PHOTORESPONSES OF LARVAL ATLANTIC MENHADEN (BREVOORTIA-TYRANNUS LATROBE) IN OFFSHORE AND ESTUARINE WATERS - IMPLICATIONS FOR TRANSPORT

This study tested the hypothesis that Atlantic menhaden (Brevoortia ty rannus Latrobe) larvae have separate behaviors in offshore and estuari ne waters that are evoked by chemical cues in these waters. Atlantic m enhaden spawn offshore, and larvae are transported shoreward where the y metamorphose after entering estuaries. Field studies suggest that du ring the day larvae are at moderate depths offshore and less abundant in the water column than at night in estuaries. Since photoresponse ma y contribute to these depth distributions, they were tested in the lab oratory in a light field that mimicked the underwater angular light di stribution. Responses were quantified as the proportion of larvae swim ming in the upper portion of a water column containing either offshore or estuarine water at the same salinity and temperature. Swimming var ied significantly with water type. Young larvae (<8 mm total length (T L)) swam near the surface in estuarine water and lower in offshore wat er. Older larvae (14-16 mm TL) showed the opposite responses. Since th e test waters only differed in chemical composition, the results suppo rted the hypothesis. In addition, the study characterized the predator avoidance shadow response, in which larvae descend in response to a d ecrease in light intensity. The shadow response did not change with la rval age and was very similar in offshore and estuarine water. The min imum percent decrease in light intensity to evoke the descent response ranged from 37-50%. Young larvae descended primarily by passive sinki ng. Older larvae sank upon small decreases in intensity, but actively swam down in response to large intensity decreases. Thus, water type i nduced positional changes in a water column which may reflect the vert ical position necessary for horizontal transport in offshore and estua rine areas by different age larvae. In contrast, the shadow response r emained consistent, which suggests the predator threat is the same in offshore and estuarine areas and does not change with larval age.