Anders Røstad

Large mesopelagic fishes biomass and trophic efficiency in the open ocean

With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters.

Large scale patterns in vertical distribution and behaviour of mesopelagic scattering layers

Recent studies suggest that previous estimates of mesopelagic biomasses are severely biased, with the new, higher estimates underlining the need to unveil behaviourally mediated coupling between shallow and deep ocean habitats. We analysed vertical distribution and diel vertical migration (DVM) of mesopelagic acoustic scattering layers (SLs) recorded at 38 kHz across oceanographic regimes encountered during the circumglobal Malaspina expedition. Mesopelagic SLs were observed in all areas covered, but vertical distributions and DVM patterns varied markedly. The distribution of mesopelagic backscatter was deepest in the southern Indian Ocean (weighted mean daytime depth: WMD 590 m) and shallowest at the oxygen minimum zone in the eastern Pacific (WMD 350 m). DVM was evident in all areas covered, on average ~50% of mesopelagic backscatter made daily excursions from mesopelagic depths to shallow waters. There were marked differences in migrating proportions between the regions, ranging from ~20% in the Indian Ocean to ~90% in the Eastern Pacific. Overall the data suggest strong spatial gradients in mesopelagic DVM patterns, with implied ecological and biogeochemical consequences. Our results suggest that parts of this spatial variability can be explained by horizontal patterns in physical-chemical properties of water masses, such as oxygen, temperature and turbidity.

Winter and spring diving behavior of bowhead whales relative to prey

BackgroundLittle is known about bowhead whale (Balaena mysticetus) foraging behavior and what concentrations of prey are required to balance the energetic trade-offs of feeding. We used satellite telemetry, archival depth recorders, and water column echo sounding data to study bowhead whale diving behavior relative to prey depth and concentration in Disko Bay, West Greenland.ResultsBetween March and May 2008 to 2011, nine bowhead whales were tagged in Disko Bay, West Greenland with instruments that collected data on location and diving over a period of 1 to 33 days. The frequency of U-dives (presumed to be foraging dives) was low during winter months but more than doubled in spring concurrent with a decrease in diving depth. The mean speed of the horizontal bottom phase of the U-dives was 0.9 ms-1 and on average, whales spent 37% of their time at the bottom phase of the dive. In March, bowhead whales presumably fed on copepods (Calanus spp.) close to the seabed (between 100 and 400 m). In April and May, after the copepods ascended to shallower depths, bowhead whales also dove to shallower depths (approximately 30 m) more often. However, echo sounding surveys in the vicinity of feeding whales in early May indicated that patches of copepods could still be found close to the seabed.ConclusionsThere was a marked change in diving behavior from winter through spring and this was likely in response to the changes in sea ice conditions, primary production and potential copepod abundance in the upper part of the water column. Depth and duration of dives changed significantly during this period; however, other dive parameters (for example the proportion of time spent feeding on the bottom of U-dives) remained fairly constant indicating a constant feeding effort. Bowhead whales target copepods at or close to the seabed in winter months in Disko Bay and continue feeding on copepods when they migrate to the surface. However, bowhead whales leave West Greenland before peak abundance of copepods occurs at the surface.

Vertical distribution and diel vertical migration of krill beneath snow-covered ice and in ice-free waters

A bottom mounted upward looking Simrad EK60 120-kHz echo sounder was used to study scattering layers (SLs) and individuals of the krill Meganyctiphanes norvegica. The mooring was situated at 150-m depth in the Oslofjord, connected with an onshore cable for power and transmission of digitized data. Records spanned 5 months from late autumn to spring. A current meter and CTD was associated with the acoustic mooring and a shore-based webcam monitored ice conditions in the fjord. The continuous measurements were supplemented with intermittent krill sampling campaigns and their physical and biological environment. The krill carried out diel vertical migration (DVM) throughout the winter, regardless of the distribution of potential prey. The fjord froze over in mid-winter and the daytime distribution of a mid-water SL of krill immediately became shallower associated with snow fall after freezing, likely related to reduction of light intensities. Still, a fraction of the population always descended all the way to the bottom, so that the krill population by day seemed to inhabit waters with light levels spanning up to six orders of magnitude. Deep-living krill ascended in synchrony with the rest of the population in the afternoon, but individuals consistently reappeared in near-bottom waters already <1 h after the ascent. Thereafter, the krill appeared to undertake asynchronous migrations, with some krill always being present in near-bottom waters even though the entire population appeared to undertake DVM.

Social behaviour in mesopelagic jellyfish

Gelatinous organisms apparently play a central role in deep pelagic ecosystems, but lack of observational methodologies has restricted information on their behaviour. We made acoustic records of diel migrating jellyfish Periphylla periphylla forming small, ephemeral groups at the upper fringe of an acoustic scattering layer consisting of krill. Groups of P. periphylla were also documented photographically using a remotely operated vehicle (ROV). Although the adaptive value of group formation remains speculative, we clearly demonstrate the ability of these jellyfishes to locate and team up with each other.

The submarine volcano eruption off El Hierro Island: effects on the scattering migrant biota and the evolution of the pelagic communities.

The submarine volcano eruption off El Hierro Island (Canary Islands) on 10 October 2011 promoted dramatic perturbation of the water column leading to changes in the distribution of pelagic fauna. To study the response of the scattering biota, we combined acoustic data with hydrographic profiles and concurrent sea surface turbidity indexes from satellite imagery. We also monitored changes in the plankton and nekton communities through the eruptive and post-eruptive phases. Decrease of oxygen, acidification, rising temperature and deposition of chemicals in shallow waters resulted in a reduction of epipelagic stocks and a disruption of diel vertical migration (nocturnal ascent) of mesopelagic organisms. Furthermore, decreased light levels at depth caused by extinction in the volcanic plume resulted in a significant shallowing of the deep acoustic scattering layer. Once the eruption ceased, the distribution and abundances of the pelagic biota returned to baseline levels. There was no evidence of a volcano-induced bloom in the plankton community.

Light penetration structures the deep acoustic scattering layers in the global ocean.

The depth distribution of a major marine biomass component is determined by variation in water clarity. The deep scattering layer (DSL) is a ubiquitous acoustic signature found across all oceans and arguably the dominant feature structuring the pelagic open ocean ecosystem. It is formed by mesopelagic fishes and pelagic invertebrates. The DSL animals are an important food source for marine megafauna and contribute to the biological carbon pump through the active flux of organic carbon transported in their daily vertical migrations. They occupy depths from 200 to 1000 m at daytime and migrate to a varying degree into surface waters at nighttime. Their daytime depth, which determines the migration amplitude, varies across the global ocean in concert with water mass properties, in particular the oxygen regime, but the causal underpinning of these correlations has been unclear. We present evidence that the broad variability in the oceanic DSL daytime depth observed during the Malaspina 2010 Circumnavigation Expedition is governed by variation in light penetration. We find that the DSL depth distribution conforms to a common optical depth layer across the global ocean and that a correlation between dissolved oxygen and light penetration provides a parsimonious explanation for the association of shallow DSL distributions with hypoxic waters. In enhancing understanding of this phenomenon, our results should improve the ability to predict and model the dynamics of one of the largest animal biomass components on earth, with key roles in the oceanic biological carbon pump and food web.

Microplastic in the gastrointestinal tract of fishes along the Saudi Arabian Red Sea coast

This study assesses the presence of microplastic litter in the contents of the gastrointestinal tract of 26 commercial and non-commercial fish species from four difference habitats sampled along the Saudi Arabian coast of the Red Sea. A total of 178 individual were examined for microplastics. In total, 26 microplastic fragments were found. Of these, 16 being films (61.5%) and 10 being fishing thread (38.5%). FTIR analysis revealed that the most abundant polymers were polypropylene and polyethylene. Parascolosps eriomma species sampled at Jazan registered the highest number of ingested microplastic. This fish species is benthic and feeds on benthic invertebrates. Although differences in the abundance of microplastic ingestion among species were not statistically significant, a significant change was observed when the level of ingestion of microplastics particles was compared among the habitats. The higher abundance of microplastics particles may be related to the habitats of fish and the presence of microplastics debris near the seabed. The results of this study represent a first evidence that microplastic pollution represents an emerging threat to Red Sea fishes, their food web and human consumers.

The Mesopelagic Scattering Layer: A Hotspot for Heterotrophic Prokaryotes in the Red Sea Twilight Zone

The vast majority of marine dissolved organic carbon (DOC), the largest reservoir of reduced carbon on Earth, is believed to accumulate in the abyssal layers of the ocean over timescales of decades to millennia. However, evidence is growing that small animals that migrate vertically every day from the surface to mesopelagic layers are significantly contributing to the active vertical flux of organic matter. Whether that represents an important source of carbon available for microbial production and respiration at the mesopelagic realm, and its contribution to oceanic carbon budgets and energy flows, is yet to be explored. Here we present data suggesting that Red Sea migrating animals may produce an overlooked source of labile DOC (used at a mean rate of 2.1 µmol C L-1 d-1) that does not accumulate but fuels the metabolism of prokaryotic heterotrophs in the twilight zone, generating a disregarded hotspot for heterotrophic prokaryotes.

Far-field super-resolution imaging of resonant multiples

Seismic resonant multiples, which can yield resolutions more than twice the classical resolution limit, are used for far-field super-resolution imaging. We demonstrate for the first time that seismic resonant multiples, usually considered as noise, can be used for super-resolution imaging in the far-field region of sources and receivers. Tests with both synthetic data and field data show that resonant multiples can image reflector boundaries with resolutions more than twice the classical resolution limit. Resolution increases with the order of the resonant multiples. This procedure has important applications in earthquake and exploration seismology, radar, sonar, LIDAR (light detection and ranging), and ultrasound imaging, where the multiples can be used to make high-resolution images.