Water Circulation Patterns Detected in AVHRR-SST Imagery in Relation to Larval Menhaden Abundances in the South Atlantic Bight

Abstract

Sea surface temperature (SST) imagery from the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-9 and NOAA-11 satellites was used to observe surface circulation patterns in shelf waters off the coast of North Carolina. I hypothesized that interannual variation in the frequency and spatial extent of Gulf Stream induced onshore pulses of warm water are a cause of inter-annually varying larval fish abundances arriving at the mouths of North Carolina estuaries. The Atlantic menhaden, Brevoortia tyrannus, is an estuarine-dependent species which spawns offshore in 18-22°C water over a protracted period from November to April. A major menhaden spawning area is found along the shoreward Gulf Stream edge in the South Atlantic Bight. The developing larvae must travel approximately 100-150km to coastal inlets where they enter estuarine nurseries. A probable mechanism facilitating this migration is the onshore forcing of warm water masses due to Gulf Stream fluctuations. Using SST imagery, I tracked the shoreward progression of l 8°C-22°C water as a potential location of menhaden larvae over the spawning season. The spatial location of this l 8°C water is followed over the November - April spawning period to detect frequency and arrival of these warm water "pulses." Based on the inter-annual variability observed in a larval menhaden data set, I focused on the cross-shelf circulation of the following four spawning periods: two periods coinciding with times of relatively very high larval abundances (Nov.-Mar. 1987-88 and 1985-86), and two with very low larval abundances (Nov.-April, 1990-91, and 1991-1992). Cross-shelf circulation analysis revealed that periods of maximum spawning were associated with large magnitude, shoreward progressing, l 8°C water pulses. Although the results showed that the frequency of pulses over the protracted spawning period did not explain the interannual variability, there was a clear association between onshore pulse movement and maximum spawning periods supporting the theory that these pulses may be a significant larval transport vehicle. This association was most notably evident from an event during December 1987, part of the highest larval abundance season. This particular event may have governed the relatively high number of larvae to the reach the inlet in the spring of 1988. Thus, larger magnitude and longer duration pulses may aid in larval transport and estuarine recruitment. Rates derived from SST gradients over time coincided with the range of the transport rates estimated from larval ages and the distance from offshore spawning grounds to Beaufort Inlet. The association