The importance of time of day in structuring demersal ichthyofaunal assemblages on the West Florida Shelf

Although turnover of reef-fish assemblages based on time of day has been generally well documented, much less is known regarding diel dynamics of demersal fish communities along the continental shelf of the US Gulf of Mexico. We analyzed trawl data collected as part of the Gulf of Mexico Southeast Area Monitoring and Assessment Program (SEAMAP) to document diel changes in fish abundance and assess the relative importance of space and time of day in structuring demersal fish assemblages. Trawling operations were conducted day and night in neritic waters from southern Florida to eastern Alabama (West Florida Shelf). Ichthyofaunal assemblages differed by region and depth, but also by time of day (day, night, crepuscular) within region and depth categories. Both fish abundance and diversity were generally highest at night. Some day/night differences in abundance for particular species can be attributed to specific behaviors, such as nighttime foraging in open areas coupled with cryptic habits (e.g., burrowing, shelter-seeking). Prominent examples of such behavior-associated abundance patterns include cusk-eels (Ophidiidae), which are burrowers, and grunts (Haemulidae), which are closely associated with structural habitats during daytime. We document these strong diel trends in the SEAMAP trawl data and interpret them based on current understanding of the behavior of fish species and families and in relation to the development of ecosystem models for management purposes.