites located on Cable
3 and Control 2 transects, respectively (see example on Fig. 2; note
some image distortion). Each mosaic was cropped to the largest
area captured consistently across all eight consecutive surveys
(mean area: 2.6 m2, mean width perpendicular to the cable: 1.5 m).
Live sponge area was manually digitized in ArcGIS 10 (ESRI Inc.),
converted to raster format, and processed in Idrisi Selva 17 (Clark
Labs, Clark University, USA) to calculate live sponge area. Mean
percent changes in live sponge area were plotted for cable and
control transects to assess sponge growth and mortality over time.
The effects of cables on live sponge area were tested using an
ANCOVA. To meet the assumption of linearity, the data set was split
into two segments, October 2008 to February 2010 and October
2010 to February 2012; for each, initial live sponge area values (for
October 2008 and October 2010, respectively) were used as
covariates.