Cross-shelf larval fish assemblage structure and trophic niche partitioning, condition, and growth of Sciaenidae larvae in the North-Western Gulf of Mexico

Fluctuations in physical and biological parameters can have a profound influence on larval fish survivorship and recruitment success. The purpose of this dissertation was to investigate how differences in physical and biological parameters along cross-shelf gradients influence larval fish community assemblage structure, niche partitioning, nutritional condition and growth. It was hypothesized that sudden changes in cross-shelf temperature and salinity regimes caused by episodes of strong physical forcing, such as storms, would alter larval fish assemblage structure. To address this overarching hypothesis, Hurricane Harvey (2017) was used as a case study to investigate the effects of a large-scale flood plume on larval fish community assemblage structure in coastal north-western GOM waters. Results indicated a significant increase in sciaenids one month following the storm, with the overall assemblage composition during September more closely resembling assemblages typically found during October. This may have been related to an earlier peak in adult spawning cued by cooler than normal water temperatures following the storm. Within the sciaenids, a total of eight co-occurring species were found across September and October assemblages. To investigate factors which may support the co occurrence of multiple species with similar spatial distribution and phenology, niche partitioning among larval Cynoscion nothus, Larimus fasciatus, and Micropogonias undulatus was compared. It was hypothesized that diet composition would differ between species, development stages, and distance to shore. The study found evidence of differences in intra- and interspecific diet composition, ontogenetic dietary shifts (ODS’s), and trophic interactions differing between nearshore and mid-shelf. It was suggested that the inclusion of alternative food sources like cyclopoid crustaceans and non-copepod prey taxa alleviated potential interspecific competition allowing for larval co-occurrence. Finally, larval fish feeding success, condition, and growth, were investigated and related to changes in cross-shelf physical and biological parameters one and two months following a large-scale disturbance. It was hypothesized that higher plankton density, warmer water temperature, and lower salinity during September, especially nearshore, would be beneficial for larval fish feeding, condition, and growth compared to October. The results indicated temperature as the main driver supporting higher larval growth nearshore during September. Contrary to the overarching hypothesis, salinity had a negligible effect on larval fish assemblage structure. In summary the results of the different studies highlight post-storm temperature regimes as being an important factor influencing differences in assemblage composition and larval survivorship.