The oceanic Gulf is a unique ecoregion recognized for its diverse assemblage of mesopelagic fishes. These fishes, which exhibit specialized morphological traits and behaviors, play a crucial role in the region’s food web dynamics. While the Gulf is well studied in terms of mesopelagic diversity and abundance, there is a limited understanding of how these patterns change geographically, particularly in relation to the outer continental shelf. The oceanic rim, described here as a transitional zone over the upper slope (to 1000 - 1500 m water depth), provides an ecotone in which pelagic assemblages may vary from ‘typical’ open-ocean assemblages. This region is also an important one for economically valued predators (e.g., tunas), many of whom may rely on mesopelagic prey. This study investigates whether proximity to the shelf break influences the faunal composition, abundance, and diversity of mesopelagic fishes in the northern Gulf. Specimens were collected using a 10-m2 Multiple Opening/Closing Net and Environmental Sensing System at discrete-depth intervals twice a day. Stations were categorized into hypothesis groups (rim vs. offshore) based on abiotic factors such as distance to isobaths and bottom depth. A permutational multivariate analysis of variance (PERMANOVA+) was used to calculate dissimilarity between stations. Where significant differences occurred, Hill-Shannon diversity values and K-dominance curves were generated to assess species diversity and abundance distribution. Results revealed a statistical difference in assemblage structure between the classified rim and offshore stations, with generally higher diversity observed at the offshore stations and the disappearance of several deep-pelagic taxa at the rim stations that are dominant offshore, (e.g., Cyclothone obscura).