SELECTIVE TIDAL STREAM TRANSPORT OF SPOT (LEISTOMUS XANTHURUS-LACEPEDE) AND PINFISH [LAGODON-RHOMBOIDES (LINNAEUS)] LARVAE - CONTRIBUTION OF CIRCATIDAL RHYTHMS IN ACTIVITY

Spot (Leistomus xanthusus Lacepede) and pinfish (Lagodon rhomboides Li nnaeus) spawn offshore and their larvae are then transported shoreward where they migrate into estuaries for continued development. The pres ent study determined (1) whether larvae of these species use selective tidal stream transport for up-estuary movement, and, (2) whether an e ndogenous rhythm in activity contributes to this movement. Field studi es determined the abundance of spot and pinfish larvae over time at tw o locations in a tidal estuary as related to tidal phase and the light -dark cycle. At one location, both species were statistically most abu ndant in the water column during flood tide at night, which indicates the presence of selective tidal stream transport. Since results were i nconsistent at the other location, the effectiveness of selective tida l stream transport for up-estuary movement varies with location in an estuary. Endogenous rhythms in activity of estuarine caught larvae wer e measured under constant conditions in the laboratory. Both species h ad circatidal rhythms with period lengths close to a tidal cycle and t wo activity peaks over a lunar day. In most cases maximum activity cor responded to the times of ebb tide in the field. During selective tida l stream transport, increased activity could represent the behavior ne eded to remain low in the water column or move laterally in order to r educe horizontal movements during ebb tides. During flood tides larvae could move passively with up-estuary directed currents. (C) 1998 Else vier Science B.V.