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Fishery Bulletin 104(2) 



Cross-shelf 



if different habitat types are con- 

 sidered (Ross and Epperly, 1985; 

 Hettler, 1989; Nelson et al., 1991). 

 Thus, unconsolidated sediments 

 are as rich a habitat in terms of the 

 number of species, as are pelagic, 

 rocky reef, and estuarine habitats 

 in the southeast U.S. continental 

 shelf ecosystem. 



Cross-shelf regions were defined 

 on the Georgia shelf from spatial 

 patterns in juvenile fish distribu- 

 tions. The number and extent of 

 cross-shelf regions varied season- 

 ally, but in general, three regions 

 were identified: inner-shelf, mid- 

 shelf, and outer-shelf regions. Ju- 

 venile assemblages were associated 

 with the regions, yet some individ- 

 ual species were distributed across 

 regions. Juveniles of year-round 

 residents (e.g.. Ophidian selenops, 

 Diplectrum formosum. and Pn- 

 onotus carolinus) were usually the 

 most abundant species, although 

 transient juveniles (e.g., Leiostomus 

 xanthiirus, Lagodon rhomboides, 

 and Brevoortia tyrannus) were sea- 

 sonally abundant. Cross-shelf gra- 

 dients in species distribution have 

 been found for other organisms and 

 life stages of fish on the southeast 

 U.S. shelf (macroinfauna: Atkinson 

 et al., 1985; larval fish; Marancik 

 et al, 2005; adult reef fish; Ches- 

 ter et al., 1984; adult demersal 

 fish: Wenner et al.'*'' " "). Cross- 

 shelf gradients also are common 

 in demersal juvenile fish distribu- 

 tions in other continental shelf ecosystems (northwest 

 U.S.: Norcross et al., 1997; Toole et al., 1997; Abookire 

 and Norcross, 1998; Bailey et al., 2003; Johnson et al., 

 2003; northeast U.S.: Steves et al., 2000; Sullivan et 

 al., 2000; southwest U.S.: Johnson et al., 2001). 



The cause of cross-shelf gradients in juvenile fish 

 distribution is difficult to determine. The primary envi- 

 ronmental variable correlated with cross-shelf juvenile 

 fish assemblages is often depth (Table 6; Norcross et 

 al., 1997; Steves et al., 2000; Sullivan et al., 2000; 

 Johnson et al., 2001; Johnson et al., 2003), although 

 temperature, salinity, and sediment grain size also are 

 correlated with depth (Norcross et al., 1997; Steves et 

 al., 2000; Sullivan et al., 2000). On the Georgia shelf, 

 salinity, density, and stratification correlated with the 

 distribution of juvenile assemblages during the spring, 

 fall, and winter (Table 6), and along with temperature 

 (Figs. 2 and 6) likely influenced the distribution of ju- 

 venile fish, but the causative mechanisms remain unre- 

 solved. On the northeast U.S. shelf, cold bottom water 

 left from winter resides on the mid-shelf during summer 



Along-shelf 



CA 1 



CA1 



Figure 7 



Correspondence analysis ordinations (portraying the first and second dimen- 

 sion scores) of the offshore juvenile fish community data showing cross-shelf 

 and along-shelf station groups in fall (A and Bl and winter (C and D). Solid 

 lines enclose the boundary of each station group with three or more stations. 

 Station groups comprising one or two stations are not enclosed by a solid line. 

 Each station group is labeled and portrayed with a different symbol. The 

 dashed lines intersect at the origin of the plot. Analyses were conducted by 

 using juvenile fish abundance data only. 



and fall (cold pool; Ketchum and Corwin, 1964). Steves 

 et al. (2000) and Sullivan et al. (2000) hypothesized 

 that cross-shelf patterns in settlement and juvenile 

 fish distributions were caused by cross-shelf tempera- 

 ture gradients related to the presence of the cold pool 

 on the mid-shelf Similarly, juvenile flatfish species in 

 Alaska exhibited cross-shelf gradients in habitat use 

 that were influenced by temperature-depth interactions 

 (Norcross et al., 1997). Hippoglossoides elassodon were 

 most abundant in the colder, deeper locations, and Hip- 

 poglossus stenolepis were more abundant in the warmer, 

 shallow locations (Norcross et al., 1997). On the Georgia 

 shelf, juvenile assemblages were less distinct during 

 summer, when environmental gradients were weakest, 

 providing some support for the hypothesis that cross- 

 shelf patterns in juvenile distribution were caused by 

 environmental factors. Alternatively, in other studies 

 it has been hypothesized that juvenile distribution re- 

 sults from selection of specific habitat characteristics 

 within large-scale environmental gradients (Stoner 

 and Abookire, 2002). For example, laboratory and field 



