266 



Fishery Bulletin 102(2) 



Habitat mapping 



The underlying spatial framework for incorporating 

 model predictions into the GIS consisted of six maps: 

 four salinity periods, one bathymetric map, and one 

 map defining bottom-type distribution. All GIS maps 

 were developed in Universal Transverse Mercator 

 projection, UTM, datum-1983, zone-15, using ArcView 

 3.1 (Redlands, CA) software. Each map consisted of 

 10 x 10 m grid cells where each cell contained pertinent 

 salinity, depth, or bottom-type information. 



Salinity maps were developed from depth-aver- 

 aged salinity models by using historical Galveston 

 Bay data collected during 1979-90 (Orlando et al., 

 1993). Four salinity periods were identified to rep- 

 resent typical salinity conditions under average sea- 

 sonal freshwater inflow: low (March-June), increas- 

 ing (July), high (August-October), and decreasing 

 (November-February). Five isohalines were developed to 

 display spatial salinity distribution (Christensen et al. 1 ): 

 0-0.5, 0.51-5, 5.1-15, 15.1-25, and >25 parts per thou- 

 sand (ppt) (Fig. 3). 



Bottom types from the drop sample database were di- 

 vided into three categories: 



Marsh edge (ME) 



Submerged 

 aquatic 

 vegetation (SAV) 



Shallow non- 

 vegetated 



bottom (SNB) 



intertidal marsh within 5 meters of 

 open water habitat. This category 

 consisted primarily of saltmarsh cord 

 grass (Spartina alterniflora), and 

 smaller proportions of salt meadow- 

 grass {Spartina patens), black needle- 

 rush {Juncus roemerianus), salt grass 

 (Distichlis spicata), bullrushes iScir- 

 pus spp.), and cattails (Typha spp.); 



consisted primarily of shoalgrass 

 {Halodule wrightii), wigeongrass 

 iRuppia maritima), and a sporadic 

 distribution of wild celery (Vallisneria 

 americana); 



generally restricted to waters less 

 than 1 meter deep, including creeks, 

 ponds, shoreline, and open bay habitat. 



Density data for other bottom types were limited and were 

 not used in the analysis. 



Wetland maps, used in the creation of the bottom type 

 map in the GIS, were obtained from the U.S. Fish and Wild- 

 life Service's national wetland inventory (NWI). The NWI 

 maps were obtained as vector files, created by digitizing 

 boundaries between wetland types from 1989 aerial photo- 

 graphs and classified by using the classification scheme of 

 Cowardin et al. ( 1979). Regularly flooded emergent vegeta- 

 tion and submerged aquatic vegetation distributions from 



Christensen, J. D., T. A. Battista, M. E. Monaco, and C. J. 

 Klein. 1997. Habitat suitability modeling and GIS technol- 

 ogy to support habitat management: Pensacola Bay, Florida 

 Case Study, 58 p. NOAA/NOS Strategic Environmental 

 Assessments Division, Silver Spring, MD. 



40 50 60 70 

 Total length (mm) 



Figure 2 



Total-length frequency distribution for juvenile brown shrimp 

 captured in drop traps within Galveston Bay (1982-971. 



the NWI maps of Galveston Bay were chosen to represent 

 ME and SAV, respectively, from the drop sample database. 

 Nonvegetated open water areas with depths greater than 

 1 m were eliminated throughout the bay to reflect depth 

 range from the drop sample database. This elimination 

 was done by plotting approximately 400,000 depth sound- 

 ings obtained from the National Geophysical Data Center 

 (NGDC ), and a bathymetric grid map was developed in 1-m 

 contours with ArcView 3.1 (6 nearest neighbors, power=2). 

 The nonvegetated open water map from NWI was overlaid 

 with the bathymetric map and only those areas within the 

 1-m contour were extracted and added to the bottom-type 

 map (Fig. 4). 



Two maps were used to plot (map) seasonal model 

 predictions, bottom type, and the respective salinity 

 period. The salinity maps did not completely correspond 

 temporally with seasons defined by cluster analysis of in 

 situ temperature recordings from the density database. 

 Salinity periods were chosen to correlate with temporal 

 seasons based on maximum monthly overlap to develop the 

 seasonal prediction maps: low salinity (spring); increas- 

 ing salinity (summer); high salinity (fall); and decreasing 

 salinity (winter). 



The total area of Galveston Bay (2020 km 2 ) was deter- 

 mined by combining the total areas for regularly flooded 

 emergent vegetation, irregularly flooded emergent vegeta- 

 tion, SAV, and open water classifications from NWI data. 

 The bottom-type map reflects the study area and totaled 

 565.6 km 2 after excluding all areas >1 m in depth and with 

 irregularly flooded emergent vegetation: SNB = 476.2 km 2 . 

 ME = 84.9 km 2 , and SAV = 4.5 km 2 . Initially, NWI's SAV 

 classification totaled 5.7 km 2 , but the final SAV coverage 

 was reduced to 4.5 km 2 based on SAV mapping by White 

 etal. (1993). 



Regression modeling 



ANOVA and Tukey-Kramer multiple means comparisons 

 were used to determine if mean density varied significantly 

 by bottom type, salinity zone, and season. Multiple regres- 

 sion with significant predictors was used to predict mean 

 log density. The model was then applied to the GIS maps 



