A numerical study of larval fish retention along the southeast Florida coast

Abstract

The possibility that the eggs and larvae of southeast Florida fishes can be retained by local hydrodynamic processes is examined using simple idealizations of the local currents, oceanic turbulence, Ekman drift, horizontal larval swimming and larval mortality. The simulated settling rates were greatly affected by the interaction between the hydrodynamics and the degree of spatial heterogeneity in the mortality fields. In the presence of an onshore Ekman drift, for example, settling rates tended to increase unless the mortality rate over the reef was much higher than elsewhere. Overall, the simulations suggest that the meanders, eddies and gyres associated with the Florida current front can retain between 0.07 and 41% of the larvae with planktonic life spans on the order of 1 month. However, only a recirculating gyre enabled large numbers of larvae to settle within a few kilometers of their birth place. Assuming the mortality rate near the reef is 20–90% per day, but lower offshore, between 0.1 and 7% of the larvae retained are likely to settle. This implies that southeast Florida reef fish populations should be able to replenish themselves without relying on upstream sources of recruitment, provided they exhibit normal fecundities. The simulations also suggest that natural levels of variation in the mortality field or local currents can both cause large fluctuations in recruitment.