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Comparison of average larval fish 



vertical distributions among species 



exhibiting different transport pathways 



on the southeast United States continental shelf 



Jonathan A. Hare 



John J. Govoni 



Center for Coastal Fisheries and Habitat Research 



101 Pivers Island Road 



Beaufort. North Carolina 28516 



Present address (for J A. Hare): Narragansett Laboratory 



Northeast Fisheries Science Center 



28 Tarzwell Drive 



Narragansett, Rhode Island 02882 

 E-mail address (for J A Hare) ion hareia'noaa.gov 



Water currents are vertically struc- 

 tured in many marine systems and 

 as a result, vertical movements by 

 fish larvae and zooplankton affect 

 horizontal transport (Power. 1984). 

 In estuaries, the vertical movements 

 of larvae with tidal periods can result 

 in their retention or ingress (Fortier 

 and Leggett, 1983; Rijnsdorp et al., 

 1985; Cronin and Forward. 1986; For- 

 ward et al., 1999). On the continental 

 shelf, the vertical movements of organ- 

 isms interact daily and ontogeneti- 

 cally with depth-varying currents to 

 affect horizontal transport (Pillar et 

 al., 1989; Barange and Pillar, 1992; 

 Cowen et al., 1993, 2000; Batchelder 

 et al., 2002). 



A suite of fish species, which use 

 estuaries during the juvenile stage, 

 spawn during winter on the mid- and 

 outer continental shelf of the south- 

 east United States (Fig. 1A): Brevoor- 

 tia tyrannus (Atlantic menhaden), 

 Leiostomus xanthurus (spot), Micropo- 

 gonias undulatus (Atlantic croaker), 

 Paralichthys albiguta (Gulf flounder), 

 P. dentatus (summer flounder), and P. 

 lethostigma (southern flounder). Ver- 

 tically structured flow is a major part 

 of proposed larval transport mecha- 

 nisms for these species from offshore 

 spawning areas to estuarine nurs- 

 ery habitats (Govoni and Pietrafesa, 

 1994; Hare et al., 1999). Brevoortia 

 tyrannus, however, is found higher 

 in the water column on average than 

 the other species that use estuaries 



during their juvenile stage (Miller 

 et al., 1984; Govoni and Pietrafesa, 

 1994; Govoni and Hoss, 2001). Fur- 

 ther, larvae of B. tyrannus apparent- 

 ly exhibit a difference in horizontal 

 transport compared to other winter- 

 spawning species that use estuarine 

 habitats as juveniles; B. tyrannus lar- 

 vae spawned on the southeast U.S. 

 shelf may be transported to estuarine 

 nursery habitats along the northeast 

 U.S. shelf (Warlen et al., 2002). The 

 effects of differences in vertical lar- 

 val distribution on cross-shelf lar- 

 val transport are unknown, and the 

 transport pathways from shelf spawn- 

 ing areas to estuarine nursery areas 

 remain unclear. 



Other species also spawn during 

 winter on the southeast United States 

 continental shelf. Some species settle 

 to benthic habitats on the shelf (e.g., 

 Etr'opus cyclosquamus [shelf floun- 

 der], E. microstomus [smallmouth 

 flounder], and E. rimosus [grayfloun- 

 der], Leslie and Stewart, 1986) or re- 

 main on the shelf in pelagic habitats 

 (e.g., Etrumeus teres [round herring], 

 Crawford, 1981; Schwartz, 1989). 

 However, some species are regularly 

 advected offshore, entrained into the 

 Gulf Stream, and exported north- 

 wards (e.g., Bothus spp. [peacock, 

 eyed, and spotted flounders], Pepri- 

 lus triacanthus [butterfish], Syacium 

 papillosum [dusky flounder], Xyrich- 

 tys novacula [pearly razorfish]; Hare 

 and Cowen, 1991; Cowen et al., 1993; 



Rotunno and Cowen, 1997; Grothues 

 and Cowen, 1999). 



The purpose of our study was to ex- 

 amine associations between average 

 larval fish vertical distributions and 

 general larval transport pathways 

 on the southeast United States conti- 

 nental shelf during winter. Our goal 

 was to determine if larval vertical 

 distributions differed among species 

 that exhibit different outcomes of lar- 

 val transport: export from the local 

 shelf, arrival at local estuaries, and 

 retention on the shelf. Our approach, 

 however, was unconventional. Rather 

 than couple detailed descriptions of 

 the flow field with detailed describi- 

 tions of larval vertical distributions 

 (including diel variation), we chose to 

 compare average vertical distributions 

 among species that exhibit overall 

 differences in larval transport. Verti- 

 cal distribution data were collected in 

 three separate years, over periods of 

 time ranging from 24 to 96 hours. If 

 average larval vertical distributions 

 are different among species, and these 

 differences occur consistently among 

 the various sampling times and in 

 concordance with the general outcome 

 of transport, then we conclude that 

 larval vertical distributions are an 

 important part of larval transport on 

 the southeast U.S. shelf. 



Our specific objectives were two- 

 fold: 1) to test the null hypothesis 

 that there are no differences in lar- 

 val fish vertical distributions between 

 species, and 2) to evaluate significant 

 differences in larval depth distribu- 

 tion in relation to the a priori clas- 

 sification of the outcome of transport. 

 Vertically discrete data from six sam- 

 pling times were analyzed, and ow- 

 ing to differences in protocols among 

 sampling times, comparisons of lar- 

 val vertical distributions were made 

 within sampling times only. The re- 

 sults of these comparisons were then 

 combined to evaluate whether there 

 were consistent differences in larval 

 vertical distributions among sampling 

 times related to the outcome of larval 

 transport. 



Manuscript submitted 5 April 2004 

 to the Scientific Editor's Office. 



Manuscript approved 30 March 2005 

 by the Scientific Editor. 



Fish. Bull 103:728-736(2005). 



