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



121 



Relation between seasonal larval assemblages and 

 environmental variables 



The seasonal pattern in the larval concentration data 

 described above was maintained when constrained by 

 environmental variables in the CCA. The community 

 data clearly showed a seasonal influence on the first 

 dimension in ordination space; winter taxa were sepa- 

 rate from taxa collected during the rest of the seasons. 

 This seasonal pattern was also reflected in the environ- 

 mental data (Fig. 11). Salinity, density, temperature, 

 depth, and stratification of the water column were again 

 the most significant environmental variables for explain- 

 ing variance in the species data (P<0.05, Monte Carlo 

 permutation test, Table 6). The warm weather stations 

 and taxa coincided with higher water temperature, 

 lower density, and a lower density gradient. In addition, 

 the cross-shelf pattern evident in the second and third 

 dimensions of the full larval concentration data (Fig. 10, 

 A and B) appeared to correlate with depth of the water 

 column, the degree of stratification in the water column, 

 and salinity (Fig. 11). 



Implications for larval transport 



The structure of larval assemblages was linked to water 

 mass distributions and the cross-shelf zonation of physi- 

 cal circulation processes. Three cross-shelf zones of 

 physical dynamics have been defined previously (Atkin- 

 son and Menzel, 1985; Pietrafesa et al., 1985a, 1985b; 

 Lee et al., 1991; Boicourt et al., 1998). Three analogous 

 cross-shelf zones were delineated in the larval com- 

 munity data. The cross-shelf larval assemblages were 

 linked to three water masses with cross-shelf structure, 

 and to the physical-chemical characteristics of the region 

 (temperature, salinity, density, and stratification of the 

 water column). The three cross-shelf zones identified pre- 

 viously in terms of physical dynamics coincided with the 

 station groups and larval assemblages identified in our 

 study. Thus, larval distribution and physical properties 

 of the ocean are linked and indicate a strong influence 

 of physical properties and processes on the distribution 

 of larval fish on the southeast United States continental 

 shelf. 



Retention on the inner-shelf was a clear larval trans- 

 port pattern identified in the analyses. Menticirrhus 

 americanus represents the inner-shelf group (Table 5) 

 and were always found inshore of the 20-m isobath in 

 inner-shelf water, in inner-shelf-mid-shelf mixed water, 

 or in mid-shelf water, (Fig. 12). Spawning likely occurs 

 on the inner-shelf (Cowan and Shaw, 1988), and larvae 

 are retained in the inner-shelf region. 



The analyses also demonstrated that transport from 

 offshore onto the shelf is limited on the continental 

 shelf off the coast of Georgia. Ceratoscopelus maderensis 

 and Auxis rochei were found only at offshore stations 

 (Fig. 13), representing the outer-shelf group (Table 5) 

 and the mid-shelf-Gulf Stream mixed water mass. The 

 presence of C. maderensis identified transport of a me- 

 sopelagic fish to waters inshore of the shelf break; how- 



_M3GS 



MSGS 



ISMS 



D 



ISMS 



'■1 -■■'•.- 



CA1 

 Figure 9 



Correspondence analysis (CA) ordinations (portraying the first 

 and second dimension scores) of the larval fish community 

 data showing the full ten-percent data set: (A) spring, iBi 

 summer, (C) fall, and (D) winter. The points represent stations 

 classified by water mass. 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. Stations with inner-shelf water are labeled with IS 

 (inner-shelf), inner-shelf-mid-shelf mixed water with ISMS, 

 mid-shelf water with MS, and mid-shelf-Gulf Stream mixed 

 water with MSGS. The dashed lines intersect at the origin of 

 the plot. Analyses were conducted using larval data only. 



