Estuarine Ingress of the Blue Crab Callinectes Sapidus

Abstract

This dissertation investigated ingress of postlarval blue crabs Callinectes sapidus to the Newport River estuary, North Carolina, USA. Data from C. similis, Menippe mercenaria, Pachygrapsus transversus, and Arenaeus cribrarius are included in some chapters for comparison. Changes in tolerance to low salinity were examined by: 1) exposing postlarvae (megalopae) collected in coastal and estuarine areas to a range of salinities and 2) determining the cue that stimulates acclimation of coastal megalopae to low salinities, the time to acclimation, and the decrease in salinity necessary for acclimation. Coastal megalopae were less tolerant to salinities of 5 and 10 than megalopae from the estuary. Coastal megalopae became acclimated to low salinities within 12 h when salinity was reduced from 35 to 31.

Spatial patterns in abundance during ingress were investigated simultaneously in coastal and estuarine areas. Coastal distributions were determined using nighttime surface plankton tows at slack water after ebb tide and slack water after flood tide on four nights; two each during spring and neap tides. Estuarine distributions were determined using nightly settlement on 'hog's hair' collectors. C. sapidus megalopae were most abundant at the coast east of Beaufort Inlet, but settlement was restricted to western channels of the estuary. Species-specific patterns in abundance were maintained during two spring/neap cycles, possibly due to interactions between larval behavior and physical forcing.

Biophysical mechanisms of estuarine ingress were investigated by comparing nightly abundance in coastal and estuarine areas with environmental variables. Comparisons were made using cross-correlation and cross-fourier analyses. High estuarine abundances were associated with wind-driven estuarine inflow and nighttime flood tides. The seasonal pattern of estuarine ingress was strongly associated with the seasonal pattern of alongshore wind stress, suggesting that inter-annual variations in atmospheric forcing may determine the yearly abundance of megalopae arriving in estuarine nursery habitats.

The effect of sampling interval on annual megalopal abundance estimates was determined using an 11-year dataset of nightly settlement. Variability in abundance estimates increased with increasing sampling interval. Switching from a one day to two day sampling interval resulted in a 20 % decrease in the likelihood of detecting a significant correlation between annual abundance and CPUE in the North Carolina blue crab pot fishery.