Estimating Zooplankton Biomass Distribution in the Water Column Near the Endeavour Segment of Juan de Fuca Ridge Using Acoustic Backscatter and Concurrently Towed Nets

Logistical challenges, time, and the cost of towed net surveys make it difficult to obtain estimates of secondary biomass and production in the open ocean outside the summer sampling season. Alternate approaches are sometimes needed. This study examines the relationship between biomass obtained from 197 mixedspecies zooplankton net samples and proximate acoustic backscatter data collected over six summers, a depth range of 3,000 m, and a spatial scale of 200 km centered at a major hydrothermal region in the Northeast Pacific Ocean. Results show that the acoustic backscatter data from a single-frequency (150 kHz) acoustic Doppler current profiler mounted near the opening of the towed net system accounts for 84% of the variance in total net biomass, despite the remarkable mix of faunal types and depth range, and the broad spatial and temporal extent of the study. We discuss the potential reasons for the minor remaining variance in net biomass. The present findings demonstrate that profiling or moored acoustic backscatter instrumentation can provide a less-challenging methodology than net tows for obtaining bulk estimates of deep-sea zooplankton biomass in the open waters of the Northeast Pacific.