Distribution of Fall/Winter-spawned Larval Fish in Relation to Hydrographic Fronts on the North Carolina Shelf: Implications for Larval Transport Mechanisms

Abstract

Fish population dynamics are partially determined by successful larval transport. Many fish in the southeast U.S. use estuaries as juvenile nursery habitat, but are spawned on the continental shelf. Favorable cross-shelf larval transport pathways for fall ⁄ winter-spawned species off North Carolina have been theorized to occur as a result of seasonal circulation patterns of currents and episodic onshore movement of water masses. As a result, hydrographic fronts which separate the water masses; including the outer-shelf front and mid-shelf front have been linked to cross-shelf larval transport for several species. The objective of this study was to determine whether fronts on the continental shelf off the southeast U.S. influence larval fish distribution and cross-shelf transport. Ichthyoplankton and hydrographic data were analyzed in concert to determine if the presence of fronts on the shelf influenced larval distribution patterns or mean distance from shore of larvae. In addition, "average" larval distribution patterns for each species were compared with the results from when fronts were present. Results indicated larval fish were not aggregated at fronts, though abundance and size distribution patterns and mean distance from shore of several species-size classes analyzed were significantly different when the outer-shelf front, mid-shelf front, warm Gulf Stream filament front, or Hatteras front (i.e., intrusions of Virginia coastal water) were present on the shelf. Average cross-shelf distributions of several species were similar to previous studies which led to the conclusion that cross-shelf larval transport was predominately driven by seasonal circulation patterns; however, present results verify that episodic events, such as fronts modify seasonal transport pathways.