Long-term measurements of fish backscatter from Saildrone unmanned surface vehicles and comparison with observations from a noise-reduced research vessel

Abstract

Two Saildrone unmanned surface vehicles (USVs) were instrumented with echosounders and deployed in the Bering Sea to make acoustic observations of walleye pollock for 103 days. The Saildrones proved to be a suitable platform for measurement of fish backscatter: they produced high-quality measurements at wind speeds of <10 m s−1. Pollock backscatter measured from the Saildrones was compared to backscatter measured by a noise-reduced research vessel during two “follow-the-leader” comparisons. In a location where pollock were shallowly distributed (30–100 m), there was evidence of depth-dependent avoidance reactions to the ship. This behaviour was not evident in a second comparison, where the fish were primarily deeper than 90 m. Opportunistic comparisons indicate that backscatter where the ship and USVs crossed paths was similar. However, the Saildrones observed higher densities of shallow fish, which is consistent with the diving response inferred in the first follow-the-leader comparison. USVs equipped with echosounders, like all platforms, have inherent strengths (endurance) and limitations (species identification) that should be carefully considered for a given application. USVs can complement traditional ship-based surveys by increasing the spatial and temporal extent of acoustic observations, and their use is likely to become more widespread.

Detecting Mesopelagic Organisms Using Biogeochemical‐Argo Floats

“…The first two hypotheses could explain why the depths are not consistent but not the presence of a consistent bias. The third hypothesis is unlikely as, to our knowledge, no deepening of scattering layers at such depths was reported (De Robertis & Handegard, 2013;De Robertis et al, 2019;Ona et al, 2007;Peña, 2019). The fourth hypothesis cannot be rejected and is discussed further below.…”

Test of unmanned surface vehicles to conduct remote focal follow studies of a marine predator

“…In the study region, acoustic backscatter is dominated by scattering from pollock (De Robertis et al 2019), the northern fur seal's primary prey when foraging on the Bering Shelf (Zeppelin & Ream 2006). Acoustic-trawl surveys conducted from a research vessel in July 2016 confirmed that acoustic backscatter could be primarily attributed to pollock (De Robertis et al 2019).…”

“…This feasibility study occurred during a largescale, ecosystem-based research mission to map walleye pollock Gadus chalcogrammus ('pollock') abundance and depth distribution in the fur seal's foraging range (Mordy et al 2017, De Robertis et al 2019. The aim was to follow an individual for 2 d and collect data on oceanographic conditions and prey availability along the foraging path.…”

“…During the Saildrone deployments, 19 (2016) and 17 (2017) oceanographic and atmospheric parameters were measured (see Supplement). Echosounder data, used to measure fish depth distribution and abundance, were processed using methods developed specifically for Saildrone data (De Robertis et al 2019). In the study region, acoustic backscatter is dominated by scattering from pollock (De Robertis et al 2019), the northern fur seal's primary prey when foraging on the Bering Shelf (Zeppelin & Ream 2006).…”

“…Recently, unmanned wind-and solar-powered surface vehicles (USVs), called Saildrones (Saildrone Inc.), have successfully demonstrated their ability to conduct long-duration missions (>100 d), collecting data on oceanographic and atmospheric conditions and mapping fish depth distribution and abundance using a scientific echosounder (Cokelet et al 2015, Mordy et al 2017, De Robertis et al 2019. We tested the feasibility of using Saildrones to conduct remote focal follow studies of northern fur seals Callorhinus ursinus over long distances (>100 km) and multiple days, to gain information about relationships between northern fur seals and their prey.…”