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Fishery Bulletin 104(1) 



pink shrimp postlarvae react to changes in salinity by 

 changing swimming direction. Postlarvae were more 

 active in the water column with increases in salinity, 

 and this finding implies a shoreward displacement with 

 the flood tide. Similar responses to salinity changes 

 have been found for postlarvae oi Farfantepenaeus cali- 

 forniensis, Farfantepenaeus brevirostris, Litopenaeus 

 stylirostris, and Litopenaeus vannamei from the Mexi- 

 can Pacific (Mair, 1980). The importance of a salinity 

 cue for transport has been questioned for other penaeid 

 species that inhabit hypersaline estuaries (southeast 

 African, western Australia) in which penaeid postlar- 

 vae would need to move against a salinity gradient 

 (Penn, 1975; Forbes and Benfield, 1986; Rothlisberg 

 et al., 1995). Our simulations of transport guided by 

 salinity changes indicated that planktonic stages could 

 travel distances in the range of only 30 km in 30 days. 

 This result may indicate that salinity is not the only 

 environmental factor controlling long cross-shelf migra- 

 tions of pink shrimp. However, Hughes (1969a, 1969b) 

 in early experiments suggested that a salinity cue 

 could apply to postlarvae near the nursery grounds. 

 Changes in water pressure have been proposed as the 

 only environmental factor that triggers the vertical 

 migration of postlarval shrimps in the Gulf of Car- 

 pentaria, Australia (Penn, 1975; Forbes and Benfield, 

 1986; Rothlisberg et al., 1995). Laboratory experiments 

 and numerical models have shown that tiger shrimps 

 iPenaeus semisulcatus and Penaeus esculentus) larvae 

 switch behavior when the change in water pressure 

 with tides becomes a significant fraction of the total 

 pressure (Rothlisberg et al., 1996; Condie et al., 1999; 

 Vance and Pendrey''^). This behavior only occurred in 

 larvae above a certain size. However, it still remains to 

 be determined whether, in a natural ecological context, 

 the rates of relative changes of pressure are consistent 

 with the tidal cycle periodicity and are detectable at 

 absolute amounts in order to permit a behavioral re- 

 sponse. 



By means of simulations of transport, we have identi- 

 fied a potential STST mechanism for planktonic pink 

 shrimp to migrate the estimated 150 km in 30 days 

 from spawning to nursery grounds over the Florida 

 shelf Organisms inhabiting coastal ecosystems domi- 

 nated by tides have the potential to control their cross- 

 shelf movement through STST (Shanks, 1995). The 

 extent of the transport depends on the speed of the 

 tidal current and the time that organisms spend in the 

 water column. Success in reaching the nursery grounds 

 depends upon the stage in larval or postlarval develop- 



^ Vance D. J., and R. C. Pendrey. 2001. Vertical migration 

 behaviour of postlarval penaeid prawns: a laboratory study 

 of the effect of tide, water depth and day/night. In The 

 definition of effective spawning stocks of commercial tiger 

 prawns in the Northern prawn fishery and king prawns in the 

 eastern king prawn fishery-behaviour of postlarval prawns, 

 p. 28-52. Fisheries Research and Development Corporation 

 (FRDC ) Final Report ( Project 97/108 ). 68 p. CSIRO Marine 

 Research Laboratories, P.O. Box 120, Cleveland, Qld. 4163, 

 Australia. 



ment when the tidal behavior is added. A dependence 

 on tidal currents for the entire larval transport period 

 was postulated for Melicertus latisultacus in Western 

 Australia (Penn, 1975). The shrimp Lucifer faxoni is 

 the only species for which a STST mechanism across 

 the shelf has been shown (Woodmansee, 1966). Strong 

 tidal currents and several coastal sources of fresh wa- 

 ter define the spawning grounds of the wide and shal- 

 low SW Florida shelf (Lee et al., 2001; Jurado, 2003). 

 Under these conditions, parts of the SW Florida shelf 

 may behave as an estuary in which planktonic pink 

 shrimp may easily recognize tides by means of endog- 

 enous behavior or environmental variables. 



From this study we determined that the greatest 

 influx of postlarval pink shrimp occurred at the north- 

 western border of Florida Bay in summer. Postlarvae 

 entering Florida Bay through the channels of the Mid- 

 dle Florida Keys occurred at a much lower magnitude 

 and there were only sporadic peaks and no appar- 

 ent seasonality in influx. The transport mechanism 

 of planktonic stages of pink shrimp across the SW 

 Florida shelf seems to depend heavily on semidiurnal 

 tides and larval behavior, and much less on seasonal 

 winds. The response of postlarvae to the tidal currents 

 was clearly observed at the western margin of Florida 

 Bay. Such behavior needs to be explored in early stages 

 to define the age at which larvae begin to respond to 

 tides, the location on the Florida shelf at which this 

 response occurs, and the specific environmental cues 

 linked to such behavior. With this information, more 

 realistic simulations of transport can be made with a 

 complete hydrodynamic model that would incorporate 

 spatial and vertical variations in currents. Depend- 

 ing on the resulting transport, the location of spawn- 

 ing grounds may be better defined, leading to better 

 protection of this valuable fishery resource. Informa- 

 tion on recruitment variability and key environmen- 

 tal factors affecting larval transport are essential to 

 accurately interpret stock assessments, maintain the 

 ecological integrity of both spawning grounds and 

 nursery grounds, and to effectively manage the pink 

 shrimp fishery. 



Acknowledgments 



We are especially grateful to Thomas Lee and Elizabeth 

 Williams (RSMAS, University of Miami), Elizabeth 

 Johns, Ryan Smith, Shailer Cummings, and Nelson Melo 

 (NOAA/AOML, Miami), and Ned Smith (Harbor Branch 

 Oceanographic Institute) for providing ADCP data and 

 valuable comments; to William Richards (NMFSC/ 

 NOAA Miami) and Robert Cowen (RSMAS, University 

 of Miami) for their constructive comments and support; 

 to Hernando Cardenas (NMFSC) for his assistance 

 sorting plankton samples; to Andre Daniels (USGS, 

 Miami), for his valuable support of fieldwork; and to 

 the personnel involved in collecting and managing data 

 from C-MAN and COMP stations. This research was 

 funded by the NOAA South Florida Ecosystem Resto- 



