Ontogenetic Changes in Behavioral and Energetic Responses of Juvenile Atlantic Croaker to an Estuarine Salinity Gradient

Larval Atlantic Croakers Micropogonias undulatus ingress into estuaries from offshore spawning sites during fall and winter. Larvae and early juveniles migrate up-estuary to oligohaline nurseries where they reside for several months before emigrating seaward. We examined the ontogenetic changes in behavioral salinity preference and avoidance as potential contributors to these movement patterns along the estuarine salinity gradient. Four size-classes (26-40, 41-55, 56-70, and 71-85 mm SL) were acclimated to 10 parts per thousand and 18 degrees C and exposed to horizontal salinity gradients providing five discrete salinity choices: 2, 6, 10, 14, and 18 parts per thousand. Behavioral salinity preference varied ontogenetically: 26-40-mm juveniles avoided salinities <= 10 parts per thousand and preferred higher salinities, 41-55-mm juveniles showed a shift toward lower salinities, and 56-70-mm fish continued this trend by preferring 2 parts per thousand and avoiding 18 parts per thousand. The largest fish tested (71-85 mm) showed no significant preference or avoidance behavior, although they tended to avoid the low and prefer the high salinities that were tested. The preference for higher salinities among early (26-40 mm) juveniles may facilitate orientation in higher-salinity bottom waters by assisting up-estuary immigration to oligohaline nurseries via the residual bottom-layer inflow. The preference for low salinity of 56-70-mm juveniles is consistent with their residence in oligohaline areas. We also conducted feeding and growth experiments, at 18 degrees C, on 30-40-mm and 75-85-mm fish, corresponding to the smallest and largest fish in the salinity preference trials. Smaller juveniles showed no significant energetic advantage in oligohaline versus mesohaline conditions. However, larger juveniles grew significantly faster at 18 parts per thousand than at 2 parts per thousand, indicating that down-estuary movement during summer and fall improves growth capacity later in the nursery season. The increased precipitation and river discharge that is predicted from climate change and its associated stronger net up-estuary flow of saline bottom waters may facilitate the up-estuary immigration of early juvenile Atlantic Croakers through their attraction to higher salinity, as is demonstrated by the present work.

Automated summary

Behavioral salinity preference and avoidance of larval Atlantic Croakers varied ontogenetically along the estuarine salinity gradient, potentially aiding in their migration to oligohaline nurseries. Larger juveniles showed faster growth at higher salinities, indicating a possible advantage for down-estuary movement later in the nursery season. The study suggests that climate change may affect the movement patterns of early juvenile Atlantic Croakers by influencing their attraction to higher salinities.