Svenja Christiansen

• Dornan, Tracey; Smith, Abigail J.R.; Bahlburg, Dominik; Cariou, Thibault; Christiansen, Svenja & Demer, David A. [Show all 17 contributors for this article] (2026). Applications of active-acoustic and complementary technologies in marine ecosystem science. ICES Journal of Marine Science. ISSN 1054-3139. 83(4). doi: 10.1093/icesjms/fsag044. Full text in Research Archive Show summary

  Since the invention of echosounders and sonars in the early twentieth century, the applications of active-acoustic instruments have expanded from single-frequency range detection and fisheries stock estimates to wider explorations of ecosystem dynamics using increasingly advanced tools. Echosounders are now integrated with ships, nets, probes, uncrewed and autonomous vehicles, moorings, landers, and marine mammals, to sample the oceans on ecologically relevant spatial and temporal scales. Yet the uptake of these technologies by ecologists is hindered by steep learning curves, data post-processing techniques and availability, and funding challenges, as well as persistent knowledge gaps within the field of acoustics. An interdisciplinary approach, joining expertise in sampling technology and ecology, coalesces the people, ideas, resources, and data needed to conduct priority science. Here, we present the current and anticipated applications of active-acoustic and complementary instruments deployed on a variety of platforms to conduct interdisciplinary marine ecosystem science—beyond estimates of fisheries stock biomass. We also discuss the skill development, resource and data access, and interdisciplinary collaboration necessary to address challenges to marine ecosystems and progress the priorities identified in the ICES Science Plan.

• Salvanes, Anne Gro Vea; Gallo, Natalya; Solås, Martine Røysted; Saltalamacchia, Francesco; Aksnes, Dag Lorents & Darelius, Elin Maria K. [Show all 19 contributors for this article] (2025). Deep Fjords Are Excellent Natural Infrastructure for Climate Impact Studies. Fish and Fisheries. ISSN 1467-2960. 26(2), p. 270–277. doi: 10.1111/faf.12879. Full text in Research Archive Show summary

  Fjords provide valuable research opportunities for marine scientists. They are excellent natural infrastructure for climate impact studies associated with hypoxic episodes and consequences for mesopelagic and deep-sea ecosystems involving oceanographic circulation processes and basin water renewals. Repeated sampling from the same populations is possible, making fjords excellent systems for developing time series of data for climate impact studies. We provide an overview of the 14 years of data from Norwegian West Coast fjords, focusing on Masfjorden, and report major findings from Oslofjorden in Eastern Norway, exhibiting recurrent hypoxia in the basin waters. We document that the oxygen levels in Masfjorden decreased rapidly by over 60% at 450 m depth in < 8 years, which is much faster than the average rate of deoxygenation in the global ocean. We also discuss the increase in the deep-sea and low-light-adapted coronate jellyfish Periphylla periphylla in view of altered optical conditions of the basin water potentially related to deoxygenation. We argue that fjords like Masfjorden and Oslofjorden are not only macrocosms for ecological processes but also are likely an accelerated version of deep oceans with respect to climate impacts.

• Kaartvedt, Stein; Christiansen, Svenja & Røstad, Anders (2024). Mesopelagic fishes in the Red Sea. In Rasul, Najeeb M. A. & Stewart, Ian C.F. (Ed.), Coral Reefs and Associated Marine Fauna Around the Arabian Peninsula. CRC Press. ISSN 9781032342900. p. 93–100. doi: 10.1201/9781003321392-10. Full text in Research Archive Show summary

  The Red Sea twilight zone (200-1000 m) is a unique environment characterized by extraordinarily high temperatures, low zooplankton concentrations, and strata with low oxygen contents. This harsh habitat houses few species of mesopelagic fishes, yet individuals are numerous, and distinct mesopelagic acoustic scattering layers occur throughout the ocean. The scattering fish layers extend down to 600-800 m during the day. The fishes carry out rapid nocturnal vertical migrations to upper waters, with limited seasonal variations. However, behavioural plasticity includes examples of strikingly short-term and rapid darts into upper waters at dusk and dawn. Such behaviour may relate to the rapid shifts between day and night at low latitudes, the respective light sensitivities of the fish species and varying presence of predators. Foraging mainly happens in upper waters at night, yet the fishes are active at day in their mesopelagic habitat. Excretion in the daytime habitat fuels microbial processes, generating a hotspot for heterotrophic prokaryotes. The fishes’ role as prey in the Red Sea food chain remains to be established. The Red Sea mesopelagic fishes appear to spawn throughout the year, though apparently with minima in mid-summer.

• Christiansen, Svenja; Bråthen, Mabel Pernille; Moksnes, Fredrik Lund; Schmedling, Charlotte Aurora; Thorsby, Sara Jegstad & Titelman, Josefin [Show all 7 contributors for this article] (2024). Extensive avoidance behaviour of krill from predators and nets. ICES Journal of Marine Science. ISSN 1054-3139. 81(9), p. 1764–1773. doi: 10.1093/icesjms/fsae123. Full text in Research Archive Show summary

  We used echosounders to assess krill (Meganyctiphanes norvegica) responses to sampling gear and predators. Towing pelagic trawls above autonomous submerged, upward-looking echosounders revealed instantaneous diving by the krill during daytime but not at night. The krill dived at 10–30 cm s−1 with a near-simultaneous response throughout the krill layer below the gear (>10 m). Traversing the paths of the previous sampling revealed long-lasting voids in the krill distribution during the daytime. Such voids were less apparent at night. Backscatter increased in the wake of the gear during nocturnal sampling, possibly due to changes in individual backscatter related to variation in swimming behaviour. During daytime, krill responded to fish schools with instantaneous coherent diving, spanning tens of meters at speeds of 10–27 cm s−1. Coordinated and far-ranging responses indicate that even loose krill aggregations may facilitate predator avoidance by enabling efficient signal transmission between individuals. The krill antipredator behaviour instigated marked daytime krill patchiness. Krill patchiness will vary relative to the predator regime and reflect recent encounters.

• Kaartvedt, Stein; Christiansen, Svenja & Titelman, Josefin (2023). Mid-summer fish behavior in a high-latitude twilight zone. Limnology and Oceanography. ISSN 0024-3590. 68(7), p. 1654–1669. doi: 10.1002/lno.12374. Full text in Research Archive Show summary

  Abstract The behavior of the mesopelagic fish Benthosema glaciale was studied at 60°N in mid‐summer. We hypothesized that diel vertical migration (DVM) is constrained by short and dusk nights (surface illumination &gt; 10 −2   μ mol m −2 s −1 ) and that individuals are active at depth during the long summer days. Submerged echosounders provided high‐resolution data throughout the water column. During the day, a part of the population ascended toward the increasing daylight. Short vertical relocations were followed by minutes of vertical inactivity. Swimming included horizontal turns and loops associated with the vertical steps. Normal DVM was initiated ~ 4 h before sunset and reflected independent individual decisions. The fish initially ascended stepwise but switched to mostly straight upwards swimming attaining 3–4 cm s −1 . Their vertical speed was faster than the slow ascent of isolumes and even the deepest living fish potentially could reach upper layers shortly after sunset. However, many individuals aborted their ascent and returned to depth before the darkest time of the night, while others returned downward closer to sunrise. The daytime swimming and individual variability in diel migration behavior have implications for encounters with prey and predators in the twilight zone and the biological carbon pump. A principal conclusion is that mesopelagic fishes can modify their behavior and migration patterns to suit a wide range of changing conditions.

• Christiansen, Svenja; Langangen, Øystein Ole Gahr; Titelman, Josefin; Vøllestad, Leif Asbjørn & Kaartvedt, Stein (2022). Three-dimensional swimming behavior and activity of a mesopelagic fish. Limnology and Oceanography. ISSN 0024-3590. 67, p. 2677–2690. doi: 10.1002/lno.12230. Full text in Research Archive Show summary

  Knowledge of the three-dimensional behavior and activity patterns of mesopelagic fishes is crucial for understanding their role in ecological and biogeochemical processes in the ocean. We here show that the use of submerged stationary echosounders enables detailed analysis of three-dimensional swimming behavior of small mesopelagic fishes in situ. We explore a 4-month continuous dataset containing more than 100,000 nocturnal tracks of juvenile Maurolicus muelleri, obtained from a surface-facing, moored echosounder in a Norwegian fjord. We extend previous observations of vertical nocturnal swimming and step-wise (stop-and-go) dusk and dawn migrations with the corresponding horizontal and turning behavior. In the middle of the night, the M. muelleri mainly drifted with the currents. Horizontal swimming speeds were then usually below 0.5 body length s−1 but increased substantially during the step-wise dusk and dawn vertical migration. Most vertical relocations were followed by a change in horizontal direction. Our observations are in line with hypotheses of growth maximization in the juvenile fish, and of reducing detection and encounters with predators.

• Kaartvedt, Stein; Christiansen, Svenja; Røstad, Anders & Aksnes, Dag Lorents (2022). Mesopelagic fishes in a hurry at low latitudes. Marine Ecology Progress Series. ISSN 0171-8630. 694, p. 149–156. doi: 10.3354/meps14114. Full text in Research Archive
• Kiko, Rainer; Picheral, Marc; Antoine, David; Babin, Marcel; Berline, Léo & Biard, Tristan [Show all 42 contributors for this article] (2022). A global marine particle size distribution dataset obtained with the Underwater Vision Profiler 5. Earth System Science Data. ISSN 1866-3508. 14, p. 4315–4337. doi: 10.5194/essd-14-4315-2022. Full text in Research Archive Show summary

  Marine particles of different nature are found throughout the global ocean. The term “marine particles” describes detritus aggregates and fecal pellets as well as bacterioplankton, phytoplankton, zooplankton and nekton. Here, we present a global particle size distribution dataset obtained with several Underwater Vision Profiler 5 (UVP5) camera systems. Overall, within the 64 µm to about 50 mm size range covered by the UVP5, detrital particles are the most abundant component of all marine particles; thus, measurements of the particle size distribution with the UVP5 can yield important information on detrital particle dynamics. During deployment, which is possible down to 6000 m depth, the UVP5 images a volume of about 1 L at a frequency of 6 to 20 Hz. Each image is segmented in real time, and size measurements of particles are automatically stored. All UVP5 units used to generate the dataset presented here were inter-calibrated using a UVP5 high-definition unit as reference. Our consistent particle size distribution dataset contains 8805 vertical profiles collected between 19 June 2008 and 23 November 2020. All major ocean basins, as well as the Mediterranean Sea and the Baltic Sea, were sampled. A total of 19 % of all profiles had a maximum sampling depth shallower than 200 dbar, 38 % sampled at least the upper 1000 dbar depth range and 11 % went down to at least 3000 dbar depth. First analysis of the particle size distribution dataset shows that particle abundance is found to be high at high latitudes and in coastal areas where surface productivity or continental inputs are elevated. The lowest values are found in the deep ocean and in the oceanic gyres. Our dataset should be valuable for more in-depth studies that focus on the analysis of regional, temporal and global patterns of particle size distribution and flux as well as for the development and adjustment of regional and global biogeochemical models. The marine particle size distribution dataset (Kiko et al., 2021) is available at https://doi.org/10.1594/PANGAEA.924375.

• Benguria, Beatriz Sobradillo; Christiansen, Svenja; Røstad, Anders & Kaartvedt, Stein (2022). Individual daytime swimming of mesopelagic fishes in the world's warmest twilight zone. Deep Sea Research Part I: Oceanographic Research Papers. ISSN 0967-0637. 190. doi: 10.1016/j.dsr.2022.103897. Full text in Research Archive Show summary

  We assessed the activity and swimming patterns of mesopelagic fishes in the Red Sea using bottom-moored, upward-facing echosounders deployed at 555 and 700 m depth. The vertically migrating mesopelagic scattering layer descended close to the bottom during daytime. This permitted assessment of behavior at mesopelagic depths by applying acoustic target tracking for individuals traversing the acoustic beam. Swimming activity did not fit the notion of torpid behavior in the daytime habitat. The fishes were moving continuously, with a prevailing downward direction before noon and upward after, though individuals were swimming in both directions at all times. They moreover were swimming horizontally at estimated speeds of ∼2.1 cm s−1, suggesting ∼0.5–1 body length s−1, intermittently turning. High activity at high temperatures suggests high respiration at depth, considered a key element for the active carbon pump.

• Christiansen, Svenja; Klevjer, Thor Aleksander; Røstad, Anders; Aksnes, Dag Lorents & Kaartvedt, Stein (2021). Flexible behaviour in a mesopelagic fish (Maurolicus muelleri). ICES Journal of Marine Science. ISSN 1054-3139. 78(5), p. 1623–1635. doi: 10.1093/icesjms/fsab075. Full text in Research Archive Show summary

  Variability of mesopelagic scattering layers is often attributed to environmental conditions or multi-species layer composition. Yet, little is known about variation in behaviour among the individuals forming scattering layers. Based on a 10 months high-resolution dataset from stationary echosounders in a Norwegian fjord, we here assess short-term and long-term behaviour of a single mesopelagic fish species, the pearlside Maurolicus muelleri. The daytime vertical extension of the monospecific pearlside scattering layers spanned four orders of magnitude ambient light in the autumn and winter and less than one order of magnitude in summer. While the main layers tracked relatively stable light levels over daytime, some individuals actively crossed light gradients of up to 1.5 orders of magnitude. This included individuals that moved between scattering layers, and apparently bold individuals that made regular upward excursions beyond the main population distribution. During the daytime, M. muelleri mitigated the risk of predation by forming tight groups in the upper scattering layer and, at light levels >10−6 µmol m−2 s−1, by instantly diving into deeper waters upon encounters with predators. Our observations suggest that individual, and probably state-dependent, decisions may extend the pearlsides’ vertical distribution, with implications for predator–prey interactions.

• Kaartvedt, Stein; Røstad, Anders; Christiansen, Svenja & Klevjer, Thor Aleksander (2020). Diel vertical migration and individual behavior of nekton beyond the ocean's twilight zone. Deep Sea Research Part I: Oceanographic Research Papers. ISSN 0967-0637. 160. doi: 10.1016/j.dsr.2020.103280. Full text in Research Archive Show summary

  Diel vertical migration (DVM) is normally limited to the upper 1000 m. However, the use of ship-borne and stationary submerged echosounders in the Red Sea unveiled consistent formation of daytime patches extending to 1200-1300 m, the patches dissolving at night when individuals migrated shallower. The diel vertical migration was not synchronized, and in the evening 4–5 h passed from the first to the last individual leaving their continuously dark daytime habitat. The mean ascent and descent speeds were ~15 cm s−1. Individuals were active upon return to deep water in the morning, some swimming rapidly up and down at speeds of up to ~25 cm s−1, with abrupt changes in swimming direction. Patch formation was swift, in spite of dilute overall population density, suggesting remote signaling to team up with conspecifics. Vertical swimming subsided as patches became established and persisted at 2–3 cm s−1 within patches. The acoustic target strength of the individuals peaked at ~ -55 dB (38 kHz) and was constant regardless of vertical swimming directions. Based on the behavioral and acoustic evidence squids (Sthenoteuthis sp) are likely acoustic targets.

• Kiko, Rainer; Brandt, Peter; Christiansen, Svenja; Faustmann, Jannik; Kriest, Iris & Rodrigues, Elizandro [Show all 8 contributors for this article] (2020). Zooplankton-mediated fluxes in the eastern tropical North Atlantic. Frontiers in Marine Science. 7. doi: 10.3389/fmars.2020.00358. Full text in Research Archive Show summary

  Zooplankton organisms are a central part of pelagic ecosystems. They feed on all kinds of particulate matter and their egested fecal pellets contribute substantially to the passive sinking flux to depth. Some zooplankton species also conduct diel vertical migrations (DVMs) between the surface layer (where they feed at nighttime) and midwater depth (where they hide at daytime from predation). These DVMs cause the active export of organic and inorganic matter from the surface layer as zooplankton organisms excrete, defecate, respire, die, and are preyed upon at depth. In the Eastern Tropical North Atlantic (ETNA), the daytime distribution depth of many migrators (300–600 m) coincides with an expanding and intensifying oxygen minimum zone (OMZ). We here assess the day and night-time biomass distribution of mesozooplankton with an equivalent spherical diameter of 0.39–20 mm in three regions of the ETNA, calculate the DVM-mediated fluxes and compare these to particulate matter fluxes and other biogeochemical processes. Integrated mesozooplankton biomass in the ETNA region is about twice as high at a central OMZ location (cOMZ; 11° N, 21° W) compared to the Cape Verde Ocean Observatory (CVOO; 17.6° N, 24.3° W) and an oligotrophic location at 5° N, 23° W (5N). An Intermediate Particle Maximum (IPM) is particularly strong at cOMZ compared to the other regions. This IPM seems to be related to DVM activity. Zooplankton DVM was found to be responsible for about 31–41% of nitrogen loss from the upper 200m of the water column. Gut flux and mortality make up about 31% of particulate matter supply to the 300–600 m depth layer at cOMZ, whereas it makes up about 32% and 41% at CVOO and 5N, respectively. Resident and migrant zooplankton are responsible for about 7–27% of the total oxygen demand at 300–600 m depth. Changes in zooplankton abundance and migration behavior due to decreasing oxygen levels at midwater depth could therefore alter the elemental cycling of oxygen and carbon in the ETNA OMZ and impact the removal of nitrogen from the surface layer.

• Hoving, Henk-Jan T.; Neitzel, Philipp; Hauss, Helena; Christiansen, Svenja; Kiko, Rainer & Robison, Bruce H. [Show all 8 contributors for this article] (2020). In situ observations show vertical community structure of pelagic fauna in the eastern tropical North Atlantic off Cape Verde. Scientific Reports. 10. doi: 10.1038/s41598-020-78255-9. Full text in Research Archive Show summary

  Abstract Distribution patterns of fragile gelatinous fauna in the open ocean remain scarcely documented. Using epi-and mesopelagic video transects in the eastern tropical North Atlantic, which features a mild but intensifying midwater oxygen minimum zone (OMZ), we established one of the first regional observations of diversity and abundance of large gelatinous zooplankton. We quantified the day and night vertical distribution of 46 taxa in relation to environmental conditions. While distribution may be driven by multiple factors, abundance peaks of individual taxa were observed in the OMZ core, both above and below the OMZ, only above, or only below the OMZ whereas some taxa did not have an obvious distribution pattern. In the eastern eropical North Atlantic, OMZ expansion in the course of global climate change may detrimentally impact taxa that avoid low oxygen concentrations ( Beroe , doliolids), but favour taxa that occur in the OMZ ( Lilyopsis , phaeodarians, Cydippida, Colobonema , Haliscera conica and Halitrephes) as their habitat volume might increase. While future efforts need to focus on physiology and taxonomy of pelagic fauna in the study region, our study presents biodiversity and distribution data for the regional epi- and mesopelagic zones of Cape Verde providing a regional baseline to monitor how climate change may impact the largest habitat on the planet, the deep pelagic realm.

• Hoving, Henk-Jan; Christiansen, Svenja; Fabrizius, Eduard; Hauss, Helena; Kiko, Rainer & Linke, Peter [Show all 9 contributors for this article] (2019). The Pelagic in situ Observation System (PELAGIOS) to reveal biodiversity, behavior, and ecology of elusive oceanic fauna. Ocean Science. ISSN 1812-0784. 15(5), p. 1327–1340. doi: 10.5194/os-15-1327-2019. Full text in Research Archive Show summary

  There is a need for cost-efficient tools to explore deep-ocean ecosystems to collect baseline biological observations on pelagic fauna (zooplankton and nekton) and establish the vertical ecological zonation in the deep sea. The Pelagic In situ Observation System (PELAGIOS) is a 3000 m rated slowly (0.5 m s−1) towed camera system with LED illumination, an integrated oceanographic sensor set (CTD-O2) and telemetry allowing for online data acquisition and video inspection (low definition). The high-definition video is stored on the camera and later annotated using software and related to concomitantly recorded environmental data. The PELAGIOS is particularly suitable for open-ocean observations of gelatinous fauna, which is notoriously under-sampled by nets and/or destroyed by fixatives. In addition to counts, diversity, and distribution data as a function of depth and environmental conditions (T, S, O2), in situ observations of behavior, orientation, and species interactions are collected. Here, we present an overview of the technical setup of the PELAGIOS as well as example observations and analyses from the eastern tropical North Atlantic. Comparisons to data from the Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) net sampling and data from the Underwater Vision Profiler (UVP) are provided and discussed.

• Christiansen, Svenja; Titelman, Josefin & Kaartvedt, Stein (2019). Nighttime Swimming Behavior of a Mesopelagic Fish. Frontiers in Marine Science. 6, p. 1–12. doi: 10.3389/fmars.2019.00787. Full text in Research Archive Show summary

  Nocturnal migration of mesopelagic fish into surface waters is well-documented. Yet, although there is increasing evidence of individual-based deviations from average population migrations and of the importance of small-scale individual behavior for predator-prey interactions and energetic cycling, little is known about what mesopelagic animals do when in upper waters. Using high-resolution data from an upward-facing, moored, split-beam echosounder we analyzed the night-time individual vertical swimming behavior of pearlsides (Maurolicus muelleri) over one winter. The population behavior is characterized by migration to the surface after sunset, “midnight-sinking” and another migration to the surface in the morning, followed by return to the daytime habitat. Focusing on individuals unveiled diverse behavioral patterns during different phases of the migration. After ascending to upper layers at dusk, M. muelleri leaves the surface waters, not by sinking, but by actively swimming in a step-wise pattern characterized by relocations alternated by pauses. Following the descent, vertical swimming is sustained at lower levels. Around midnight, the vertical swimming direction changes from predominantly downward to upward. Several hours before dawn, the fish start ascending toward the surface in a step-wise pattern. During population ascent in the afternoon and descent in the morning, some individuals at the fringes of schools migrate without intermittent pauses. This study documents the feasibility of using submerged, stationary echosounders in unveiling the individual behavior of mesopelagic fish.

• Christiansen, Svenja; Hoving, Henk-Jan; Schütte, Florian; Hauss, Helena; Karstensen, Johannes & Körtzinger, Arne [Show all 14 contributors for this article] (2018). Particulate matter flux interception in oceanic mesoscale eddies by the polychaete Poeobius sp. Limnology and Oceanography. ISSN 0024-3590. 63(5), p. 2093–2109. doi: 10.1002/lno.10926. Full text in Research Archive

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• Nyeggen, Margot Ulfsdatter; Christiansen, Svenja; Garvang, Even Sletteng; Andersen, Tom; Heuschele, Jan David & Titelman, Josefin (2025). Zooplankton community dynamics in Oslofjorden. Full text in Research Archive
• Kaartvedt, Stein; Christiansen, Svenja & Titelman, Josefin (2024). Internal waves and fine-scale zooplankton sampling: Comment on Dewar-Fowler et al. (2023). Marine Ecology Progress Series. ISSN 0171-8630. 734, p. 173–175. doi: 10.3354/meps14577. Full text in Research Archive
• Moy, Frithjof Emil; Kroglund, Tone; Hestenes, Arne; Endregard, Geir; Akerø, Edith & Christiansen, Svenja (2024). Sluttrapport for Frisk Oslofjord - 2018-2024. Havforskningsinstituttet. Full text in Research Archive Show summary

  Det overordnede målet for «Frisk Oslofjord» prosjektene har vært å skape varig miljøbevissthet og engasjement for Oslofjorden gjennom undervisning og formidling bygget på kunnskap fra forskning, overvåkning og miljøkartlegging, samt legget et grunnlag for nye prosjekt og permanente løsninger.

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