BBP in Brief, Issue 2, April 2004

Connectivity Between The Bahamas and the Greater Caribbean
Donald B. Olson (University of Miami-RSMAS) and Dan Brumbaugh (AMNH-CBC)

[Click to enlarge]

Almost all of the Bahamian marine fauna are also found throughout at least portions of the insular Caribbean Sea (bounded by South and Central America to the south and west, and the Greater and Lesser Antilles to the north and east). The Bahamas and the Florida coast represent the northernmost outpost for these species (with the exception of the small island of Bermuda), though certain species' distributions differ somewhat among subregions of the greater Caribbean. Given that one objective of many MPAs is to protect populations in order to build or simply sustain fisheries stocks, what is the importance of ecological connectivity to the design of MPA networks? Fundamentally, wide-scale connections are important to fisheries management because of the traditional argument that fisheries, whether measured at maximum sustainable yield (MSY) or some more conservative measure, rely on ecosystem surpluses. In other words, individuals taken from the wild will not harm the long-term productivity of populations because either the exploited numbers are less than what could cut into the local reproductive output of the population, or the exploited portion is downstream and distant from the reproductive source. For example, if juvenile lobsters in Florida waters originate from the insular Caribbean, then they are all surplus, at least up to the point that their removal does not disrupt the supply of larvae to lobster populations further downstream or some important local ecosystem functions. In this case of supply to downstream sites, a network of MPAs may be warranted to provide a series, analogous to stepping stones, of upstream refuges for breeding populations. If, however, local reproduction is important for the Florida population, then it is important to maintain a local breeding stock. Creating individual MPAs in Florida (in this case, lobster reserves) is one mechanism for achieving this. The outcome depends strongly on whether the species in question is present due to distant or local sources.

With the goal of addressing how populations within The Bahamas are connected to ones outside, the Connectivity Working Group has attempted to quantify the origin of near surface waters in the vicinity of The Bahamas. The observational tools used include satellite imagery, surface drifters, and an analysis of the waters themselves in terms of their temperature and salinity. The satellite data on sea surface temperature and ocean color (a measure of phytoplankton biomass in the open ocean and as well as the sea floor in shallow bank areas) provides information on the flows out of the Caribbean through the Yucatan Channel and into the Florida Current and some idea of the flows through the Providence Channel. There is also a fairly extensive dataset of satellite-tracked drifters that documents the through-Caribbean flow as well as the flow to the east of The Bahamas. Each drifter includes a radio-transmitting unit tied to a surface buoy which is attached to a fabric cylinder called the drogue (see photo). The drogue drops 15 meters below the surface so that the buoy moves with the water's currents at that depth. In addition to these oceanographic data, the group is using simulations of dispersing particles in a numerical circulation model of the Atlantic in order to investigate the possible pathways that larval animals might take to disperse from the insular Caribbean into The Bahamas (see map).

Results to date suggest that only the southern and eastern Bahamas below San Salvador Island are connected ecologically to the northern Antilles, unless there are periodic flow reversals northward through the Windward Passage (as has been suggested but not well documented). A stronger connection between The Bahamas and the insular Caribbean occurs through the Gulf Stream, but this influence seems to only introduce Caribbean waters to the northwestern edges of the Grand and Little Bahama Banks. These patterns are partially consistent with morphological and genetic patterns in the sharknose goby (Elacatinus [Gobiosoma] evelynae) presented by Taylor and Hellberg in a 2003 Science article. The connectivity group is currently working on further genetic sampling on queen conch (Strombus gigas), spiny lobster (Panulirus argus), land crabs (Cardisoma guanhumi and Gecarcinus lateralis), staghorn coral (Acropora cervicornis), and bonefish (Albula vulpes). The Connectivity Working Group results suggest that the Bahamian archipelago is largely divided into two regions with somewhat different connections to the Caribbean. The next step is to understand in finer detail the connections within The Bahamas themselves.