VARIABILITY OF SEA-SURFACE TEMPERATURE IN THE SOUTH-ATLANTIC BIGHT ASOBSERVED FROM SATELLITE - IMPLICATIONS FOR OFFSHORE-SPAWNING FISH

We examined full-resolution (1 x 1 km) satellite images of sea-surface temperature (SST) over five consecutive years (1981-1986) covering th e Atlantic menhaden (Brevoortia tyrannus) recruitment period (November -April) in the SABRE (South Atlantic Eight recruitment experiment) stu dy site. The results of our image rime series indicated two processes which could be possible mechanisms for the onshore transport of fish l arvae into coastal regions. One is the influx of warm Gulf Stream wate r that oscillates in and out of the Carolina Bays. These oscillations occurred throughout the study period over distances of 20-40 km and on time-scales as short as two days. The other is a tongue of relatively cold water located adjacent to the Virginia coast that moved southwar d and penetrated into Onslow Bay between January and March. Previous s tudies showed that Atlantic menhaden preferentially spawn in 18-22 deg rees C waters on the outer shelf. On the assumption that the 18 degree s C isotherm (18DI) indicates where high larval abundance may occur, w e used AVHRR-SST imagery to track the onshore-offshore movement of the 18DI along a transect extending onshore-offshore in Onslow Bay. Owing to seasonal warming and cooling, this isotherm was always found close st to the coast in early November, reached maximum offshore displaceme nt by January/March, and then moved onshore again in April/May. Our re sults also showed that the position of this isotherm can move offshore or onshore in a matter of a few days. An important influence and poss ibly the major cause of the higher frequency displacements of the 18DI are Gulf Stream meanders or filaments moving through Onslow Bay. Our estimates of onshore isotherm speeds as determined from satellite SST ranged from 2 to 25 cm s(-1) and are within the same order as those ca lculated by physical models or larval age determinations. If the onsho re pulses of warm Gulf Stream water are indeed a mode by which menhade n larvae are transported cross-shelf, then the use of satellite-based observations to determine their frequency and onshore extent, as done in the present study, is a useful tool to study variations in fish rec ruitment.