Recruitment of coral-reef fishes to Bermuda: local retention or long-distance transport?

Abstract

The benthic marine fauna of isolated oceanic islands may be self-seeding, or alternatively may be continually supplied with recruits advected from elsewhere by ocean currents. Estimates of the time required for transport of larvae from elsewhere was combined with information on the larval durations of fishes inhabiting the reefs of Bermuda, to test the hypothesis that other populations are sources for recruits to Bermuda. Specifically, we tested the prediction that transport occurs frequently enough to sustain local populations of reef fishes. Transport of larvae was modelled as a 2-step process in several numerical simulations. The first step of transport was assumed to occur via the Gulf Stream. The second step, in which larvae are transported across the northern Sargasso Sea to Bermuda, was modelled in several different ways, Involving either cold-core rings thrown off by the Stream, or mixtures of Gulf Stream and Sargasso water entrained by cold-core rings. Travel distances and speeds were estimated from a variety of sources, focusing on satellite imagery of sea-surface temperature Pelag~c larval durations (PLDs) for reef f~sh on Bermuda were analyzed from the daily age record in the otoliths of 6 wrasse and 1 parrotfish species. When PLDs were compared with transport times, it was determined that transport events would occur too infrequently to sustain Bermudian populations of reef fishes. The PLDs for most of these species are similar to results obtained elsewhere in their geographic range. It is evident that these populations must be mainly replenished via a pool of larvae that are spawned locally and retained in the vicinity of the Bermuda islands.