Photoresponses of larval Atlantic menhaden ( Brevoortia tyrannus Latrobe) in offshore and estuarine waters: implications for transport

Abstract

This study tested the hypothesis that Atlantic menhaden ( Brevoortia tyrannus Latrobe) larvae have separate behaviors in offshore and estuarine waters that are evoked by chemical cues in these waters. Atlantic menhaden spawn offshore, and larvae are transported shoreward where they metamorphose after entering estuaries. Field studies suggest that during the day larvae are at moderate depths offshore and less abundant in the water column than at night in estuaries. Since photoresponse may contribute to these depth distributions, they were tested in the laboratory in a light field that mimicked the underwater angular light distribution. Responses were quantified as the proportion of larvae swimming in the upper portion of a water column containing either offshore or estuarine water at the same salinity and temperature. Swimming varied significantly with water type. Young larvae (< 8 mm total length (TL)) swam near the surface in estuarine water and lower in offshore water. Older larvae (14–16 mm TL) showed the opposite responses. Since the test waters only differed in chemical composition, the results supported the hypothesis. In addition, the study characterized the predator avoidance shadow response, in which larvae descend in response to a decrease in light intensity. The shadow response did not change with larval age and was very similar in offshore and estuarine water. The minimum percent decrease in light intensity to evoke the descent response ranged from 37–50%. Young larvae descended primarily by passive sinking. Older larvae sank upon small decreases in intensity, but actively swam down in response to large intensity decreases. Thus, water type induced positional changes in a water column which may reflect the vertical position necessary for horizontal transport in offshore and estuarine areas by different age larvae. In contrast, the shadow response remained consistent, which suggests the predator threat is the same in offshore and estuarine areas and does not change with larval age.