Flexible behaviour in a mesopelagic fish (Maurolicus muelleri)

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.publishedVersio

Vertical movements of a pelagic thresher shark (Alopias pelagicus): insights into the species' physiological limitations and trophic ecology in the Red Sea

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Arostegui, M. C., Gaube, P., Berumen, M. L., DiGiulian, A., Jones, B. H., Rostad, A., & Braun, C. D. Vertical movements of a pelagic thresher shark (Alopias pelagicus): insights into the species' physiological limitations and trophic ecology in the Red Sea. Endangered Species Research, 43, (2020): 387-394, https://doi.org/10.3354/esr01079.The pelagic thresher shark Alopias pelagicus is an understudied elasmobranch harvested in commercial fisheries of the tropical Indo-Pacific. The species is endangered, overexploited throughout much of its range, and has a decreasing population trend. Relatively little is known about its movement ecology, precluding an informed recovery strategy. Here, we report the first results from an individual pelagic thresher shark outfitted with a pop-up satellite archival transmitting (PSAT) tag to assess its movement with respect to the species’ physiology and trophic ecology. A 19 d deployment in the Red Sea revealed that the shark conducted normal diel vertical migration, spending the majority of the day at 200-300 m in the mesopelagic zone and the majority of the night at 50-150 m in the epipelagic zone, with the extent of these movements seemingly not constrained by temperature. In contrast, the depth distribution of the shark relative to the vertical distribution of oxygen suggested that it was avoiding hypoxic conditions below 300 m even though that is where the daytime peak of acoustic backscattering occurs in the Red Sea. Telemetry data also indicated crepuscular and daytime overlap of the shark’s vertical habitat use with distinct scattering layers of small mesopelagic fishes and nighttime overlap with nearly all mesopelagic organisms in the Red Sea as these similarly undergo nightly ascents into epipelagic waters. We identify potential depths and diel periods in which pelagic thresher sharks may be most susceptible to fishery interactions, but more expansive research efforts are needed to inform effective management.This research was funded by a KAUST Center Part-nership Fund award (4107.3 to the Red Sea Research Cen-ter) and KAUST baseline funding (B.H.J. and M.L.B.).M.C.A. and P.G. acknowledge support from NOAA projectNA15OAR4320063. This study was conducted under a pro-tocol approved by the University of Washington’s Institu-tional Animal Care and Use Committee

Diel dynamics of dissolved organicmatter and heterotrophic prokaryotes reveal enhanced growth at the ocean's mesopelagic fish layer during daytime

Contrary to epipelagic waters, where biogeochemical processes closely follow the light and dark periods, little is known about diel cycles in the ocean's mesopelagic realm. Here, we monitored the dynamics of dissolved organic matter (DOM) and planktonic heterotrophic prokaryotes every 2 h for one day at 0 and 550 m (a depth occupied by vertically migrating fishes during light hours) in oligotrophic waters of the central Red Sea. We additionally performed predator-free seawater incubations of samples collected from the same site both at midnight and at noon. Comparable in situ variability in microbial biomass and dissolved organic carbon concentration suggests a diel supply of fresh DOM in both layers. The presence of fishes in the mesopelagic zone during daytime likely promoted a sustained, longer growth of larger prokaryotic cells. The specific growth rates were consistently higher in the noon experiments from both depths (surface: 0.34 vs. 0.18 d-1, mesopelagic: 0.16 vs. 0.09 d-1). Heterotrophic prokaryotes in the mesopelagic layer were also more efficient at converting extant DOM into new biomass. These results suggest that the ocean's twilight zone receives a consistent diurnal supply of labile DOM from the diel vertical migration of fishes, enabling an unexpectedly active community of heterotrophic prokaryotes

The Simrad EK60 echosounder dataset from the Malaspina circumnavigation

The Malaspina Expedition was funded by the Spanish Ministry of Economy and Competitiveness through the Malaspina 2010 expedition project (Consolider-Ingenio 2010, CSD2008-00077), the Fundación BBVA, CSIC, the Spanish Institute of Oceanography, AZTI Foundation, the universities of Granada, Cadiz, Basque Country and Barcelona and the King Abdullah University of Science and Technology. Data have been made available through the EU funded project SUMMER (H2020-BG-2018-2, proposal number: 817806-2).We provide the raw acoustic data collected from the R/V Hesperides during the global Malaspina 2010 Spanish Circumnavigation Expedition (14th December 2010, Cádiz-14th July 2011, Cartagena) using a Simrad EK60 scientific echosounder operating at 38 and 120 kHz. The cruise was divided into seven legs: leg 1 (14th December 2010, Cádiz-13th January 2011, Rio de Janeiro), leg 2 (17th January 2011, Rio de Janeiro-6th February 2011, Cape Town), leg 3 (11th February 2011, Cape Town-13th March 2011, Perth), leg 4 (17th March 2011, Perth-30th March 2011, Sydney), leg 5 (16th April 2011, Auckland-8th May 2011, Honolulu), leg 6 (13th May 2011, Honolulu-10th June 2011, Cartagena de Indias) and leg 7 (19th June 2011, Cartagena de Indias-14th July 2011, Cartagena). The echosounder was calibrated at the start of the expedition and calibration parameters were updated in the data acquisition software (ER60) i.e., the logged raw data are calibrated. We also provide a data summary of the acoustic data in the form of post-processed products.Publisher PDFPeer reviewe

The Submarine Volcano Eruption off El Hierro Island: Effects on the Scattering Migrant Biota and the Evolution of the Pelagic Communities

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Behavior of individual mesopelagic fish in acoustic scattering layers of Norwegian fjords

Mesopelagic acoustic scattering layers (SLs) in 2 fjords were studied from a stationary research vessel. Diel vertical movements of SLs were assessed by hull-mounted transducers, while in situ behavior of individuals constituting the SLs was resolved by a submerged echo sounder. The study focused on SLs made up of the lightfish Maurolicus muelleri and the lanternfish Benthosema glaciale. Individual fish migrated in a pronounced stepwise manner, alternating between vertical movements and stationary phases both during ascent and descent. Mean lengths of steps varied between 2.01 and 0.40 m, and mean duration of stationary phases between 69 and 36 s for fish in different SLs. Such travel-pause behavior concords with saltatory search, where fish scan the water for prey during the stationary phases, relocate and scan a new water parcel. Little activity was recorded among individuals in deep water, apart from infrequent, short shifts in vertical distribution. This study shows that stationary submerged echo sounders can provide detailed information on in situ behavior of mesopelagic fish

Stationary echosounder (EK60, Simrad, 38 & 200 kHz) recordings from the central Red Sea near Thuwal, Saudi Arabia, between January 2014 and February 2016

Simrad EK60 echosounders (38 and 200 kHz) were deployed in the central Red Sea during several multiple-days periods between January 2014 and February 2016. Records were made in the deepest part of a ~700 m deep basin (22.5°N, 39.03°E) referred to as the Economic City Deep (ECDEEP) and at ~555 m along its sloping edge, and at a nearby 880 m deep station (22.08⁰N, 38.71⁰E). All locations were situated near the King Abdullah University of Science and Technology (KAUST) campus, Saudi Arabia. In the events marked by “bottom-mounted deployment” the echosounders were deployed on the bottom facing the surface. The floating mooring was connected to concrete weights by steel wires ensuring vertical orientation such that the transducers were located ca. 7 m off the bottom. The bottom-mounted deployments continuously recorded the movement and abundance of scattering layers and organisms over depth and time at a time resolution of ~0.5 pings/s. In the events marked by “profile”, the echosounders were deployed facing downwards in a profile stopping at different depths for each ca. 15-30 minutes. During the profiles, a tungsten carbide (38.1 mm; theoretical target strength -42.3 dB and -38.9 dB on 38 and 200 kHz, respectively) calibration sphere was attached 10 m or 16 m below the echosounder rig as target strength reference. The dataset contains 12 events with in total 1317 raw proprietary Simrad EK60 data files with file sizes between 0.7 and 168 MB. The raw files include transducer configuration data and sample data (raw power (W) and angles (alongship and athwartship, °))

Sleep walking copepods? Calanus diapausing in hypoxic waters adjust their vertical position during winter

Abstract While hypoxia is generally associated with negative connotations, some animals may also take advantage of reduced oxygen concentrations. However, the dynamics of such processes for zooplankton are poorly understood. We made continuous acoustic studies of Calanus helgolandicus overwintering in hypoxic waters (Oslofjorden, Norway). Their apparent minimum oxygen tolerance was 0.2–0.3 mL O2 L−1 at 8°C. The copepods adjusted their vertical distribution in concert with the upward progression of hypoxia as oxygen contents declined in the course of winter. The hypoxic overwintering habitat largely excluded potential predators and mortality appeared low in early winter. As the copepod distribution shallowed in phase with declining oxygen contents at depth, mortality increased. In contrast to recent predictions, C. helgolandicus had sufficient energy reserves to sustain long-term overwintering. Termination of the overwintering phase in spring was gradual but appeared to accelerate during the development of the spring bloom. Enhanced oceanic deoxygenation with climate change may affect seasonally migrating copepods in unpredictable ways

Ecology of overwintering sprat (Sprattus sprattus)

We used moored upward-facing echosounders in combination with field campaigns to address the overwintering ecology of the clupeid sprat (Sprattus sprattus) throughout four separate winters in a Norwegian fjord. The stationary echosounders were cabled to shore and provided continuous measurements at a temporal resolution of seconds. The long-term coverage of several winters enabled study of the sprat behavior in relation to different biotic parameters like abundance, vertical distribution and taxonomic composition of potential prey and predators, as well as environmental conditions like ice-free vs. ice-covered waters and hypoxic- vs. normoxic conditions. Also the size distribution of the sprat differed significantly between years. The majority of the large-size classes had empty stomachs, particularly prominent in one winter. Otherwise, the diet of the sprat seemed to vary according to the fluctuating mesozooplankton community, yet with calanoid copepods being the most common prey in the sprat stomachs all winters. Krill were not common prey apart for the largest sprat in one winter, but particularly large concentrations of krill appeared to mitigate predation pressure from gadoids, which then preferred krill as prey. During daytime, sprat distribution and swimming behavior varied according to the oxygen conditions. Solitary swimming in near-bottom-waters (~150 m) prevailed in moderate hypoxia (30% O2 saturation) as opposed to schooling in mid-waters when the deep waters were oxygen depleted (0–7% O2 saturation). Nevertheless, a bimodal vertical distribution with an additional part of the sprat population distributed in upper waters was common in all years. The sprat carried out diel vertical migration (DVM) in all winters, but the patterns varied, and included both normal and asynchronous DVM, including fish with a somewhat deeper nocturnal than daytime distribution. Moreover, individual sprat carried out short and rapid excursions to the surface during the night in all years, likely for gulping atmospheric air. Ice conditions imposed a behavioral response with the sprat moving to shallower depths after the ice covering. The varied ecology and behavior observed throughout the course of four consecutive years underlines the importance of conducting long-term studies for the understanding of overwintering strategies. Overall, this study provided unique insight into the dynamic conditions that a population of fish may encounter while overwintering, providing novel information on a scarcely described phase in the life history of fish at high latitudes

Herding mesopelagic fish by light

To assess organisms forming mesopelagic scattering layers in the Red Sea, we took advantage of their reactions to light. We used a remotely operated vehicle (ROV) equipped with LED lamps for herding the acoustic targets down to the bottom (700 m), while concurrently monitoring the event by shipborne and deployed echosounders as well as video footage from the ROV. In essence, entire mesopelagic scattering layers at 38 kHz were moved downwards until the organisms became trapped and concentrated at the bottom and identified as fish from video images. However, responses to the artificial light source appeared to include both repulsion and attraction. An individual-based model reproduced the herding event by assuming a dichotomous response to light where targets close to the light source are attracted, while targets further away are repulsed. We hypothesize that attraction is associated with the artificial light acting as a point source (beam light), while the repulsion is associated with the artificial light acting as diffuse light