Meeting Abstract
Mesopelagic fauna are known to follow distinct vertical distribution patterns, ubiquitously concentrating in one or more layers between 300-1000 meters throughout the open ocean. This high density of midwater fauna, known as the deep scattering layer (DSL), consists of a variety of hard and soft-bodied organisms that contribute to the DSL’s characteristic acoustic backscatter. Most DSL backscatter is attributed to midwater fish; however, gelatinous scatterers like gas-containing physonect siphonophores also play a role. Exploratory remotely operated vehicle (ROV) transects of the midwater environment allow us to gain insight into the distribution of a wider breadth of midwater organisms than traditional trawling studies, which struggle to recover delicate gelatinous fauna. Exploratory midwater transect video footage and active acoustic data collected during the Windows to the Deep 2019 expedition (EX1903), which focused on the EEZ off the Southeastern coast of the United States, were used to study the distribution of mesopelagic taxa relative to the DSL and quantify the ROV avoidance of midwater taxa during visual surveys. Sighting frequencies of most groups of mesopelagic fauna, with the exception of physonect siphonophores, were greater during transects within the DSL compared to transects outside of it. There was evidence of ROV avoidance on several transects, determined by measuring local acoustic backscatter before and after transects occurred. Quantification of ROV avoidance is crucial to further contextualizing visual surveys, but data collected during EX1903 points towards faunal distribution patterns that aid in our understanding of community structure in this enigmatic ecosystem.
