Sound Scattering Layers

As sound energy propagates through the sea, sound spreading, absorption, reflection, refraction, and scattering can impact the direction of underwater sound propagation and/or cause signal attenuation. In particular, scattering occurs when an underwater sound strikes inhomogeneities, such as the uneven seafloor, sea surface, and objects in the water column. Some of the sound may reflect back to the sound source. This is called backscatter.

Some backscattered energy is known as reverberation. Reverberation is the total of all scattering received at a particular location. When using active sonar, reverberation can be a limiting factor in the detection of the echo signal, as the reverberation can be more intense than the returning echo and the ambient noise (see sonar equation page). Reverberation alters the characteristics of the original sound signal, and it tends to persist over time intervals longer than the duration of the original signal.

Reverberation is divided into three classes: volume, sea-surface, and bottom[1]Urick, R. J. (1983). Principles of Underwater Sound, Third Edition (3rd edition, Reprint 2013). McGraw-Hill, Inc.. This section focuses on volume reverberation. For more information on surface and bottom reverberation, please visit the Science of Sound page on Reverberation. Volume reverberation is caused by various reflectors, most notably marine life. The amount of scattering is affected by the size, density, and concentration of the objects in the sound path.

Target Strength: Reverberation results in acoustic energy scattering in a variety of directions, including reflecting back toward the sonar (backscatter). However, specific objects, such as fish or submarines, create echoes that can be distinctively identified from the reverberation. Measures of the reflected acoustic energy of an object (target strength) can provide information about its size, shape, and composition. One example is fisheries scientists using echosounders and their knowledge of the target strength of fish species for research and management.

Volume reverberation usually decreases with increasing ocean depth, because at greater depth fewer particles exist and density gradients and other inhomogeneities are reduced. One exception is the Deep Scattering Layer (DSL), a concentrated layer of marine organisms that creates strong scattering that can sometimes resemble scattering from a surface, such as the seafloor.

The DSL (used interchangeably with sound scattering layer (SSL)) was originally described in the Pacific Ocean in the early 1940’s during World War II. During submarine detection exercises, broad zones of acoustic backscattering were recorded, confounding underwater acoustic experiments. Initially, sonar analysts mistakenly interpreted the echoes to be caused by the seafloor. However, other data suggested this sonar-detected ‘seafloor’ to be much shallower than that shown in maps and recorded via direct depth measurements. Ships sitting in one place also found the sonar-detected seafloor depth to change throughout the course of a day. Scientific observations then revealed backscattering from horizontal layers at depth during the day. These initial observations found that whatever was causing the backscattering rose towards the surface at nightfall and migrated back to depth at dawn[2]Eyring, C. F., Christensen, R. J., & Raitt, R. W. (1948). Reverberation in the Sea. The Journal of the Acoustical Society of America, 20(4), 462–475. https://doi.org/10.1121/1.1906399.[3]Hersey, J. B., & Backus, R. H. (1954). New evidence that migrating gas bubbles, probably the swimbladders of fish, are largely responsible for scattering layers on the continental rise south of New England. Deep Sea Research (1953), 1(3), 190–191. https://doi.org/10.1016/0146-6313(54)90050-7..

Backscatter returns from an echosounder on a stationary vessel in the Gulf of California. Three different scattering layers are shown, comprised of a mix of small squid, krill, myctophid fishes, and/or other organisms. Movements of predators, including sperm whales and larger squid, can also be seen in the echosounder output. Modified with permission from the Annual Review of Marine Science, Volume 8 © 2016 by Annual Reviews, https://www.annualreviews.org.

DSLs are found across the global ocean[4]Dietz, R. S. (1948). Deep scattering layer in the Pacific and Antarctic Oceans. Journal of Marine Research, 7(3). https://elischolar.library.yale.edu/journal_of_marine_research/680[5]Hersey, J. B., & Moore, H. B. (1948). Progress report on scattering layer observations in the Atlantic Ocean. Transactions, American Geophysical Union, 29(3), 341. https://doi.org/10.1029/TR029i003p00341[6]Chapman, R. P., & Marshall, J. R. (1966). Reverberation from Deep Scattering Layers in the Western North Atlantic. The Journal of the Acoustical Society of America, 40(2), 405–411. https://doi.org/10.1121/1.1910087[7]Chapman, R. P., Bluy, O. Z., Adlington, R. H., & Robison, A. E. (1974). Deep scattering layer spectra in the Atlantic and Pacific Oceans and adjacent seas. The Journal of the Acoustical Society of America, 56(6), 1722–1734. https://doi.org/10.1121/1.1903504[8]Urick, R. J. (1983). Principles of Underwater Sound, Third Edition (3rd edition, Reprint 2013). McGraw-Hill, Inc.[9]Ariza, A., Landeira, J. M., Escánez, A., Wienerroither, R., Aguilar De Soto, N., Røstad, A., Kaartvedt, S., & Hernández-León, S. (2016). Vertical distribution, composition and migratory patterns of acoustic scattering layers in the Canary Islands. Journal of Marine Systems, 157, 82–91. https://doi.org/10.1016/j.jmarsys.2016.01.004[10]Boersch-Supan, P. H., Rogers, A. D., & Brierley, A. S. (2017). The distribution of pelagic sound scattering layers across the southwest Indian Ocean. Deep Sea Research Part II: Topical Studies in Oceanography, 136, 108–121. https://doi.org/10.1016/j.dsr2.2015.06.023[11]Gjøsæter, H., Wiebe, P. H., Knutsen, T., & Ingvaldsen, R. B. (2017). Evidence of Diel Vertical Migration of Mesopelagic Sound-Scattering Organisms in the Arctic. Frontiers in Marine Science, 4, 332. https://doi.org/10.3389/fmars.2017.00332[12]Blanluet, A., Doray, M., Berger, L., Romagnan, J.-B., Le Bouffant, N., Lehuta, S., & Petitgas, P. (2019). Characterization of sound scattering layers in the Bay of Biscay using broadband acoustics, nets and video. PLOS ONE, 14(10), e0223618. https://doi.org/10.1371/journal.pone.0223618. There is often more than one SSL in a location, the depths of which vary globally between 200 and 1000 m. Scattering layer depth is influenced by environmental conditions such as light intensity, oxygen concentration, water temperature, and mixing.

Scattering layers typically rise at sunset and descend at sunrise (diel vertical migration, DVM), remaining at constant depths during daytime and nighttime hours. Volume reverberation associated with DSLs reduces during the night as the animals that comprise the layers migrate to the surface[13]Dietz, R. S. (1962). The Sea’s Deep Scattering Layers. Scientific American, 207(2), 44–51.The distribution of species within some scattering layers may not be random, but rather, may be aggregates of similar organisms[14]Benoit-Bird, K. J., Moline, M. A., & Southall, B. L. (2017). Prey in oceanic sound scattering layers organize to get a little help from their friends: Schooling within sound scattering layers. Limnology and Oceanography, 62(6), 2788–2798. https://doi.org/10.1002/lno.10606.

Mesopelagic myctophid fishes, such as this lanternfish, are a major component of sound scattering layers in the ocean. Image credit: NOAA.

Scientists initially suspected the DSL to be biological in nature, first suggesting fish and their gas-filled swim bladders to be the scatterers[15]Marshall, N. B. (1951). Bathypelagic fishes as sound scatterers in the ocean. Journal of Marine Research, 10(1). https://elischolar.library.yale.edu/journal_of_marine_research/733[16]Hersey, J. B., & Backus, R. H. (1954). New evidence that migrating gas bubbles, probably the swimbladders of fish, are largely responsible for scattering layers on the continental rise south of New England. Deep Sea Research (1953), 1(3), 190–191. https://doi.org/10.1016/0146-6313(54)90050-7. However, it is now known that discrete aggregations of diverse marine species with soft bodies or other structures that resonate when ensonified (e.g., swim bladders, gas bubbles, and hard shells) produce the distinct echoes associated with scattering layers. Plankton, mesopelagic fish (esp. myctophid fishes), other fish larvae or juveniles, pelagic invertebrates (e.g., krill and pteropods) and gelatinous creatures (e.g., siphonophores, jellyfish) can be found in scattering layers and are often detected with sonars at higher frequencies[17]Moore, H. B. (1950). The relation between the scattering layer and the Euphausiacea. The Biological Bulletin, 99(2), 181–212. https://doi.org/10.2307/1538738[18]Barham, E. G. (1966). Deep Scattering Layer Migration and Composition: Observations from a Diving Saucer. Science, 151(3716), 1399–1403. https://doi.org/10.1126/science.151.3716.1399[19]Dietz, R. S. (1962). The Sea’s Deep Scattering Layers. Scientific American, 207(2), 44–51[20]Holliday, D. V., & Pieper, R. E. (1980). Volume scattering strengths and zooplankton distributions at acoustic frequencies between 0.5 and 3 MHz. The Journal of the Acoustical Society of America, 67(1), 135–146. https://doi.org/10.1121/1.384472[21]Green, C. H., Widder, E. A., Youngbluth, M. J., Tamse, A., & Johnson, G. E. (1992). The migration behavior, fine structure, and bioluminescent activity of krill sound-scattering layers. Limnology and Oceanography, 37(3), 650–658. https://doi.org/10.4319/lo.1992.37.3.0650[22]Davison, P. C., Koslow, J. A., & Kloser, R. J. (2015). Acoustic biomass estimation of mesopelagic fish: Backscattering from individuals, populations, and communities. ICES Journal of Marine Science, 72(5), 1413–1424. https://doi.org/10.1093/icesjms/fsv023.

Understanding the structure and timing of scattering layers is important for locating and tracking underwater targets with active sonar. Submarines may hide within the DSL. Active sonar signals may not penetrate the layer to detect the target of interest (e.g., the submarine), and/or returning signals may be too weak to discern targets below the scattering layer.

Defense applications are a key motivation for studying scattering layers. However, much scientific research is also motivated by understanding the composition and movements of scattering layers, especially when considering the biogeochemical and ecological impacts associated with DVM. Daytime DSL depth, which determines migration extent, varies across the global ocean, and is influenced by water mass properties such as dissolved oxygen and/or light levels. Acoustic data show downward migrations, which precede sunrise, happen at a faster rate than upward migrations prior to sunset[23]Bianchi, D., & Mislan, K. A. S. (2016). Global patterns of diel vertical migration times and velocities from acoustic data: Global patterns of diel vertical migration. Limnology and Oceanography, 61(1), 353–364. https://doi.org/10.1002/lno.10219. Not all animals migrate, and species within a layer may migrate at different times[24]Dietz, R. S. (1962). The Sea’s Deep Scattering Layers. Scientific American, 207(2), 44–51[25]De Robertis, A. (2002). Size-dependent visual predation risk and the timing of vertical migration: An optimization model. Limnology and Oceanography, 47(4), 925–933. https://doi.org/10.4319/lo.2002.47.4.0925[26]Benoit‐Bird, K. J., & Moline, M. A. (2021). Vertical migration timing illuminates the importance of visual and nonvisual predation pressure in the mesopelagic zone. Limnology and Oceanography, 66(8), 3010–3019. https://doi.org/10.1002/lno.11855. Vertical migration is also dynamic over a wide range of time scales. In some ecosystems, animals cease migration seasonally[27]Bandara, K., Varpe, Ø., Wijewardene, L., Tverberg, V., & Eiane, K. (2021). Two hundred years of zooplankton vertical migration research. Biological Reviews, 96(4), 1547–1589. https://doi.org/10.1111/brv.12715. Scientists have also observed scattering layers to restructure within seconds in response to the presence of predators. In California, sound scattering layers were found to be compressed and abruptly increased their depth during bouts of echolocating clicks produced by Pacific white-sided dolphins and Risso’s dolphins[28]Benoit-Bird, K. J., & Au, W. W. L. (2004). Diel migration dynamics of an island-associated sound-scattering layer. Deep Sea Research Part I: Oceanographic Research Papers, 51(5), 707–719. https://doi.org/10.1016/j.dsr.2004.01.004[29]Benoit-Bird, K. J., Moline, M. A., & Southall, B. L. (2017). Prey in oceanic sound scattering layers organize to get a little help from their friends: Schooling within sound scattering layers. Limnology and Oceanography, 62(6), 2788–2798. https://doi.org/10.1002/lno.10606.

Echograms showing four deep scattering layers (DSL) in waters near the Canary Islands. The scattering layers were observed from 400 to 1000 m, with migrating and non-migrating layers attributed to different fish and decapod species. Species displayed different migration speeds (thick black and red dashes), with short (400 m) and long-range (700 m) migration rates estimated at 5 and 12 cm s− 1, respectively. Reprinted with permission from Ariza, A., Landeira, J. M., Escánez, A., Wienerroither, R., Aguilar De Soto, N., Røstad, A., Kaartvedt, S., & Hernández-León, S. (2016). Vertical distribution, composition and migratory patterns of acoustic scattering layers in the Canary Islands. Journal of Marine Systems, 157, 82–91. https://doi.org/10.1016/j.jmarsys.2016.01.004.

DVM provides a critical energy link between surface waters and the deep ocean. Vertically migrating marine animals transport carbon away from the surface to the deep ocean (biological carbon pump). This is important in regulating global climate. The mesopelagic zone (200 to 1,000 meters beneath the ocean surface) is also one of the Earth’s largest habitats and includes an estimated 11–15 gigatons (Gt) of fish biomass[30]Irigoien, X., Klevjer, T. A., Røstad, A., Martinez, U., Boyra, G., Acuña, J. L., Bode, A., Echevarria, F., Gonzalez-Gordillo, J. I., Hernandez-Leon, S., Agusti, S., Aksnes, D. L., Duarte, C. M., & Kaartvedt, S. (2014). Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nature Communications, 5(1), 3271. https://doi.org/10.1038/ncomms4271. Many of these species are commercially important as well as being food sources for marine animals. Understanding DSL distributions and properties are therefore important in understanding the structure and function of pelagic ecosystems, behavior and distribution of highly migratory marine predators (e.g. tuna, whales, and sharks), predator-prey interactions, and fisheries productivity.

Biological Pump: Phytoplankton convert inorganic carbon, as carbon dioxide (CO2), into organic matter through photosynthesis. A small part of this organic matter moves into the deep ocean through one of several processes: particulate sinking, vertical migration, or transport by downwelling water. The transport of carbon to the deep ocean ultimately removes and isolates (sequesters) CO2 from the atmosphere and is an important part of the carbon cycle and the global climate system.

DVM provides a critical energy link between surface waters and the deep ocean. Vertically migrating marine animals transport carbon away from the surface to the deep ocean (biological carbon pump). This is important in regulating global climate. The mesopelagic zone (200 to 1,000 meters beneath the ocean surface) is also one of the Earth’s largest habitats and includes an estimated 11–15 gigatons (Gt) of fish biomass[31]Irigoien, X., Klevjer, T. A., Røstad, A., Martinez, U., Boyra, G., Acuña, J. L., Bode, A., Echevarria, F., Gonzalez-Gordillo, J. I., Hernandez-Leon, S., Agusti, S., Aksnes, D. L., Duarte, C. M., & Kaartvedt, S. (2014). Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nature Communications, 5(1), 3271. https://doi.org/10.1038/ncomms4271. Many of these species are commercially important as well as being food sources for marine animals. Understanding DSL distributions and properties are therefore important in understanding the structure and function of pelagic ecosystems, behavior and distribution of highly migratory marine predators (e.g. tuna, whales, and sharks), predator-prey interactions, and fisheries productivity.

To better understand sound scattering layers, scientists use a variety of techniques, many of which include active and passive acoustics. Early research utilized explosive sound sources and hydrophones[32]Hersey, J. B., Backus, R. H., & Hellwig, J. (1961). Sound-scattering spectra of deep scattering layers in the western North Atlantic Ocean. Deep Sea Research (1953), 8(3–4), 196–210. https://doi.org/10.1016/0146-6313(61)90021-1.. Modern day efforts largely involve narrowband and/or broadband acoustics. Echosounders (single beam, split-beam, multibeam) can be ship-based, deployed upward-facing on the seafloor, incorporated into acoustic moorings and cabled observatories, and integrated into remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs). Collecting acoustic data using echosounders carried by vehicles capable of diving directly into scattering layers greatly increases the spatial resolution needed to distinguish individual animals in a layer. However, marine animals causing acoustic backscatter are difficult to identify via acoustic outputs alone. Studies to verify acoustic data with net samples and video observations are often necessary.

Top: diagram showing collaborative operations of the LRAUV Makai, Wave Glider (surface vehicle), Tiny, and the seafloor-mounted EK60 echosounder. Bottom: Echosounder data showing Makai’s sampling stations (white arrows) within seven different depth bins per mission (white arrows) The returns next to the white arrows are associated with the AUV, and are not part of the SSL. Zhang et al. 2020- Figures 2 and 9. Images CC BY 4.0.

New approaches are being developed to further enhance acoustic observations with direct samples from the DSL. In one study, an AUV and an autonomous surface vehicle (ASV) were deployed in tandem over a seabed-mounted, upward-facing echosounder. The AUV collected and processed water samples within each scattering layer; real-time data received from the bottom-mounted echosounder determined the depth at which the AUV would sample. The AUV maintained its position within the seafloor-mounted echosounder beam by acoustically tracking the station-keeping ASV. Water samples collected by the AUV were analyzed for eDNA, which enabled the identification of animals present at precise locations within the water column[33]Zhang, Y., Kieft, B., Hobson, B. W., Raanan, B.-Y., Urmy, S. S., Pitz, K. J., Preston, C. M., Roman, B., Benoit-Bird, K. J., Birch, J. M., Chavez, F. P., & Scholin, C. A. (2021). Persistent Sampling of Vertically Migrating Biological Layers by an Autonomous Underwater Vehicle Within the Beam of a Seabed-Mounted Echosounder. IEEE Journal of Oceanic Engineering, 46(2), 497–508. https://doi.org/10.1109/JOE.2020.2982811.

Additional Links on DOSITS

References

  • Barham, E. G. (1963). Siphonophores and the Deep Scattering Layer. Science, 140(3568), 826–828.
  • Benoit-Bird, K. J., & Au, W. W. L. (2009). Cooperative prey herding by the pelagic dolphin, Stenella longirostris. The Journal of the Acoustical Society of America, 125(1), 125–137. https://doi.org/10.1121/1.2967480.
  • Benoit-Bird, K. J., & Lawson, G. L. (2016). Ecological insights from pelagic habitats acquired using active acoustic techniques. Annual Review of Marine Science, 8(1), 463–490. https://doi.org/10.1146/annurev-marine-122414-034001.
  • Johnson, M. W. (1948). Sound as a tool in marine ecology, from data on biological noises and the deep scattering layer. Journal of Marine Research, 7(3). https://elischolar.library.yale.edu/journal_of_marine_research/681.
  • Kuperman, W. A., & Roux, P. (2014). Underwater Acoustics. In T. D. Rossing (Ed.), Springer Handbook of Acoustics (pp. 157–212). Springer New York. https://doi.org/10.1007/978-1-4939-0755-7_5.
  • Pearre, S. (2003). Eat and run? The hunger/satiation hypothesis in vertical migration: history, evidence and consequences. Biological Reviews of the Cambridge Philosophical Society, 78(1), 1–79. https://doi.org/10.1017/S146479310200595X.
  • Urmy, S. S., Benoit-Bird, K. J., Ryan, J. P., & Horne, J. K. (2019). Mesopelagic predator-prey interactions revealed by joint passive and active acoustic observations. The Journal of the Acoustical Society of America, 146(4_Supplement), 2899–2899. https://doi.org/10.1121/1.5137058.
  • Wiebe, P. H., Greene, C. H., Stanton, T. K., & Burczynski, J. (1990). Sound scattering by live zooplankton and micronekton: Empirical studies with a dual-beam acoustical system. The Journal of the Acoustical Society of America, 88(5), 2346–2360. https://doi.org/10.1121/1.400077.

Cited References

Cited References
⇡1, ⇡8 Urick, R. J. (1983). Principles of Underwater Sound, Third Edition (3rd edition, Reprint 2013). McGraw-Hill, Inc.
⇡2 Eyring, C. F., Christensen, R. J., & Raitt, R. W. (1948). Reverberation in the Sea. The Journal of the Acoustical Society of America, 20(4), 462–475. https://doi.org/10.1121/1.1906399.
⇡3 Hersey, J. B., & Backus, R. H. (1954). New evidence that migrating gas bubbles, probably the swimbladders of fish, are largely responsible for scattering layers on the continental rise south of New England. Deep Sea Research (1953), 1(3), 190–191. https://doi.org/10.1016/0146-6313(54)90050-7.
⇡4 Dietz, R. S. (1948). Deep scattering layer in the Pacific and Antarctic Oceans. Journal of Marine Research, 7(3). https://elischolar.library.yale.edu/journal_of_marine_research/680
⇡5 Hersey, J. B., & Moore, H. B. (1948). Progress report on scattering layer observations in the Atlantic Ocean. Transactions, American Geophysical Union, 29(3), 341. https://doi.org/10.1029/TR029i003p00341
⇡6 Chapman, R. P., & Marshall, J. R. (1966). Reverberation from Deep Scattering Layers in the Western North Atlantic. The Journal of the Acoustical Society of America, 40(2), 405–411. https://doi.org/10.1121/1.1910087
⇡7 Chapman, R. P., Bluy, O. Z., Adlington, R. H., & Robison, A. E. (1974). Deep scattering layer spectra in the Atlantic and Pacific Oceans and adjacent seas. The Journal of the Acoustical Society of America, 56(6), 1722–1734. https://doi.org/10.1121/1.1903504
⇡9 Ariza, A., Landeira, J. M., Escánez, A., Wienerroither, R., Aguilar De Soto, N., Røstad, A., Kaartvedt, S., & Hernández-León, S. (2016). Vertical distribution, composition and migratory patterns of acoustic scattering layers in the Canary Islands. Journal of Marine Systems, 157, 82–91. https://doi.org/10.1016/j.jmarsys.2016.01.004
⇡10 Boersch-Supan, P. H., Rogers, A. D., & Brierley, A. S. (2017). The distribution of pelagic sound scattering layers across the southwest Indian Ocean. Deep Sea Research Part II: Topical Studies in Oceanography, 136, 108–121. https://doi.org/10.1016/j.dsr2.2015.06.023
⇡11 Gjøsæter, H., Wiebe, P. H., Knutsen, T., & Ingvaldsen, R. B. (2017). Evidence of Diel Vertical Migration of Mesopelagic Sound-Scattering Organisms in the Arctic. Frontiers in Marine Science, 4, 332. https://doi.org/10.3389/fmars.2017.00332
⇡12 Blanluet, A., Doray, M., Berger, L., Romagnan, J.-B., Le Bouffant, N., Lehuta, S., & Petitgas, P. (2019). Characterization of sound scattering layers in the Bay of Biscay using broadband acoustics, nets and video. PLOS ONE, 14(10), e0223618. https://doi.org/10.1371/journal.pone.0223618
⇡13, ⇡19, ⇡24 Dietz, R. S. (1962). The Sea’s Deep Scattering Layers. Scientific American, 207(2), 44–51
⇡14 Benoit-Bird, K. J., Moline, M. A., & Southall, B. L. (2017). Prey in oceanic sound scattering layers organize to get a little help from their friends: Schooling within sound scattering layers. Limnology and Oceanography, 62(6), 2788–2798. https://doi.org/10.1002/lno.10606
⇡15 Marshall, N. B. (1951). Bathypelagic fishes as sound scatterers in the ocean. Journal of Marine Research, 10(1). https://elischolar.library.yale.edu/journal_of_marine_research/733
⇡16 Hersey, J. B., & Backus, R. H. (1954). New evidence that migrating gas bubbles, probably the swimbladders of fish, are largely responsible for scattering layers on the continental rise south of New England. Deep Sea Research (1953), 1(3), 190–191. https://doi.org/10.1016/0146-6313(54)90050-7
⇡17 Moore, H. B. (1950). The relation between the scattering layer and the Euphausiacea. The Biological Bulletin, 99(2), 181–212. https://doi.org/10.2307/1538738
⇡18 Barham, E. G. (1966). Deep Scattering Layer Migration and Composition: Observations from a Diving Saucer. Science, 151(3716), 1399–1403. https://doi.org/10.1126/science.151.3716.1399
⇡20 Holliday, D. V., & Pieper, R. E. (1980). Volume scattering strengths and zooplankton distributions at acoustic frequencies between 0.5 and 3 MHz. The Journal of the Acoustical Society of America, 67(1), 135–146. https://doi.org/10.1121/1.384472
⇡21 Green, C. H., Widder, E. A., Youngbluth, M. J., Tamse, A., & Johnson, G. E. (1992). The migration behavior, fine structure, and bioluminescent activity of krill sound-scattering layers. Limnology and Oceanography, 37(3), 650–658. https://doi.org/10.4319/lo.1992.37.3.0650
⇡22 Davison, P. C., Koslow, J. A., & Kloser, R. J. (2015). Acoustic biomass estimation of mesopelagic fish: Backscattering from individuals, populations, and communities. ICES Journal of Marine Science, 72(5), 1413–1424. https://doi.org/10.1093/icesjms/fsv023
⇡23 Bianchi, D., & Mislan, K. A. S. (2016). Global patterns of diel vertical migration times and velocities from acoustic data: Global patterns of diel vertical migration. Limnology and Oceanography, 61(1), 353–364. https://doi.org/10.1002/lno.10219
⇡25 De Robertis, A. (2002). Size-dependent visual predation risk and the timing of vertical migration: An optimization model. Limnology and Oceanography, 47(4), 925–933. https://doi.org/10.4319/lo.2002.47.4.0925
⇡26 Benoit‐Bird, K. J., & Moline, M. A. (2021). Vertical migration timing illuminates the importance of visual and nonvisual predation pressure in the mesopelagic zone. Limnology and Oceanography, 66(8), 3010–3019. https://doi.org/10.1002/lno.11855
⇡27 Bandara, K., Varpe, Ø., Wijewardene, L., Tverberg, V., & Eiane, K. (2021). Two hundred years of zooplankton vertical migration research. Biological Reviews, 96(4), 1547–1589. https://doi.org/10.1111/brv.12715
⇡28 Benoit-Bird, K. J., & Au, W. W. L. (2004). Diel migration dynamics of an island-associated sound-scattering layer. Deep Sea Research Part I: Oceanographic Research Papers, 51(5), 707–719. https://doi.org/10.1016/j.dsr.2004.01.004
⇡29 Benoit-Bird, K. J., Moline, M. A., & Southall, B. L. (2017). Prey in oceanic sound scattering layers organize to get a little help from their friends: Schooling within sound scattering layers. Limnology and Oceanography, 62(6), 2788–2798. https://doi.org/10.1002/lno.10606
⇡30 Irigoien, X., Klevjer, T. A., Røstad, A., Martinez, U., Boyra, G., Acuña, J. L., Bode, A., Echevarria, F., Gonzalez-Gordillo, J. I., Hernandez-Leon, S., Agusti, S., Aksnes, D. L., Duarte, C. M., & Kaartvedt, S. (2014). Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nature Communications, 5(1), 3271. https://doi.org/10.1038/ncomms4271
⇡31 Irigoien, X., Klevjer, T. A., Røstad, A., Martinez, U., Boyra, G., Acuña, J. L., Bode, A., Echevarria, F., Gonzalez-Gordillo, J. I., Hernandez-Leon, S., Agusti, S., Aksnes, D. L., Duarte, C. M., & Kaartvedt, S. (2014). Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nature Communications, 5(1), 3271. https://doi.org/10.1038/ncomms4271
⇡32 Hersey, J. B., Backus, R. H., & Hellwig, J. (1961). Sound-scattering spectra of deep scattering layers in the western North Atlantic Ocean. Deep Sea Research (1953), 8(3–4), 196–210. https://doi.org/10.1016/0146-6313(61)90021-1.
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