Maranak et al.: Fish assemblages on the southeast United States continental shelf 



113 



for managing reef fish on the southeast United States 

 continental shelf (see Powell and Robbins, 1994; 1998). 

 Serraninae were also included because the majority of 

 these larvae are likely one type: Serranus subligarius. 

 In contrast, larvae identified to some genera were ex- 

 cluded because there are multiple species common in 

 the area within each genus, and each species likely 

 has different larval distributions: Etropus spp. (3 spe- 

 cies), Prionotus spp. (14 species), Sphoeroides spp. (11 

 species), Symphurus spp., (22 species), and Syngnathus 

 spp. (10 species). In summary, 86 taxa were included in 

 the one percent data set, and 16 taxa were included in 

 the ten percent data set (Table 2). 



Preparation of environmental data 



Season, water mass, and eight environmental variables 

 (mostly derived from temperature and salinity data) 

 were chosen in an attempt to explain variation in the 

 ichthyoplankton data (Table 3). For subsequent use in 

 multivariate analyses, all environmental variables were 

 standardized to a mean of zero and a standard devia- 

 tion of one. 



CTD data were processed with the manufacturer's 

 software (Seasave vers. 5.3, Seabird Electronics, Inc., 

 Bellevue, WA) and averaged into 0.5-m bins. Two pa- 

 rameters were derived to describe each hydrographic 

 variable (salinity, temperature, density): an average 

 value through the entire water column and a horizontal 

 gradient value (calculated as the difference in value 

 between the two adjacent stations). Vertical stratifica- 

 tion was estimated by using Simpson's stratification 

 parameter (Simpson and James, 1986): 



<t> = l/h j (p-p)gzdz, 



where /) = water column depth; 



7? = average water column density; 

 p = water density; 

 g = acceleration due to gravity; and 

 z = depth. 



The stratification parameter, <f>(jowles/m 3 ), is a measure 

 of the resistance of water to mixing; higher numbers 

 signify higher resistance to mixing. 



Temperature and salinity data were further used to 

 define water masses on the continental shelf off the 

 coast of Georgia. Pietrafesa et al. (1994) defined four wa- 

 ter masses on the southeast U.S. continental shelf: Geor- 

 gia Bight Water, Carolina Capes Water, Virginia Coastal 

 Water, and Gulf Stream Water. However, temperature 

 data collected on the continental shelf off the coast of 

 Georgia exhibited greater seasonal variability (10-29°C) 

 than reported by Pietrafesa et al. (1994; 14-29°C). As 

 a result, water mass definitions for our study, although 

 based largely on the definitions of Pietrafesa et al. 

 (1994), reflect the greater range of temperature and 

 reflect the natural breaks in temperature, salinity, and 

 stratification data. Specifically, two water masses (inner- 

 shelf water and mid-shelf water) and two mixes (inner- 

 shelf-mid-shelf mixed water and mid-shelf-Gulf Stream 

 mixed water) were defined (Fig. 2). Inner-shelf water was 

 characterized by salinities <35 ppt and seasonally vari- 

 able temperatures. This water mass was found during 

 winter and spring and was distributed inside the 20-m 

 isobath (Fig. 3). Mid-shelf water, with salinities >36 

 (Fig. 2), was typically well mixed vertically (Simpson's 

 stratification parameter value <10). Mid-shelf water 

 was found year round over large sections of the shelf, 

 particularly in the fall (Fig. 3). A mixture between in- 

 ner-shelf and mid-shelf water was defined with salinities 

 between 35 and 36 (Fig. 2). A mixture was also defined 



