Larval transport and recruitment
Recruitment pathways in the Florida Keys:
We have been actively investigating the physical processes contributing to the recruitment of fishes to the Florida Keys. Although the Florida Keys is the only coral reef system in the contiguous U.S., very little is known about how populations of reef organisms are replenished and sustained over time. We have conducted several time series collections of larval arrival along the Florida Keys which have shown that (1) near-reef larval fish assemblages differ markedly from offshore assemblages and their abundance is strongly event-related (Sponaugle et al. 2003), (2) very large multi-taxa pulses of settlement stage fish occur in association with the nearshore passage of mesoscale Florida Current eddies, and (3) such mesoscale eddies can also flush larvae out of the system (Sponaugle et al. 2005a, D'Alessandro et al. 2007).
Reef fish recruitment to the Florida Keys:
Without fundamental data on patterns of recruitment, it is difficult to examine the underlying processes creating such pattern. My lab began a year-round monthly reef fish recruitment survey in 2003, which now forms a five-year baseline time series on recruitment magnitude and timing. Monthly underwater SCUBA-based surveys are conducted across the upper Florida Keys shelf encompassing replicate fringing reef, patch reef, seagrass, and mangrove sites. Students continue to incorporate the recruitment dynamics of particular species into their research (Paddack & Sponaugle 2008), including comparison of larval supply and recruitment among protected (reserve) and non-protected sites in the Florida Keys National Marine Sanctuary (Grorud-Colvert & Sponaugle 2009).
Population connectivity
Larval linkages: an interdisciplinary approach
The degree to which populations of marine organisms are connected via the dispersal of larval propagules is a central unanswered ecological and oceanographic question. The complex oceanography of marine systems, and high mortality and diffuse concentrations of larvae make direct measurement of larval sources generally not feasible, particularly for marine populations distributed along open coastlines. Furthermore, ecological population connectivity is not only a function of the physical transport of the larvae, but also the interaction of factors influencing larval growth, survival, and condition at settlement (Pineda et al . 2007; Cowen & Sponaugle 2009). A major ongoing effort in my lab is our research into connectivity of reef fishes in the Florida Keys. This is a large interdisciplinary NSF-sponsored project designed to integrate intensive field sampling with biophysical modeling to begin to define dispersal kernels for reef fish populations in oceanographically dynamic regions. High-resolution shipboard ichthyoplankton and physical oceanographic sampling along and upstream of the Florida Keys will be linked with simultaneous reef-based sampling of larval supply and juvenile recruitment. Fishes will be "tracked" across this transitional period by analysis of their otolith growth trajectories. We plan to test whether larval growth varies with distance to shore and water mass (specifically, larval encounter with mesoscale eddies). These empirical data will be incorporated into a coupled biophysical model (a comprehensive three-dimensional hydrodynamic model coupled with a Lagrangian particle tracking model; see Claire Paris' website http://www.rsmas.miami.edu/personal/cparis/ and Villy Kourafalou's website http://www.vortexclientsite.com/umrsmas/ ), which will be run iteratively to quantify the relative contribution of local (near-field) and upstream (far-field) larval sources. Co-PIs on this project are Robert Cowen (MBF/RSMAS), Claire Paris (AMP/RSMAS), and Villy Kourafalou (MPO/RSMAS). Field and shipboard efforts coordinated by Senior Research Associates Kristen Delano Walter and Cedric Guigand, respectively. Morgan Witman, a high school science teacher was selected by the NSF ARMADA program to participate in our second cruise in 2007. Her experience is fully documented in her online journal at http://www.armadaproject.org/journals/2007-2008/hardwick-witman/7-29-30.htm
Other fish movement studies:
A recent collaborative Sea Grant funded project with Jiangang Luo (RSMAS) and Joe Serafy (NOAA/NMFS Miami Lab) focused on the older stages of gray snapper ( Lutjanus griseus ) and involved tracking the diel and seasonal movement of gray snapper among nearshore habitats of the Florida Keys. We used conventional, acoustic, and archival tagging techniques as well as video observations to directly track snapper within and among mangroves, seagrasses, and coral reefs near Biscayne Bay (Luo et al. 2009 ).
Larval growth and survivorship
Reef fishes:
Successful settlement of reef fishes is more than successful larval transport between spawning sites and juvenile habitat. My lab is actively involved in quantifying aspects of larval growth and survivorship for several model reef fishes. The analysis of fish otoliths is a central tool in this research as a method for comparing relative growth rates and condition among individuals. Building upon previous work on the relationship between variation in early life history traits and survival of fishes in Barbados (Searcy & Sponaugle 2000, 2001), and the effect of distinct physical oceanographic features on larval growth and transport (Sponaugle & Pinkard 2004a,b), we have been investigating natural variability in early life history traits and the consequences of that variability to recruitment (Sponaugle et al. 2006), early juvenile growth, and survival of a common reef fish, Thalassoma bifasciatum, in the Florida Keys (Sponaugle et al. 2006, Sponaugle & Grorud-Colvert 2006). Experimental studies have been used to further tease apart critical ecological processes occurring during the transition between the larval and juvenile stages (Grorud-Colvert & Sponaugle 2006). As part of their dissertation work, Tauna Rankin is investigating carryover traits in another model species, the bicolor damselfish, Stegastes partitus , and Evan D'Alessandro is expanding this work beyond our model reef fish taxa to the commercially important yellowtail snapper, Ocyurus chrysurus .
Much of our above work has focused on hindcasting the otolith growth records of juveniles (i.e. successful settlers), but we are also examining larval growth directly by comparing otolith growth rates of larvae collected at different distances offshore. Two undergraduates (Lisa Havel and Jennie Boulay) are examining aspects of Thalassoma bifasciatum growth off the Florida Keys as part of their Honors Theses. Recent ichthyoplankton collections will be used by my PhD student, Katie Shulzitski, to test whether larval growth of several species varies with distance from shore and with position inside and outside of mesoscale eddies.
Billfishes:
As part of a collaborative NSF-sponsored study of the transport, growth, and fate of billfish larvae in the highly dynamic Florida Straits (see Cowen webpage for more detail: http://www.rsmas.miami.edu/groups/larval-fish/Ongoing.cfm), we are investigating whether growth of larval sailfish and blue marlin varies spatially. Larval otoliths are examined from monthly ichthyoplankton collections to measure habitat-specific growth rates of these billfishes. Our goal is to examine the relative contributions of pelagic habitats to the growth and survival of these important oceanic predators. Our earlier aging work of larval blue marlin demonstrated that larvae in Exuma Sound, Bahamas, grow significantly faster than larvae in the Straits of Florida (Sponaugle et al. 2005b).
Fish behavior
Larval reef fish behavior:
An essential component of larval reef fish transport and survival is their capacity for active behavior. My PhD student, Klaus Huebert, is completing a series of experiments to examine the behavioral capabilities of larvae as they near settlement. Initial experiments conducted at sea used wild-caught larvae to test whether pressure influences vertical positioning (Huebert 2008). SCUBA divers also tracked larvae in situ seaward of Florida Keys reefs to measure active orientation behavior of late-stage larvae. These experimental data are being augmented by the analysis of vertical positioning of natural populations of larvae caught during targeted 48-hr cruises.
Juvenile reef fish behavior:
We have been interested in how recruitment and early juvenile survival are influenced by variation in the local abundance of predators, as occurs in predator-rich marine protected areas. As part of her dissertation, Kirsten Grorud-Colvert conducted a series of mesocosm and laboratory experiments to quantify the influence of predation on trait selection in young reef fishes using the bluehead wrasse, Thalassoma bifsciatum, as a model species (Grorud-Colvert & Sponaugle 2006; Grorud-Colvert 2006). Another PhD student, Tauna Rankin, is conducting in situ experiments with the bicolor damselfish, Stegastes partitus , to examine the mechanisms underlying observed patterns in otolith growth. As part of his dissertation, Sean Bignami plans to conduct experiments using reared larvae of various condition levels to examine the specifics of how traits carryover from one life stage to the next.
Invertebrates
While clearly not a primary research focus in my lab, the larval supply and recruitment of marine invertebrates is also of broad interest. Projects have ranged from extensive nightly time-series of larval supply of brachyuran crabs to Barbados (Reyns & Sponaugle 1999) to a shorter time series investigation of patterns of stomatopod larval supply (undergraduate Honors Thesis of Paige Roberts). We continue to retain all plankton and light trap samples with the optimistic expectation that time and resources will eventually enable the analysis of these data for invertebrates.