WEINSTEIN ET AL.: RETENTION OF THREE TAXA OF POSTLARVAL FISHES 



most species (including those studied here) in day- 

 light entrainment collections at a power plant lo- 

 cated on the river near Southport (Copeland et al. 

 see footnote 8) imply that this hypothesis is not 

 tenable and support the contention that diel mi- 

 grations actually do take place. If larvae were 

 similarly stratified in the water column with re- 

 spect to tide, a result paralleling that of the photo- 

 period response would be expected. By resting on 

 the substratum during ebb, or at least by moving 

 downward in the water column (below the level of 

 no net motion), larvae would be transported in the 

 landward direction. 



Behavioral responses of spot, Atlantic croaker, 

 and flounders to photoperiod and tide are sum- 

 marized in Figure 8. Several important differ- 

 ences delineate ultimate habitat utilization by 

 these species. Flounders apparently reacted to 

 tidal flows by settling to the bottom, as has been 

 suggested for oysters and shrimps (Carriker 1951; 

 Hughes 1969a, b, 1972). When the lack of sig- 



nificant tide by depth interaction and the presence 

 of a tidal main effect are considered together, this 

 hypothesis seems more tenable. To a degree, seek- 

 ing boundary layers may be a general tidal re- 

 sponse exhibited by all three species, making 

 them difBcult to sample on ebb (especially since 

 the bottom nets were set 1 m above the substrate). 

 The ability of flounders to effectively penetrate 

 freshwaters also is enhanced by this behavior. 

 Tidal flows above the salt boundary are substan- 

 tial, certainly greater than the ability of flounder 

 postlarvae to negotiate them directly. Saltatory 

 movement upriver by "riding out" ebb on the bot- 

 tom and responding to currents on flood would 

 then be a primary mechanism for continued up- 

 stream migration. 



Both spot and flounders were also observed to 

 migrate toward the surface at night; significantly 

 larger numbers of individuals were captured in 

 this stratum both in midchannel and on the 

 shoals, while Atlantic croaker tended to remain 



DAY 



SPOT - PARALICHTHYS SPR 



NIGHT 



ATLANTIC CROAKER 

 DAY NIGHT 



SURFACE 



• '••••••'•• 



BOTTOM ♦••»«'.'»•.•«• 



• • . •• 



OR 



• • • 



• . • . • 



SURFACE 



NET NON-TIDAL FLOW 

 (UPPER LAYER) 



A and/or B 



BOTTOM 



DOWNSTREAM 



UPSTREAM NET NON-TIDAL FLOW DOWNSTREAM 



(LOWER LAYER) 



UPSTREAM 



A - TIDAL RESPONSE (MOVEMENT TOWARD BOTTOM ON EBB) 



A' - TIDAL RESPONSE (MOVEMENT TOWARD SURFACE ON FLOOD) 



B - PHOTOPERIOD RESPONSE (BOTTOM ORIENTATION DURING DAY) 



B' - PHOTOPERIOD RESPONSE (SURFACE ORIENTATION AT NIGHT) 



Figure 8. — Conceptual model for a larval retention mechanism based on response to photoperiod and tide. Details of the response differ 



for the three taxa ( see text). 



431 



