ENDOGENOUS SWIMMING RHYTHMS OF LARVAL ATLANTIC MENHADEN, BREVOORTIA-TYRANNUS LATROBE - IMPLICATIONS FOR VERTICAL MIGRATION

Atlantic menhaden, Brevoortia tyrannus Latrobe, spawn on the continent al shelf. Larvae are transported shoreward where they enter estuaries and metamorphose. Field studies suggest that while offshore, larvae ma y undergo nocturnal diel vertical migration (DVM), in which they are n ear the surface during the night and at depth during the day. The DVM pattern is more pronounced in estuaries but the timing of the nocturna l ascent may be related to tides. Larvae appear to swim in the water c olumn during nocturnal rising tide and are less abundant at all other times. This migration pattern would result in up-estuary movement by s elective tidal stream transport. The present study measured endogenous swimming rhythms of Atlantic menhaden larvae and related them to poss ible vertical migration patterns in offshore and estuarine areas. Larv ae reared in the laboratory on a diel light:dark cycle were measured a t three sizes (7-9 mm, 14-17 mm and 23-27 mm total length [TL]) as rep resentative of larvae in offshore areas, while estuarine larvae (25-30 mm TL) were collected in the Newport River Estuary (N.C.). Swimming w as monitored in a column under constant conditions in the laboratory w ith a time-lapse video system. In all cases, larvae had a similar circ adian rhythm. The smallest laboratory reared larvae ascended into the upper portion of the column during the time of night and descended dur ing the time of day. The other laboratory reared and field caught larv ae had increased swimming activity during the time of night and reduce d activity during the time of day. This circadian activity rhythm woul d contribute to the nocturnal DVM in offshore and estuarine areas. How ever, there was no evidence that larvae developed a tidal rhythm in ac tivity that could contribute to selective tidal stream transport in es tuaries.