Cross-shore, seasonal, and depth-related structure of ichthyoplankton assemblages in coastal alabama

Investigations of the spatial and temporal structure of larval fish assemblages are essential for a better understanding of the dynamics of fish populations and their resilience toenvironmental change. This study provides an original typology of the spatial, seasonal, and depth-related structure of ichthyoplankton assemblages collected along a 77-km cross-shore gradient in Alabama coastal waters. This typology is based on a depth-discrete ichthyoplankton survey conducted across multiple years at a high spatial and temporal resolution in the northern Gulf of Mexico. A total of 350,766 larvae were collected, among which > 95% could be identified to families. The total density of fish larvae was significantly higher inshore, whereas the number of families increased offshore. Multivariate regression trees and Dufrêne–Legendre indices were used toidentify seven assemblages dominated by different families of larvae. These assemblages were primarily structured by distance from the shore, followed by season and depth, and were associated with different water masses characterized by distinct temperature and salinity conditions. Families Clupeidae, Bregmacerotidae, Synodontidae, Scombridae, and Ophidiidae were typical offshore, whereas families Engraulidae, Gobiidae, and Gobiesocidae were typical inshore. These observed spatial distributions likely reflected interactions between adult spawning behaviors and oceanographic processes, in particular the influence of the Mobile River. Our results thus confirm existing lines of evidence suggesting that riverine influences play a major role in fish population dynamics along the Alabama inner shelf. For many families, the observed seasonal distributions were largely consistent with the results of previous studies conducted at smaller spatial resolution in the area. However, our large-scale, high-resolution, cross-shore design clearly improved the detection of seasonal variations for inshore and offshore taxa otherwise rarely collected.