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



123 



Figure 11 



The correlation between environmental variables and station groups portrayed by canoni- 

 cal correspondence analysis (Fig. 10). (A) The proximity of seasonal station groups (black 

 polygons) and taxa (black triangles) when environmental and larval concentration data were 

 analyzed. (B) The relationship between the environmental variables (black arrows) and the 

 seasonal station groups (gray polygons). The direction of the arrows depicts the gradient of 

 each environmental variable. The dashed lines intersect at the origin of the plot. 



region was defined inshore of the 20-m isobath (Figs. 4, 

 5, 12). The inner-shelf larval assemblage was the least 

 diverse taxonomically (Table 2, Fig. 7B), and most taxa 

 in the assemblage were nearshore or estuarine spawning 

 species (e.g., Cynoscion regalis, Menticirrhus americanus. 

 Table 2). Gradients in salinity and density were associ- 

 ated with the separation of the inner-shelf region but 

 the direction of the gradient varied among seasons; in 

 the spring and summer the inner-shelf region was char- 

 acterized by lower salinity and density, whereas in the 

 fall and winter, the inner-shelf region was characterized 

 by higher salinities and densities (Fig. 8). The restricted 

 inshore distribution of the assemblage indicated mecha- 

 nisms of larval retention in the inner-shelf zone. 



The mid-shelf region was defined between the 20- and 

 40-m isobaths (Figs. 4, 5, 12). The mid-shelf larval as- 

 semblage was distributed over the widest area (Figs. 4, 

 5, 12) and species in the assemblage were found in all 

 three regions defined (Fig. 6). The mid-shelf region and 

 larval assemblage were related to the average environ- 

 mental parameters encountered on the shelf (Fig. 8), 

 which varied seasonally. The broad distribution of the 

 assemblage indicated either broad spawning distribu- 

 tions of member species or mechanism of larval trans- 

 port to both the inner- and outer-shelf regions. 



The outer-shelf region was defined as the area off- 

 shore from the 40-m isobath (Figs. 4, 5, 12). The outer- 

 shelf region was related to increased stratification of 

 the water column, which was likely a result of Gulf 

 Stream waters mixing onshore. These periodic intru- 

 sions would help explain the higher species richness of 

 rare taxa found on the outer-shelf during fall and win- 

 ter (Fig. 7B). Taxa in the outer-shelf assemblage were 

 either spawned on the outer-shelf (e.g., Hemanthias 

 vivanus), spawned offshore of the shelf break and trans- 



ported onto the shelf (e.g., Ceratoscopelus maderensis), 

 or spawned south of the study area and transported 

 onto the shelf (e.g., Abudefduf sp.). Most outer-shelf 

 taxa, however, were restricted to outer-shelf stations 

 indicating limited onshore exchange between the outer- 

 and mid-shelf regions. 



Larval assemblages on the continental shelf off the 

 coast of Georgia are derived from a combination of 

 spawning distributions and larval transport; Brevoor- 

 tia tyrannus and Bothus ocellatus I robinsi provide an 

 example. Brevoortia tyrannus spawn in water tempera- 

 tures between 16° and 23°C during winter (Checkley et 

 al. 1999); these temperatures were experienced in the 

 mid- and outer-shelf regions during winter. Bothus ocel- 

 latus/robinsi adults also occur on the mid- and outer- 

 shelf of the continental shelf off the coast of Georgia 

 (Gutherz, 1967). Thus, during winter the spawning 

 distribution of these two species are likely similar. The 

 larval distributions, however, are different: B. tyrannus 

 larvae were collected in all three regions of the shelf 

 during winter, whereas B. ocellatus /robinsi were col- 

 lected on the mid- and outer-shelf (Fig. 14). The verti- 

 cal distributions of the two species also are different. 

 B. tyrannus larvae occur higher in the water column 

 than do B. ocellatus /robsini (Hare and Govoni 1 ). The 

 observed differences in horizontal distribution could 

 result from the differences in vertical distributions. 

 Alternatively, the distributional differences could result 

 from physiological differences that allow B. tyrannus 

 larvae to survive cooler inshore waters or could result 

 from seasonal cross-shelf spawning patterns that result 



1 Hare, J. A., and J. J. Govoni. 2004. In review. Vertical 

 distribution and the outcome of larval fish transport along 

 the southeast US continental shelf during winter. 



