Deep Scattering Layer Research Articles

At the base of deep-sea food webs: Assemblage and trophic structure of suprabenthos and zooplankton in submarine canyons

Submarine canyons act as hotspots of biodiversity, hosting vulnerable marine ecosystems, and playing a fundamental role in bridging coastal zones with deeper areas. Here, we investigated the suprabenthic and Deep Scattering Layer (DSL) zooplankton fauna, that play a key role in deep-sea food webs, as main resources for both mobile and sessile megafauna, in two submarine canyons (Squillace and Amendolara) of the Ionian Sea (Central Mediterranean Sea). Our results highlighted different taxonomic and functional diversity between the two adjacent canyons: (i) biomass and abundance of suprabenthos followed an opposite trend in the two canyons, increasing both with depth in Amendolara (higher abundance and biomass in the lower part of the canyon), and decreasing with depth in Squillace (greater in the head of the canyon); (ii) DSL zooplankton abundance and biomass followed a spatial distribution, decreasing with increasing distance from the coast for both canyons (i.e. lower offshore than at the head of the canyon). Food-web structure investigated by means of stable isotope analysis of δ13C and δ15N showed a more diverse trophic niche for suprabenthos than for zooplankton. Furthermore, possible feeding modes of species with unknown feeding behaviour have been proposed. The results of the current article highlight the different ecological processes occurring within each canyon. Understanding the spatial variations of communities inhabiting submarine canyons, especially those at the base of deep-sea food webs which can act as driver of megafaunal communities (both sessile and mobile-commercial species), is essential to focalise future conservation efforts.

Daylight driven vertical migration of mesozooplankton in the Arabian sea and the Bay of Bengal

Diurnal vertical migration (DVM) of mesozooplankton in the Deep Scattering Layer (DSL) of the Indian seas is poorly studied. This cyclical vertical migration substantially controls the carbon sequestration in the ocean. The present research is a comprehensive examination to analyse the factors affecting the DVM pattern of the zooplankton community in the Arabian Sea (AS) and the Bay of Bengal (BoB). Echo sounder profiling was conducted at shallow depths (∼10–400m) of the AS (January 2023) and BoB (March 2023) with a period of 24 h to monitor the DVM pattern of the DSL. Vertical migration in both basins showcased the notable influence of the spatio-temporal contrast in the occurrence of daybreak, with the day (descend) and night (ascend) cycle of the DSL. Delayed descent was observed in the AS contrary to BoB, owing to the delayed day break in the AS relative to BoB. Intensity and temporal pattern of the incoming solar radiation were correlated with the DVM whereas diurnal variation of sea surface temperature was observed to be contrasting. The preliminary analysis is indicative of the diversified community structure of the zooplankton community in these basins resulting from the vertical migration. Furthermore, it is conclusive that the surface residence time of the zooplankton is distinct and is affirmed based on daybreak and light intensity particular for each basin. Since daybreak vary with the geolocation, sole dependence on a particular time for migration study can be erroneous, which is highlighted in the present study.

Intraseasonal variability of the deep scattering layer induced by mesoscale eddy

The deep scattering layer (DSL), a stratum of the marine diel vertical migration (DVM) organisms inhabiting the mesopelagic ocean, plays a crucial role in transporting carbon and nutrients from the surface to depth through the migration of its organisms. Using 18 months of in-situ observations and altimeter sea level data, we reveal for the first time the intraseasonal variations and underlying mechanisms of the DSL and the DVM to the east of the Taiwan Island. Substantial vertical speeds acquired from the Acoustic Doppler Current Profiler were used to examine the distribution and variation of the DVM. Innovatively, the results for the power spectrum analysis of the scattering intensity demonstrated a significant intraseasonal variability (ISV) with an 80-day period in the DSL. Furthermore, the variation in the DVM was closely linked to the DSL and showed an 80-day ISV during the observation. A dynamic relationship between the ISV of the DSL east of Taiwan Island and the westward-propagating mesoscale eddies was established. Anticyclonic (cyclonic) eddy movement toward Taiwan Island triggers downward (upward) bending of the local isotherms, resulting in a layer of DSL warming (cooling) and subsequent upper boundary layer deepening (rising). These findings underscore the substantial influence of mesoscale eddies on biological activity in the mesopelagic ocean, establishing a novel understanding of ISV dynamics in the DSL and their links to eddy-induced processes.

Characterization of pelagic communities in the Pacific sector of the Arctic Ocean using a broadband acoustic system, net samplers, and optical instruments

Rising Arctic temperatures are causing substantial declines in sea ice, altering ice retreat and formation in the Pacific Arctic and impacting marine communities. Despite the significant challenges facing the Pacific Arctic, there are still gaps in our understanding of the environmental impacts on pelagic communities, particularly sound scattering layers (SSLs), and their distributions in the southern Chukchi Sea (SCS), northern Chukchi Sea (NCS), and East Siberian Sea (ESS). This study utilized a wideband autonomous transceiver, net samplers, and optical instruments to explore SSLs in the Pacific Arctic, detailing their relationships with hydrographic properties. The findings indicated a greater vertical distribution of pelagic communities in the SCS than in the NCS and ESS. Significant differences in frequency spectra patterns were observed between the SCS and both the NCS and the ESS, though not between the NCS and the ESS. The correlations between the broadband acoustic and hydrographic values were generally weak to moderate. Elevated acoustic values in the SCS were linked to higher water temperature, dissolved oxygen, and chlorophyll and lower salinity. This study also revealed the behavioral properties of individual pelagic animals and identified Ctenophores and Copepods as the most abundant classes based on camera images and net samples. This research offers crucial insights into the distribution and interactions of pelagic communities with environmental factors, laying the groundwork for understanding climate change impacts. Additionally, this paper presents the first findings of frequency spectra from a broadband system in the Arctic Ocean.

Hydro-acoustic classification and abundance estimation of mesopelagic fish in deep scattering layers (DSL) of the Indian Ocean

Abstract Deep scattering layers (DSL) in oligotrophic systems are typically comprised of a variety of coexisting organisms, including fish, zooplankton, jellyfish, and squid, and hence there is uncertainty about the proportion of the acoustic backscatter in these layers that can be attributed to mesopelagic fish. Here, acoustic targets were classified using a multi-frequency acoustic classification algorithm based on data collected in the Indian Ocean at 18, 38, 70, and 120 kHz during three acoustic surveys in 2018. Frequency-dependent backscattering strength information (∆Sv) was extracted from acoustic data that coincided with trawl hauls dominated by mesopelagic fish. Five ∆Sv frequency pairs were used to separate the acoustic backscattering into three broad scattering categories, i.e. mesopelagic fish, crustacean-like/tunicates, and squids/others. Results indicated that the DSL is highly diverse and dominated by mesopelagic fish, with average densities at a regional scale ranging from 12.0 (±10.9) to 26.0 (±21.7) g m−2, and proportions of the acoustic backscatter attributed to mesopelagic fish ranging from 0.5 to 0.7. These estimates are generally lower than previously estimated for other regions of the Indian Ocean. The situation may well be similar elsewhere, particularly in oligotrophic systems, with potential ramifications for global mesopelagic fish biomass estimates.

Demonstrating the relevance of spatial-functional statistical analysis in marine ecological studies: The case of environmental variations in micronektonic layers

In this study, we conducted an analysis of a multifrequency acoustics dataset acquired from scientific echosounders in the West African water. Our objective was to explore the spatial arrangement of marine organism aggregations. We investigated various attributes of these intricate biological entities, such as thickness, relative density, and depth, in relation to their surroundings. These environmental conditions were represented at a fine scale using a towed multiparameter system. This study is closely intertwined with two key domains: Fisheries acoustics techniques and functional data analysis. Fisheries acoustics techniques facilitate the collection of high-resolution spatial and temporal data concerning marine organisms at various depths and spatial scales, all without causing any disturbance. On the other hand, spatial-functional data analysis is a statistical approach for examining data characterised by functional attributes distributed across a spatial domain. This analysis encompasses dimension reduction techniques, as well as supervised and unsupervised methods, which take into consideration spatial dependencies within extensive datasets.We began by applying multivariate statistical techniques and subsequently employed Functional Data Analysis (FDA). In the modeling section, we introduced the spatial dimension with the spatial coordinates as covariates in the General Additive Model (GAM) and Functional Generalized Spectral Additive Model (FGSAM) models, aiming to underscore its relevance in those contexts. In an exploratory phase, Multivariate Functional Principal Component Analysis provided detailed insights into the variations of parameters at different depths, a capability not offered by traditional Principal Component Analysis. When it came to regression tasks, we explored the interactions between descriptors of Sound Scattering Layers and key environmental variables, both with and without considering spatial dimensions. Our findings revealed significant distinctions between northern and southern Sound Scattering Layers, as well as between coastal and high-sea regions. The use of the spatial locations enhanced the performance of GAM and FGSAM, particularly in the case of salinity, reflecting the influence of water mixing and seawater temperature. The multifaceted effects of environmental variations on Sound Scattering Layers underscore the importance of spatial-functional statistical analysis in ecological studies involving complex, spatially functional objects. Beyond the scope of this specific case study, the application of functional data analysis shows promise for a wide array of ecological studies dealing with extensive spatial datasets.

Acoustic characterization of fish and macroplankton communities in the Seychelles-Chagos Thermocline Ridge of the southwest Indian ocean

In this study, we describe the dynamics of the sound scattering layers (SSLs), particularly those of fish and macroplankton communities in the epipelagic layer, in the Seychelles-Chagos Thermocline Ridge (SCTR) of the southwest Indian Ocean using hydroacoustic data, net sampling, and oceanographic information. Overall, the acoustic backscattering values of the fish community were considerably higher than those of the macroplankton. Both communities were more densely distributed in the SCTR than in its surrounding region. On the vertical profile, the acoustic peak of the fish community was at 17 m during the day; however, considerably high values of up to 82 m were observed at night. Below 26 m, macroplankton was seldom found, regardless of the time. Hydrographic properties, such as temperature, salinity, dissolved oxygen (DO), and chlorophyll fluorescence, in the SCTR, were similar; however, an area at 5–8°S, 67°E was cooler, saltier, and had slightly lower DO. Weak or moderate positive correlations were observed between acoustic and hydrographic features. Based on all net samples, the most abundant taxon in terms of the total number of samples was found to be krill (Euphausiacea, 81%), followed by lantern fish (Myctophum punctatum, 12%). Understanding the dynamics of SSLs, particularly epipelagic organisms, will help to better clarify the important ecological roles of these organisms and their ability to facilitate vertical incorporation into marine food webs.

Vertical structure characterization of acoustically detected zooplankton aggregation: a case study from the Ross Sea

Acoustic data were collected by means of Simrad EK60 scientific echosounder on board the research vessel “Italica” in the Ross Sea during the 2016/2017 austral summer as part of the P-Rose and CELEBeR projects, within the framework of the Italian National Research Program in Antarctica (PNRA). Sampling activities also involved the collection of vertical hydrological profiles using the SBE 9/11plus oceanographic probe. Acoustic data were processed to extract three specific scattering structures linked to Euphausia superba, Euphausia crystallorophias and the so called Sound-Scattering Layers (SSLs; continuous and low-density acoustic structures constituted by different taxa). Four different sectors of the study area were considered: two southern coastal sectors (between the Drygalski Ice Tongue and Coulman Island), a northern sector (~30 nmi East of Cape Hallett) and an offshore one spanning about 2 degrees of latitude from Coulman Island south to the Drygalski Ice Tongue. The vertical structure of each group in each area was then analyzed in relation to the observed environmental conditions. Obtained results highlighted the presence of different vertical structures (both environmental and acoustic) among areas, except for the two southern coastal sectors that were found similar. GAM modelling permitted to evidence specific relationships between the environmental factors and the vertical distribution of the considered acoustic groups, letting to hypothesize the presence of trophic relationships and differences in SSL species composition among areas. The advantages of acoustic techniques to implement opportunistic monitoring strategies in endangered ecosystems are also discussed.

Increased prokaryotic diversity in the Red Sea deep scattering layer

BackgroundThe diel vertical migration (DVM) of fish provides an active transport of labile dissolved organic matter (DOM) to the deep ocean, fueling the metabolism of heterotrophic bacteria and archaea. We studied the impact of DVM on the mesopelagic prokaryotic diversity of the Red Sea focusing on the mesopelagic deep scattering layer (DSL) between 450–600 m.ResultsDespite the general consensus of homogeneous conditions in the mesopelagic layer, we observed variability in physico-chemical variables (oxygen, inorganic nutrients, DOC) in the depth profiles. We also identified distinct seasonal indicator prokaryotes inhabiting the DSL, representing between 2% (in spring) to over 10% (in winter) of total 16S rRNA gene sequences. The dominant indicator groups were Alteromonadales in winter, Vibrionales in spring and Microtrichales in summer. Using multidimensional scaling analysis, the DSL samples showed divergence from the surrounding mesopelagic layers and were distributed according to depth (47% of variance explained). We identified the sources of diversity that contribute to the DSL by analyzing the detailed profiles of spring, where 3 depths were sampled in the mesopelagic. On average, 7% was related to the epipelagic, 34% was common among the other mesopelagic waters and 38% was attributable to the DSL, with 21% of species being unique to this layer.ConclusionsWe conclude that the mesopelagic physico-chemical properties shape a rather uniform prokaryotic community, but that the 200 m deep DSL contributes uniquely and in a high proportion to the diversity of the Red Sea mesopelagic.

Decomposing acoustic signal reveals the pelagic response to a frontal system

The pelagic zone is home to a large diversity of organisms such as macrozooplankton and micronekton (MM), connecting the surface productive waters to the mesopelagic layers (200-1000 m) through diel vertical migrations (DVM). Active acoustics complement net sampling observations by detecting sound-scattering layers (SL) of organisms, allowing to monitor the MM dynamics with a high spatio-temporal resolution. Multi-frequency analyses are a pertinent approach to better integrate the rich diversity of organisms composing SLs and their respective dynamics. However, analysing simultaneously emitted acoustic signals with distinct depth ranges and separating spatial from temporal variability is challenging and needs adapted tools to be fully exploited. This study examines the pelagic realm in a transition zone between the Southern Ocean and the subtropical Indian Ocean, crossing the Saint-Paul and Amsterdam islands’ natural reserve. We extended a Multivariate Functional Principal Component Analysis (mfPCA) to analyse the joint vertical variation of five frequencies from two oceanographic cruises (2016 and 2022), allowing the decomposition of the acoustic dataset into orthogonal vertical modes (VM) of variability. We found the first VM to be linked to the temporal variability due to DVM, while the following majorly depict patterns in spatial distribution. Overall, from the subantarctic to the subtropical zones, we observed (i) enrichment of densities in the surface layer (0–100 m), (ii) a decrease in densities in the intermediate layer during the daytime (100–300 m) and (iii) the apparition of an intensive deep scattering layer on the 38 kHz. We explored VMs’ connection with in-situ environmental conditions by clustering our observations into three distinct environmental-acoustic regions. These regions were compared with vertically integrated nautical area scattering coefficient distribution, a proxy for marine organisms’ biomass. Additionally, we analysed species assemblage changes from complementary cruises to further elucidate the observed acoustic distribution. We show that the mfPCA method is promising to better integrate the pelagic horizontal, vertical and temporal dimensions which is a step towards further investigating the control of the environment on the distribution and structuring of pelagic communities.

Seasonal patterns in the mesopelagic fish community and associated deep scattering layers of an enclosed deep basin

Mesopelagic fish constitute the most abundant vertebrate group in the marine environment. The current work reports on results of three seasonal acoustic cruises carried out in the Gulf of Corinth, a relatively small, deep, isolated basin located in the Central Mediterranean (Greece) that presents some unique geomorphological and ecological features. The aim of this study was to describe seasonal echo-types and the vertical distribution of the Deep Scattering Layers (DSLs) as well as to relate them with specific species or species groups. Mesopelagic fish dominated the pelagic ecosystem as confirmed by biological sampling with different gears during daytime and nighttime. In total, at least 15 species were caught, belonging to the families Myctophidae, Paralepididae, Sternoptychidae and Stomiidae, while the—elsewhere very abundant—families Gonostomatidae and Phosichthyidae were completely absent. Common echo-types included: (a) shoals and schools formed by the silvery lightfish Maurolicus muelleri, usually located along the shelf break (80–225 m), (b) a non-migrant thin DSL found at 150–280 m throughout the deep parts of the Gulf, dominated by juvenile half-naked hatchetfish Argyropelecus hemigymnus, and (c) one thick, partially migratory DSL at 250–600 m, mainly consisting of myctophids. The echo backscatter characteristics and species composition of the DSLs as well as the length distribution of the populations were found to differ seasonally. Species-specific and size related patterns in the vertical distribution of fish were detected both during daytime and nighttime. Overall, the Gulf of Corinth seems to sustain high densities of mesopelagic fish that constitute the basic food resource for the abundant dolphin populations that inhabit the area.

Suspected evening fish chorusing sounds detected in deep waters off Kauai, Hawaii

Noise analyses are crucial to understanding passive acoustic monitoring for marine mammals and their behavioral responses to natural and anthropogenic noises. Suspected biological sounds in the ambient noise data from hydrophones at the Pacific Missile Range Facility (PMRF), Kauai, Hawaii have been found. The sounds are suspected to be chorusing associated with the Deep Scattering Layer (DSL) upward daily vertical migration given they peak around 2 hours after sunset. They have been observed occurring daily including a period in January 2017 when high ambient noise levels during a storm was associated with baleen whale cessation of calling. The sounds occur from 1.4 to 1.8 kHz with the peak near 1650 Hz. They are detectable on multiple broadband hydrophones (21 to 68 km offshore) in deep water (1.5 to 4.7 km), suggesting a large spatial extent. The maximum spectrum level in the 1.6 kHz 1/3rd octave band observed to date was 71 dB re 1 μPa2/Hz. Individual sounds have not been identified to date given the geometries involved and the large spatial extent sensed using deep water hydrophones. These results have similarities to related chorusing research which has suggested a potential link with Myctophidae fishes.

The Ability to Detect Sound Scattering Layers by Interferometric Side-Scan Sonar

The Ability to Detect Sound Scattering Layers by Interferometric Side-Scan Sonar

Software Application for Automatic Detection and Analysis of Biomass in Underwater Videos

The use of underwater recording is widely implemented across different marine ecology studies as a substitute for more invasive techniques. This is the case of the Deep Scattering Layer (DSL), a biomass-rich layer in the ocean located between 400 and 600 m deep. The data processing of underwater videos has usually been carried out manually or targets organisms above a certain size. Marine snow, or macroscopic amorphous aggregates, plays a major role in nutrient cycles and in the supply of organic material for organisms living in the deeper layers of the ocean. Marine snow, therefore, should be taken into account when estimating biomass abundance in the water column. The main objective of this project is to develop a new software application for the automatic detection and analysis of biomass abundance relative to time in underwater videos, taking into consideration small items. The application software is based on a pipeline and client-server architecture, developed in Python and using open source libraries. The software was trained with underwater videos of the DSL recorded with low-cost equipment. A usability study carried out with end-users shows satisfaction with the user-friendly interface and the expected results. The software application developed is capable of automatically detecting small items captured by underwater videos. In addition, it can be easily adapted to a web application.

The Ability to Detect Sound Scattering Layers by Interferometric Side-Scan Sonar

A method and algorithm for remote detection of sound-scattering layers in the seas and oceans using strip survey data of the bottom topography by interferometric side-scan sonar (ISSS) is considered. Based on mathematical modeling of phase-difference measurements of ISSS for multilayer scattering planes in seawater, we demonstrate the possibility of detecting sound-scattering layers and measuring their depths using the proposed algorithm. The accuracy of calculating the depths of sound-dispersing layers at a constant value of the sound velocity has been evaluated and the requirements to the ISSS have been determined to ensure the necessary accuracy of detection of individual layers located at different depths. The efficiency of the proposed method is demonstrated based on the example of processing by the developed algorithm of the experimental data obtained during bottom relief studies with an 85 kHz range ISSS.

Fine-scale vertical relationships between environmental conditions and sound scattering layers in the Southwestern Tropical Atlantic.

Ocean dynamics initiate the structure of nutrient income driving primary producers, and these, in turn, shape the distribution of subsequent trophic levels until the whole pelagic community reflects the physicochemical structure of the ocean. Despite the importance of bottom-up structuring in pelagic ecosystems, fine-scale studies of biophysical interactions along depth are scarce and challenging. To improve our understanding of such relationships, we analyzed the vertical structure of key oceanographic variables along with the distribution of acoustic biomass from multi-frequency acoustic data (38, 70, and 120 kHz) as a reference for pelagic fauna. In addition, we took advantage of species distribution databases collected at the same time to provide further interpretation. The study was performed in the Southwestern Tropical Atlantic of northeast Brazil in spring 2015 and autumn 2017, periods representative of canonical spring and autumn conditions in terms of thermohaline structure and current dynamics. We show that chlorophyll-a, oxygen, current, and stratification are important drivers for the distribution of sound scattering biota but that their relative importance depends on the area, the depth range, and the diel cycle. Prominent sound scattering layers (SSLs) in the epipelagic layer were associated with strong stratification and subsurface chlorophyll-a maximum. In areas where chlorophyll-a maxima were deeper than the peak of stratifications, SSLs were more correlated with stratification than subsurface chlorophyll maxima. Dissolved oxygen seems to be a driver in locations where lower oxygen concentration occurs in the subsurface. Finally, our results suggest that organisms seem to avoid strong currents core. However, future works are needed to better understand the role of currents on the vertical distribution of organisms.

Characterizing the Sound-Scattering Layer and Its Environmental Drivers in the North Equatorial Current of the Central and Western Pacific Ocean

Acoustic technology is an essential tool for detecting marine biological resources and has been widely used in sound-scattering layer (SSL) research. The North Equatorial Current (NEC) warm pool region of the Central and Western Pacific Ocean has a vast distribution of micronekton and zooplankton; analyzing the SSL characteristics in this region is vital for monitoring the marine environment and studying the marine ecosystem. In this study, we statistically analyzed the spatiotemporal factors of 10–200 m SSL in the NEC of the Central and Western Pacific Ocean using acoustic survey data collected by the “Songhang” research vessel (RV) in 2022, and the influence of environmental factors on the scattering layer distribution was analyzed using the Generalized Additive Model (GAM). The results showed that the SSL in the warm pool area of the NEC is distributed in shallow waters above 100 m. The primary scatterers are micronekton and zooplankton, and this SSL had diel vertical migration behavior. By comparing Akaike’s Information Criterion of different GAMs, the model consisting of six factors, namely, temperature, current velocity, turbidity, solar altitude angle, longitude, and latitude, was remarkable. Each model’s factor effects primarily influence the contribution of the volume-backscatter strength (Sv). The cumulative deviation explanation rate of the Sv was 67.2%, among which the highest explanation rate of solar altitude angle variance was 35.4%, the most critical environmental factor. The results of this study can provide a reference for long-term studies on ecological changes and their effects on micronekton and zooplankton distribution.

An initial ecological characterization of mesopelagic fauna at Johnston Atoll and Musicians Seamounts

The mesopelagic region (200–1000 m) hosts a wide variety of organisms in a concentrated layer known as the deep scattering layer (DSL). Much of the mesopelagic region in the central North Pacific remains unexplored, limiting ecosystem considerations in fisheries management and other applications. The National Oceanic and Atmospheric Administration (NOAA) Office of Ocean Exploration and Research used NOAA Ship Okeanos Explorer to conduct two expeditions in the region, one to Johnston Atoll and the other to Musicians Seamounts. During these expeditions, a remotely operated vehicle collected video and CTD environmental data while the ship collected active acoustics. This project analyzed the video, CTD, and acoustic data to create an ecological characterization of the mesopelagic at Johnston Atoll and Musicians Seamounts. The initial characterization included a full inventory of mesopelagic fauna imaged, the relationship between the abundance of categorical groups (gelatinous, Chaetognatha, Crustacea, and fish) and the environmental variables of salinity, oxygen, and temperature, a comparison between the abundance of mesopelagic fauna below, within, and above the DSL, and a qualitative analysis of the spatial distribution of categorical groups. There was a significant negative relationship between the abundance of the gelatinous organisms and salinity and oxygen concentrations. Overall, there were more sightings per minute of all organisms combined below and within the DSL than above the DSL at Musicians Seamounts, while there was no difference in sightings per minute between the below, within, and above DSL groups at Johnston Atoll. Proportionally, there were more gelatinous organisms than any other single category (fish, Chaetognatha, and Crustacea) at all dive sites in the study. These initial characterizations strive to develop biodiversity concepts and the relationships between the environment and fauna. Progress in these areas can aid the management of marine national monuments and serve as a baseline for future ROV dives.

Fish Nutrition at the Underwater Mountains of the Whale Ridge (Southeastern Atlantic). 3. Structure of Fish Accumulations and Fish Feeding

Pelagic accumulations of fish are formed above the underwater mountains of the Whale Ridge in the evening and at night, but not observed during daylight hours. The study of such an accumulation above one of the mountains reveals its structure. The central part of the cluster consists of splendid alfonsino Beryx splendens, silver scabbardfish Lepidopus caudatus and oilfish Ruvettus pretiosus tend to keep on the periphery of the splendid alfonsino accumulation. Previously, it was found that food of splendid alfonsino, silver scabbardfish, rosefish Helicolenus mouchezi, Richardson’s boarfish Pentaceros richardsoni, and Cape bonnetmouth Emmelichthys nitidus consisted of organisms forming sound-scattering layers (SSL) above the underwater mountains. The composition of food and the daily dynamics of feeding of the listed commercial fish species indicate that they use two feeding tactics. In the first case, rosefish, Richardson’s boarfish, and Cape bonnetmouth forage for most of their food during the day at the bottom, when the SSL-forming organisms descend to the top of the underwater mountain during the diel migration. In the second case, splendid alfonsino, silver scabbardfish, and oilfish as part of a structured pelagic aggregation feed at night on the organisms rising into the upper layers of the water and forming sound-scattering layers. This tactic expands the possibilities for feeding fish that form pelagic aggregations in the dark hours of the day and allows the use of organisms both brought by the current to the mountain top and those that descended here earlier during the diel migration and then stayed until the beginning of the evening.

Characterizing the deep sound scattering layer with a moored upward-looking broadband split-beam echosounder

The deep scattering layer (DSL) is a ubiquitous feature of the global ocean. It consists of a large community of mesopelagic organisms which links the marine food web and has recently garnered much interest from commercial fisheries. Such biological communities are inherently coupled with oceanic physical processes such as mesoscale eddies, internal waves and boundary currents. However, very little is known about the physical consequences and biological responses in this coupled system. To address these questions, an upward-looking broadband split-beam echosounder (36kHz – 44 kHz) was moored at ∼580 m depth (bottom depth ∼2790 m) in the New England slope waters in July 2021. The time series analysis focuses on (1) volume scattering strength and spectrum shape and (2) target strength, layer density and compositions, and individual behavior in three dimensions. We demonstrate the seasonal variability in the biomass, distribution, and vertical migration patterns of the deep scattering layer, and quantify the temporal and spatial variability of the DSL under the influence of multiple oceanic physical processes, including warm core rings, semidiurnal internal tides, and near-inertial waves.