Prey in oceanic sound scattering layers organize to get a little help from their friends

Abstract

Group formation in animals is a widespread phenomenon driven by food acquisition, reproduction, and defense. Life in the ocean is characteristically aggregated into horizontally extensive layers as a result of strong vertical gradients in the environment. Each day, animals in high biomass aggregations called "deep scattering layers" migrate vertically, comprising the largest net animal movement on earth. This movement is commonly thought of as a predator avoidance tactic, however, the aggregation of animals into layers has been viewed as an incidental outcome of similar responses by many individuals to the risk of visual predation coupled with the location of resources including food and oxygen rather than active, socially mediated congregation for defense purposes. Here, using a newly adapted autonomous vehicle to measure individual characteristics, we provide the first measures of the internal layer structure, demonstrating that these features are made up of many topologically scaled, mono-specific aggregations, or "schools" rather than an indiscriminate mix of sizes and species. Schools responded to predators using behavior much like flash compression while neighboring aggregations increased their spacing to maintain coherent layers. Rather than simply an incidental outcome, the formation of layers of life in the sea is a highly organized process driven, at least in part, by biotic pressures for cohesion with broad adaptive significance for the myriad species that inhabit these ubiquitous features. These observations highlight the range of spatial scales we must examine in order to understand the strong impacts these high-biomass layers have on ecological and biogeochemical processes in the sea.

Diving Behavior and Fine-Scale Kinematics of Free-Ranging Risso's Dolphins Foraging in Shallow and Deep-Water Habitats

“…The dominant composition of layers identified from the active acoustics was determined using a net towed at relatively high speed (1-2.2 m s −1 ) that captured mobile organisms between 1 and 35 cm body length. This catch matched the composition identified acoustically (Benoit-Bird et al, 2017). Sampling of the deep-water habitat of the dolphins was coincident in space and time with tagging of two of the 33 dolphins (tag id gg13_266b and gg13_267a, Table 1).…”

“…This allowed individual animals to be observed within scattering features, whereas the ship-based sensors provided a view of entire features. Comprehensive explanation of the sensors and platforms used is provided by Moline et al (2015) while Benoit-Bird et al (2017) provide a detailed description of biological sampling and active acoustics methodology. Briefly, acoustic scattering data were processed using Echoview and individual prey detected from the AUV within scattering features were identified as single targets (Sawada et al, 1993), providing measurements of target strength at two frequencies.…”

“…Each prey layer was horizontally identified as follows: Shallow Figure 2 in Arranz et al, 2018). The shallow layer was dominated by larval fish and small crustaceans, the midwater layer by myctophids and krill, and the deep layer primarily by dragonfish, squid, shrimp, and large krill (Benoit-Bird et al, 2017). Scattered patches were not directly sampled to confirm potential composition but echo characteristics were consistent with small schooling fish.…”

Equipping an underwater glider with a new echosounder to explore ocean ecosystems

“…As a result, these biases are effectively averaged over the glider's entire dive range rather than being strictly depth‐dependent as in a fixed‐depth deployment such as on vessels or moorings. This can have substantial effects on our identification or characterization of some acoustic features (Moline et al ; Benoit‐Bird et al ).…”

Towards a macroscope: Leveraging technology to transform the breadth, scale and resolution of macroecological data

“…Sonar surveys can have global reach (Irigoien et al, 2014); however, the cost and limited temporal resolution of ship-based surveys have also led to the installation of sonars in fixed observatories (Ross, Keister, & Lara-Lopez, 2013) and AUVs (Dunlop et al, 2018). Close-in acoustic sampling with robotic vehicles enhances the possibility of distinguishing individual organisms and resolving species from echo returns, potentially enabling community structure to be inferred as a function of time (Benoit- Bird, Moline, & Southall, 2017).…”