Selective tidal stream transport of spot ( Leistomus xanthurus Lacepede) and pinfish [ Lagodon rhomboides (Linnaeus)] larvae: : Contribution of circatidal rhythms in activity

Abstract

Spot ( Leistomus xanthusus Lacepede) and pinfish ( Lagodon rhomboides Linnaeus) spawn offshore and their larvae are then transported shoreward where they migrate into estuaries for continued development. The present study determined (1) whether larvae of these species use selective tidal stream transport for up-estuary movement, and, (2) whether an endogenous rhythm in activity contributes to this movement. Field studies determined the abundance of spot and pinfish larvae over time at two locations in a tidal estuary as related to tidal phase and the light–dark cycle. At one location, both species were statistically most abundant in the water column during flood tide at night, which indicates the presence of selective tidal stream transport. Since results were inconsistent at the other location, the effectiveness of selective tidal stream transport for up-estuary movement varies with location in an estuary. Endogenous rhythms in activity of estuarine caught larvae were measured under constant conditions in the laboratory. Both species had circatidal rhythms with period lengths close to a tidal cycle and two activity peaks over a lunar day. In most cases maximum activity corresponded to the times of ebb tide in the field. During selective tidal stream transport, increased activity could represent the behavior needed to remain low in the water column or move laterally in order to reduce horizontal movements during ebb tides. During flood tides larvae could move passively with up-estuary directed currents.