Oceanic Transport and Life History of the Tropical Western Atlantic Opossum Pipefish, Microphis brachyurus lineatus
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The opossum pipefish, Microphis brachyurus lineatus, is a tropical migratory syngnathid. Adults live and breed in freshwater tributaries in specific emergent vegetative habitats. Juveniles associate with pelagic Sargassum in the tropical western Atlantic. Reproduction occurs during the wet season (May-November). Juvenile recruitment occurs during the dry season (December-April). The objective of this dissertation is to study the east Florida breeding population in the freshwater tributaries of the Indian River Lagoon and focus on: 1) Larval drift to the Gulf Stream and, 2) recruitment of pelagic juveniles from the Gulf Stream onto the freshwater breeding habitat. Pouch released larvae face sharp salinity changes as they drift from freshwater tributaries to the ocean. Two hundred-fifty aquarium released larvae were transferred from freshwater (0 ppt) to mesohaline lagoon water (18 ppt) and then to seawater (32 ppt) in two successive replicated experiments, over a two-week period. Survival was highest when larvae were transferred from 0 ppt to 18 ppt within 48 h. When transferred from 18 ppt to 32 ppt, larvae that remained at 18 ppt had the highest survival. Therefore, complete passage from freshwater to the ocean must occur at least 2 weeks after larval release. Hydrographic conditions capable of larval retention are explored using a 3-D numerical salinity model of the Indian River Lagoon. Pelagic juveniles that recruit into the population must leave a strong western boundary current, the Gulf Stream, cross the continental shelf and reach an ocean inlet. Between 27° and 31° N, the Gulf Stream flows north parallel to the contour of the continental shelf edge. Gulf Stream filaments intrude onto the continental shelf. The recruitment hypothesis states that the mechanism of juvenile cross-shelf transport is a combination of Gulf Stream filaments and onshore wind-derived Ekman drift and Stokes drift. The sampling method included three years of daily sea surface temperature satellite images and buoy wind data. Results show: 1) onshore Ekman drift lags recruitment (dry) season, 2) warm filaments are always present and are not a limiting factor for the juveniles to enter shelf waters. To determine recruitment time, all data are combined in a numerical recruitment model. The time of cross-shelf transport for the study area predicted by the model is within the limits of a pelagic fish larva/juvenile lifespan. Spawning and recruitment times suggest that the marine phase ranges from 1 to nearly 12 months. Hypotheses have been put forth regarding juvenile oceanic transport. Ocean currents transport Sargassum and the associated pelagic juveniles in the tropical western Atlantic. The Caribbean, Loop and Antilles Current may distribute juveniles within the tropical areas, and as they converge into the Gulf Stream, juveniles are transported northward along the US seaboard. If the marine phase is a year or more, juveniles could also be transported eastward to Europe by entering the north Atlantic Gyre, and eventually return to American shores. The existence of alternate oceanic migration routes is also explored.