Source, Trophic Opportunities and Fate of Billfish Larvae in the Diverse Pelagic Habitats of the Straits of Florida

Investigator(s) Robert Cowen rcowen@rsmas.miami.edu (Principal Investigator)

Kevin Leaman (Co-Principal Investigator)

Donald Olson (Co-Principal Investigator)

Sharon Smith (Co-Principal Investigator)

Su Sponaugle (Co-Principal Investigator)

Sponsor University of Miami Rosenstiel School of Marine&Atmospheric Sci

4600 Rickenbacker Causeway

Key Biscayne, FL 33149 305/361-4800

Abstract (from the National Science Foundation website)

Billfishes are a complex of large pelagic fishes that occur in the tropical and subtropical waters of the world's oceans. They are top predators in the pelagic food web and, simultaneously, the most sought-after fishes for saltwater anglers. These oceanic "giants" support a multi-million dollar sportfishing industry and comprise a significant bycatch of tuna longline efforts. Unfortunately, human activities have led to significant declines in billfish stocks; blue marlin are thought to be at less than 40% of that necessary to sustain fishing at maximum sustainable yields. Despite their critical ecological status, the biological information available on these organisms is extremely limited. Focusing on the early life history stages of billfishes is a tractable means of obtaining valuable biological data. For example, collection of early larval stages can be used, in conjunction with data on circulation, as a means of determining when and where spawning occurs. Investigation of the pelagic habitat variables associated with the occurrence of billfish larvae may provide insight into environmental conditions required for spawning and early survival. Even in their early life history, billfishes are likely important constituents of the pelagic food chain, especially as predators on zooplankton and larval fishes. The overall objective of this study is a seasonally resolved understanding of the annual cycle of billfish spawning, larval growth, feeding, and transport within the complex environment of the Straits of Florida. As top predators, billfishes play a critical yet undefined role in the pelagic food web. Here we examine the early life history of billfishes within the oceanographic context of the Straits of Florida (SSF). The SSF is a complex system of discrete, interacting water masses and pelagic environments. Preliminary data suggest that billfish larvae occur predictably in three distinct patches across the SSF. The project is designed to 1) identify the sources of the three distinct larval billfish patches within the SSF; 2) determine how the these patches differ in terms of the trophic and growth environment of the billfish larvae; and 3) investigate the transport fates of larval billfishes from these different patches. The fieldwork is divided into three separate study components: The first study will address the temporal qualities of cross-Strait features (i.e., water mass distribution, larval billfish patches, and zooplankton community structure). Monthly sampling (consisting of ichthyoplankton and zooplankton tows, CTD, fluorometry, and ADCP measurements) over two years will be linked via otolith aging studies and circulation patterns to estimates of spawning locations. Further otolith work on larval growth rates will be coupled to zooplankton work on community composition and dynamics to identify trophodynamic differences among patches. The second study will expand the spatial scale of our efforts to examine the along-Strait dynamics relating to the formation, maintenance, and fate of larval billfish patches. A single cruise will be made during the second year and will include the same suite of measurements as above as well as ARGOS-tracked drifter measurements. The third study during year 3 will incorporate Lagrangian techniques to track larval billfishes from hypothesized spawning sites through the Straits of Florida system to link these trajectories to the observed cross-Strait spatial patchiness and ultimate fate of the discrete patches. Sampling will consist of a similar set of measurements conducted at stations normal to the track of a group of drifters. Results of the proposed study will provide critical new information regarding the early life history and ecology of these important pelagic predators as well as the interaction between different water masses and the trophodynamics of pelagic food webs. A further understanding of the formation, maintenance and fate of particular chl. a/zooplankton/larval fish patches also will provide insights into understanding the role of upwelling and water mass exchange across frontal boundaries and shallow banks. [Note that this project is funded by the National Science Foundation and is scheduled for funding beginning in August 2002 and ending in July 2006]