Abundance of post-larval Callinectes sapidus, Penaeus spp., Uca spp., and Libinia spp. collected at an outer coastal site and their cross-shelf transport

Abstract

The abundance of the megalopae of Callinectes sapidus, Uca spp., and Libinia spp. and the post-larvae of Penaeus spp. settling on collectors hung from the end of the Army Corps of Engineers' Field Research Facility (FRF) pier in Duck, North Carolina, USA, was measured daily during the period 2 August through 3 November 1994. During this period the coastal oceanography was dominated by the effects of the winds; winds from the NE tended to generate surface onshore flow and downwelling while those from the SW tended to generate surface offshore flow and upwelling. If the post-larvae were transported shoreward by coastal currents then variations in their abundance should relate to the physical parameters which generate the transporting currents. Using time-series analysis the daily abundance of post-larvae was statistically analyzed and compared to along- and cross-shore wind stress, along- and cross-shore current speed near the bottom (23 m) and near the surface (4 m), and temperature, salinity, and current speed measured at the end of the FRF pier. Fourier analysis suggested that the abundance of post-larval C. sapidus and Penaeus spp. varied on a semi-lunar cycle, e.g. 15 d. No cycles were apparent in the Fourier analysis of the abundance of Uca spp. megalopae and the megalopae of Libinia spp. varied on an 18 d cycle. Significant cross-correlations were found between the log-transformed daily abundance of post-larval C. sapidus and Penaeus spp. and the maximum daily tidal range, which, coupled with the Fourier analysis, suggests that peak catches of these postlarvae tended to occur at and just after the spring tides. The cross-correlation analysis suggests that abundance of Uca spp. megalopae was weakly related to the tides, with peak catches tending to occur around the spring tides. No significant cross-correlations were found between the abundance of Libinia spp. megalopae and the maximum daily tidal range. To test for the effects of the other physical variables on post-larval abundance Autoregressive Integrated Moving Average (ARIMA) models were fitted to the biological and physical data, and cross-correlations were run between the residuals from these models. The analysis suggests that the abundance of C. sapidus megalopae did not vary with any of the measured physical variables, Penaeus spp. and Uca spp. tended to be more abundant during periods of onshore surface flow and downwelling, while Libinia spp. megalopae tended to be more abundant during periods of offshore surface flow and upwelling. The results of this study suggest that post-larval C. sapidus and Penaeus spp. were transported shoreward by tidally driven processes, both Penaeus spp. and Uca spp. were transported shoreward by wind-driven surface currents, and Libinia spp. megalopae were transported shoreward in near-bottom waters during upwelling events.