Patterns, Mechanisms and Approaches to the Study of Migrations of Estuarine-Dependent Fish Larvae and Juveniles

Five species (<10%) contribute over 85% of the annual commercial catch of finfish off North Carolina. All five spawn in winter near the Gulf Stream and migrate about 100 km to major inlets in the barrier islands and then up to another 100 km to juvenile estuarine nursery areas. The vertical distribution of pelagic larvae of menhaden (Brevoortia tyrannus) differs significantly from that of more benthic-oriented larvae such as spot (Leiostomus xanthurus), croaker (Micropogonias undulatus) and flounder (Paralichthys lethostigma and P. dentatus). Analysis of the shelf current regime suggests that differences in vertical distribution and season will subject larvae to markedly different currents, and therefore, different mechanisms for migration are required. Calculations of water movements using Ekman’s original transport equations are probably wrong in relatively shallow shelf waters subject to cross-shelf winds and density currents. Major differences probably exist, therefore, between the pelagic larvae of the west coast (e.g. Pacific sardine, Sardinops sagax) and estuarine dependent species of the east coast—both with respect to the importance of microscale processes (e.g. food patchiness) and drift. Predator avoidance is likely to be a more important determinant of migration pattern than has been suggested to date. Problems of determining migration vectors and mechanisms from highly variable larval and juvenile fish distributions are discussed. A research strategy dealing with the above is outlined, and testable implications of hypotheses are presented.