Criales et al Variability in supply and cross shelf transport of Farfanlepenaeus duorarum postlarvae into Florida Bay 



61 



ing is northeast of the Dry Tortugas. If gravid females 

 spawn northeast of the Dry Tortugas, larvae need to 

 travel up to 150 km to reach the main nursery ground 

 in western Florida Bay. Females spawn on the continen- 

 tal shelf at about 30 m of depth, where larvae develop, 

 passing through several changes in feeding habitats, 

 behavior, and physical stages (nauplii, zoeae, myses) 

 (Dobkin, 1961; Ewald, 1965; Jones et al, 1970). Postlar- 

 vae undergo between three and eight additional plank- 

 tonic stages before settlement. Larvae develop rapidly, 

 needing only about 30 days to become postlarvae ready 

 to settle to the bottom (Ewald, 1965; Dobkin, 1961). 

 Planktonic stages (larvae and postlarvae) approach 

 the coast and postlarvae enter the nursery grounds of 

 Florida Bay at about 9-10 mm total length (Tabb et al., 

 1962; Allen et al., 1980; Criales et al., 2000). Larval 

 development and ocean hydrodynamics must be tightly 

 linked to successfully bring these planktonic stages to 

 their coastal nursery grounds. 



Mechanisms of transport used by planktonic stages 

 of penaeid shrimps are highly variable, depending on 

 the species, different environmental conditions, oceanic 

 physical processes, and complexity of larval behaviors 

 (e.g., Dall, 1990; Rothlisberg et al., 1995, 1996; Wenner 

 et al., 2005). Physical oceanographic processes signifi- 

 cantly affect the transport of planktonic stages from 

 spawning to nursery grounds (Yeung and Lee, 2002; 

 Criales et al., 2003). Two main immigration routes have 

 been hypothesized for pink shrimp postlarvae entering 

 Florida Bay: 1) postlarvae may drift south-southeast 

 downstream with the Florida Current and enter Florida 

 Bay through the tidal channels of the Lower and Middle 

 Florida Keys (Rehrer et al., 1967; Munro et al., 1968), 

 and 2) postlarvae may move northeast across the SW 

 Florida shelf and enter the Bay at its northwestern 

 boundary (Jones et al., 1970; Criales and Lee, 1995). 

 The most widely recognized pathway for postlarvae 

 to reach Florida Bay up-to-now has been by transport 

 up the Atlantic side through the tidal channels of the 

 Middle Florida Keys (Munro et al., 1968; Criales and 

 McGowan, 1994; Criales et al., 2003). The favorable 

 Ekman transport generated by the southeastern winds 

 along the west-east oriented coast, and coastal counter- 

 current flow generated by cyclonic eddies provide favor- 

 able onshore transport mechanisms along the Florida 

 Keys coast (Criales and Lee, 1995; Lee and Williams, 

 1999). In contrast, larval transport across the broad, 

 shallow SW Florida Shelf has not been well studied and 

 questions exist about the feasibility of this pathway. 

 Subtidal frequency flows are weak in the SW Florida 

 shelf and mainly in the alongshore (north-south) direc- 

 tion as a direct response to wind events (Koczy et al., 

 1960; Weisberg et al., 1996; Lee et al., 2001). Tidal 

 currents are strong mainly in the cross-shelf direction 

 (Wang, 1998; Smith, 2000). Freshwater discharges from 

 the Everglades affect a broad area of the SW Florida 

 shelf (Lee et al., 2001; Jurado, 2003). Isopleths less 

 than 32 are typically confined to the region between 

 Cape Sable and Cape Romano, and from 32 to 36 extend 

 from near Cape Romano to the vicinity of Dry Tortugas 



in a highly variable annual pattern (Lee et al., 2001; 

 Johns and Szymanski'). 



For tropical penaeid shrimps that undergo larval 

 development offshore, but whose nursery grounds are 

 inshore, migratory behavior is a key factor for their 

 advection to nursery grounds (Dall et al., 1990; Shanks, 

 1995). The simplest migratory behavior is vertical 

 movement, and three types of vertical migrations are 

 known to mediate horizontal transport of larvae: on- 

 togenic, diel, and tidal (for reviews see Sponaugle et 

 al., 2002). For some Australian penaeid species (ba- 

 nana prawn [Fenneropenaeus merguiensis], grooved 

 tiger prawn [Penaeus seinisulcatus], and eastern king 

 prawn [Melicertus plebejus]) it has been shown that 

 early planktonic stages (protozoeae and myses) perform 

 diel vertical migration cued by light and that later in 

 development (as postlarvae) the migration is cued by 

 tides (Rothlisberg, 1982; Rothlisberg et al., 1983, 1995) 

 and there is no cross-shelf displacement of larvae dur- 

 ing the 15 days of diel behavior. Previous studies of 

 pink shrimp in South Florida have clearly indicated 

 ontogenic behavior for pink shrimp; postlarvae have a 

 higher degree of mobility than earlier protozoeae and 

 myses (Temple and Fischer, 1965; Eldred et al., 1965; 

 Jones et al., 1970; Criales and Lee, 1995). On the other 

 hand, diel behavior is not so well determined. Although 

 protozoeae, myses and postlarvae were more abundant 

 at the surface during the night than during the day, 

 day and night differences have not been statistically 

 significant in any of these previous studies. The effect 

 of diel, tidal, or ontogenic combinations of behavior 

 on cross-shelf transport from South Florida spawning 

 grounds to western Florida Bay nursery grounds has 

 not previously been explored. The postlarvae of many 

 penaeid shrimps, including pink shrimp, are known to 

 synchronize vertical migration with the tides at the 

 entrance to estuarine nursery grounds (for reviews see 

 Garcia and Le Reste 1981, Dall et al., 1990). This pro- 

 cess is known as selective tidal stream transport (STST) 

 (Forward and Tankersley, 2001). Penaeid postlarvae as- 

 cend in the water column during the flood and sit on the 

 bottom during the ebb to maximize up-estuary move- 

 ment (e.g., Rothlisberg et al., 1995). This behavior has 

 been shown for pink shrimp postlarvae inside Florida 

 Bay (Tabb et al., 1962; Roessler and Rehrer, 1971), but 

 not along the border of the bay with the Gulf of Mexico. 

 When during the life cycle and where on the shelf this 

 tidal behavior begins and what the environmental cues 

 are — these questions remain unanswered. 



The purpose of our research was 1) to determine pat- 

 terns of supply of pink shrimp postlarvae into Florida 

 Bay through two distinct regions, 2) to define the most 

 important transport route for planktonic stages from 

 the Dry Tortugas into Florida Bay, 3) to examine al- 

 ternative behavioral responses of larvae and postlar- 

 vae, and 4) to propose a recruitment mechanism for 



' Johns, E., and D. Szymanski. 2003. Mixing it up in Florida 

 Bay. Florida Bay News, summer 2003:1-3. 



