 QUEEN CONCH ( STROMBUS GIGAS) IN THE EXUMA CAYS, BAHAMAS by Ned Smith, Ph.D., and Patrick Pitts Division of Marine Science, HARBOR BRANCH OCEANOGRAPHIC INSTITUTION |
In the early 1990s, physical oceanographers at HARBOR BRANCH Oceanographic
Institution were able to help biologists studying the conch address two
questions concerning conch distribution. First, the biologists wanted
to know if local juvenile populations are recruiting from the larvae spawned
by local breeding adults, or if they are recruiting from adult populations
located in other regions.
Secondly, the biologists wanted to know why the juvenile aggregations are found in certain locations on the bank, while other seemingly suitable bank habitats are virtually devoid of young conch. To investigate local flow patterns, i.e., the pathways taken by drifting conch larvae, oceanographers deployed an array of recording current meters over the shelf on the Exuma Sound side of the islands, in tidal channels between islands and in the shallows of Great Bahama Bank. The data indicate that both tidal and wind-forced currents facilitate transport of conch larvae from the deep-water spawning grounds to nursery areas on Great Bahama Bank. (Above right) Divers deploy a current meter in a tidal channel. Larvae are transported along the shelf, primarily by wind-driven flow, to the mouths of inlets where they are drawn onto Great Bahama Bank by tidal currents. These tidal currents are strong enough to transport larvae considerable distances through the passes and onto the bank during the flood half of the tidal cycle. Combining average flow rates with the time the larvae spend in the water column indicates that local juveniles are recruiting from spawning stocks located no further than about 50 km (36 miles) upstream of the nursery areas. 
Before determining the habitat preferences of juvenile conch, it is important to note that certain water quality parameters differ significantly between the deep oceanic waters of Exuma Sound and the shallow waters overlying Great Bahama Bank. For example, temperatures in the sound are almost always cooler during summer and warmer during winter. (Left) Tracks taken by surface drogues on the flood tide from the mouth of the tidal passes on to the Great Bahama Bank. Also, salinity can be as much as 4 parts per thousand higher on Great Bahama Bank than in Exuma Sound. Since most of the juvenile aggregations are found on the bank within 3-4 km (2-3 miles) of the sound, biologists hypothesized that the influence of the oceanic sound water plays an important role in the juvenile distribution. |
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