Evaluation of Larval Transport Models using a Programmable, Vertically Migrating 'Zooplankton Mimic'

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Our understanding of how populations in the sea are affected by the ocean circulation to which they are subject during part or all of their life cycle is restricted at the moment to large scale circulation and biogeographic distributions. Mesoscale and small scale interactions, where the individual organism is of primary importance, are difficult to study. The vertically migrating Lagrangian drifter to be developed in this study will emulate the small-scale vertical swimming behavior of decapod larvae in the field. Drifters programmed according to laboratory and field observations of larval behavior will be released where crabs are spawning to determine if vertical migration can account for retention of larvae in estuaries and ultimate arrival at adult habitats. A larger drifter, carrying an ARGOS satellite transmitter, will track effects of behavior on transport of crab larvae exported from an esutary (Callinectes sapidus in Chesapeake Bay). Some oceanographic models predict that C. sapidus will be swept out of the mid-Atlantic Bight, but dense populations remain within 50km of the mouth of Chesapeake Bay,k apparently re-entering the Bay as megaloae and juveniles. Drifters programmed with alternative larval behaviors will help resolve whether this is caused by wind-driven advection of near- surface larvae, passive transport of megalopae in bottom currents, or transport enhanced by vertical migration into flood tidal currents. %%% One of the shellfish resources of considerable value to man is the blue crab of Chesapeake Bay and other estuaries of the southeastern United States. Studies of the distribution of the larvae have shown that they exit Chesapeake Bay during part of their life cycle, but then re-enter the Bay when they near adulthood. It is not clear how their position in the water column, their ability to move up and down in the water column, and the local currents in the Bay and mid-Atlantic Bight combine to maintain the populations in the Bay. A small drifter that can mimic larval behavior is being developed. The drifter will carry a satellite transmitter so that its position can be tracked as it is carried by the currents, and its movement up and down in the water column can be specified. The drifter will be deployed where the adult blue crabs spawn. The drifter will allow various swimming behaviors to be tested to determine which combination(s) of behavior and currents most realistically copy known larval distributions and result in maintenance of the crab population in the Chesapeake Bay.