Environmental Engineering Reference
In-Depth Information
2.2 Landscape characterization
In the second step, performed by analysts different than those whose updated and prepared
the shrimp farm polygons, the coastal landscape of four selected states before the advent of
shrimp farming were characterized by means of thematic maps generated by the
classification of Landsat TM images from 1986 to 1999, downloaded from the USGS Global
Visualization Viewer portal (http://glovis.usgs.gov/). The imagery covering the shrimp
farming area in the states of the Gulf of California comprises 14 Landsat TM images among
paths 30 to 37 and rows 39 to 45. The area of Tamaulipas was covered with three images
recorded in path 26 among rows 41 and 43. All the spectral bands except thermal infrared
were used.
The images underwent unsupervised classification using a K-means clustering technique
(Richards & Jia, 1999). A 16 spectral cluster map was produced first, which was
subsequently associated with natural covers represented by three coastal wetland types
(aquatic surfaces, saltmarsh and mangroves), while other natural vegetation (dry forest,
thorn scrub forest) and vegetation of anthropic origin (agriculture, settlements, lineal
infrastructure) were integrated into a fourth category: terrestrial covers (Fig. 3). Landsat TM
images recorded earlier than 1986 were not available, so in some cases the maps also include
a fifth land cover category corresponding to the shrimp farms present since that time.
2.3 Change detection analysis
In the third step, the changes produced by shrimp farming in the Mexican coastal
landscapes were assessed by overlaying the buffered shrimp farm polygons (t2) on the 1986-
1999 thematic maps (t1) following a post-classificatory analysis scheme (Mas, 1999,
Berlanga-Robles et al. 2010; Berlanga-Robles & Ruiz-Luna, 2011), which outputs a matrix for
change detection, identifying trends and the extent of variations on every cover presumably
produced by shrimp farming (Fig. 3). Considering just the Gulf of California region, a
similar analysis was performed only on mangroves, using a dataset produced with 1973
Landsat MSS images (60 m pixel size) developed in earlier studies (Ruiz-Luna et al., 2010).
3. Results
Based on the photo-interpretation process with Google Earth and ancillary data, a total of
273 polygons were identified, representing isolated farms or systems with more than one
farm amounting to a total of approximately 80,000 ha. All structures identified as shrimp
farms were included, even if the system was empty or out of operation. Sinaloa state has the
largest area allocated for this industry, amounting to 51% of the estimated area, followed by
Sonora, Nayarit and Tamaulipas, with 41, 6 and 1%, respectively (Table 1).
Regarding the transformations due to aquaculture, the main subsidiaries were those that
integrate anthropic and vegetation cover other than that identifiable as wetland, namely,
terrestrial coverage, with 46%, and saltmarsh, with 45%. Approximately 3% of the ponds
were built on the shallow coastal lagoons and estuaries (water surface), and mangrove was
the least modified cover (1%). The change in mangrove cover is estimated to be more than
1150 ha, mainly in Sinaloa (≈ 700 ha) and Nayarit (≈ 400), the states with the largest
mangrove cover in the Mexican Pacific, which account for approximately 70,000 ha each
(Ruiz-Luna et al., 2010). These states are also first in the execution of shrimp aquaculture
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