Dynamics of Pink Shrimp Movements, Growth, and Survival in South Florida

pink shrimp
Pink Shrimp
(Farfantepenaeus duorarum)
Photo Credit: Tom Jackson, SEFSC

Maria Criales
Maria Criales
Photo Credit: SEFSC

pink shrimp growth survival experiment setup
Pink Shrimp Growth and Survival Experiment Setup
Photo Credit: SEFSC

Pink shrimp (Farfantepenaeus duorarum) spawn on the outer continental shelf near the Dry Tortugas and migrate as postlarvae to nursery grounds in Florida Bay and other lower southwest Florida coast estuaries. After spending a few months on the nursery grounds to grow from juveniles to adults, they migrate back to the spawning grounds to produce a new generation of shrimp and support a multimillion dollar fishing industry.

The South Florida pink shrimp population and Tortugas pink shrimp fishery have historically been research topics at the Miami Laboratory of the SEFSC in partnership with the Marine Research Lab of the University of Miami. For example, Tabb et al. (1962) pointed out that northwestern Florida Bay and the nearby connected mangrove estuaries of Coot Bay and Whitewater Bay were important pink shrimp nursery grounds. A tagging study by Costello et al. (1966) indicated that, after the juvenile stage, pink shrimp migrated from Florida Bay to the Tortugas grounds. Munroe et al. (1968) and Jones et al. (1970) found pink shrimp postlarvae on the southwest shelf between the Tortugas grounds and the bay.

A series of studies began in 1996 to better define the ecology of pink shrimp (Farfantepenaeus duorarum) in south Florida and relate pink shrimp production to coastal freshwater inflows managed by the South Florida Water Management District. A computer model simulated juvenile growth and survival and potential harvests of pink shrimp as a function of salinity and temperature (Browder et al. 1999, 2002). Laboratory studies of wild-caught pink shrimp from Florida Bay provided the basic information on the relationship of growth and survival to salinity and temperature that were the foundation of the simulation model. Based on model simulations the investigators concluded that spatial and temporal variation in salinity on the nursery grounds could be a factor influencing harvests in the fishery.

On the other hand, migration success between Florida Bay and the Tortugas grounds may determine the size of pink shrimp harvests. A series of field studies were conducted by the Ecosystem Investigations Unit in collaboration with UM-RSMAS and USGS to learn more about the factors that influence migration success. Channel nets were anchored in passes on both sides of Florida Bay to catch postlarval shrimp entering the bay and young adult shrimp leaving the bay. SEFSC researchers found that the flux of incoming postlarvae is much greater in magnitude across the bay's western boundary than across it's eastern boundary (i.e., through the Upper Florida Keys). Further, migration across the western boundary is strongly seasonal, whereas migration across the eastern boundary is aperiodic--possibly episodic (Criales et al. 2006).

Peak postlarval concentrations suggesting peak postlarval transport occurred predictably within the July -September of each summer studied. Transport simulations were conducted to investigate the potential influence of tidal constituents on larval transport across the shelf to western Florida Bay. The study concluded that tidal constituents S2 and K2 in interaction with the annual cycle of night length might be responsible for the strong summer signal (Criales et al. 2005).

A cruise on the RV Gandy in the summer of 2004 yielded comparative information on behavior and potential horizontal movements of progressive larval stages of pink shrimp. These results provided the first evidence of an onshore selective tidal stream transport (STST) in decapod larvae migrating in continental shelf waters offshore (i.e., 100 km from the coast and at a depth of 20 m) while approaching their coastal nursery grounds (Criales et al. 2007)

In summer 2004 and 2005, channel nets were used to investigate the immigration of postlarvae into interior Florida Bay from the west to evaluate whether the low density of juvenile pink shrimp in the bay's interior was due to poor habitat conditions (i.e., hypersalinity and patchy seagrass) or to restrictions to the supply of postlarvae coming from offshore spawning grounds

The survival and growth model had suggested that the production of juvenile pink shrimp in the bay's interior could be increased if salinity conditions were improved, but was that realistic?

Postlarvae in the water column were sampled at six stations located at various distances into Florida Bay from its western boundary. The highest concentrations of postlarvae were found approximately midway between the westernmost and easternmost sampling sites, about 15 km from the bay's western boundary. This agreed well with calculations of the main (M2) tidal transport on the night flood tide, which suggested that pink shrimp postlarvae operating on STST could travel 15 km in 4-nights. Lack of tidal transport clearly was a constraint on larval transport beyond that point. However, principal components analyses led investigators to conclude that cross-shelf wind stress and salinity, as well as tide, influence postlarval concentrations at the sampling sites (Criales et al. 2010).

Findings of 2005-2006 field work (Criales et al submitted) provide additional evidence for flood-tide transport. In samples collected on 1- or 2-hr intervals at depth intervals in the water column, higher numbers of postlarvae per hour were found in the subsurface and intermediate strata during the flood tide vs. ebb tide. During new moon the largest collections of postlarvae occurred coincident with highest current speeds; however, during the full moon postlarvae were more abundant in the second half of the flood period near the slack tide.

Juveniles exhibited a behavior and migratory pattern opposite to that of postlarvae and were found almost exclusively near the surface on the ebb tide. Significantly larger juveniles were captured on the dark-ebb than on the dark-flood tide during both moon phases, suggesting that older juveniles were leaving the bay on the ebb tide.

Laboratory trials in the summer of 2009 examined survival and growth in relation to salinity of postlarvae of several ages (i.e., PL0, PL20, PL30, PL50) (Criales et al. 2010). One set of trials tested the hypothesis that early, relatively long-term exposure to a high or low salinity might set the salinity tolerance range of the individual later on. In postlarvae exposed for 15 days to a salinity of 35, over 80% (both Pl30 and Pl40) survived at salinities of 10, 15, 25, 35 and 45; survival declined to 50% at 5 and 55 and less than 10% at 60. PL50 previously pre-conditioned to a salinity of 15 did not survive more than 12 hr at any salinity >35, but their survival at salinities of 5, 15 and 25 was over 96% at 144 h. All PL50 pre-conditioned to 35 (250C) died at salinities of 5 and 10 after 6 h of exposure; however, survival at 55 and 60 was slightly higher than for PL30 and PL40. These results support our hypothesis that initial exposure at a given salinity determines future tolerance range.

The growth, survival, and potential harvest model, along with previous SEFSC analyses relating pink shrimp landings on the Tortugas grounds to indices of freshwater flow to coastal waters (Browder 1985, Sheridan 1996), led to adoption of pink shrimp as an indicator species in the Comprehensive Everglades Restoration Project and the South Florida ecosystem restoration effort (Browder and Robblee 2009).