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Gut contents and nitrogen isotope ratio (δ15N) analyses were conducted on splendid alfonsino Beryx splendens collected in the vicinity of Kuroshio from May 2015 to November 2016 (n = 708) to identify their feeding habits in the Western North Pacific. Micronektonic fish, shrimp, and squid were the dominant prey species, with the Decapoda Acanthephyra quadrispinosa, Prehensilosergia prehensilis, Cephalopoda Enoploteuthis chunii, Chiroteuthis calyx, Teleostei Sigmops gracilis, and Gonostoma elongatum. These micronektons are common in the Kuroshio Current; thus, B. splendens is a possible opportunistic feeder. Gut content weight (GCW), occurrence (%O) of fish and squid, and δ15N were significantly increased with the fork length (FL) of B. splendens. Only the %O of shrimp does not show a significant relationship with the FL of B. splendens. These indicated the ontogenetic shift of dominant prey from shrimp to fish. Fattened specimens could also prey on a greater number of fish. The seasonality of GCW and %O values was high in spring and autumn and low in summer and winter. This seasonality indicated that the feeding habits of B. splendens are controlled by primary productivity in the surface layer and/or by maturity. When the Kuroshio flowed in the north of Hachijojima Island, the GCW was significantly increased, and some organisms, including C. calyx, G. elongatum, and S. gracilis, were preyed on more. While the offshore area of the Kuroshio has low productivity, B. splendens can actively prey in this area, indicating that it is an important feeding ground for this species. [ABSTRACT FROM AUTHOR]

Aim: Water column acoustic backscatter is regularly registered during oceanographic surveys, providing valuable information on the composition and distribution of pelagic life in the ocean. We propose an objective approach based on functional data analysis to classify these acoustic seascapes into biogeographical regions. Location: Tropical South Atlantic Ocean off northeastern Brazil. Taxon: Sound‐scattering pelagic fauna detected with acoustic echosounders, principally small fish, crustaceans, squid and diverse gelatinous life‐forms. Methods: We use acoustic backscatter as a function of depth, simultaneously at three frequencies, to numerically describe the vertical distribution and composition of sound‐scattering organisms in the water column. This information is used to classify the acoustic seascape through functional principal component analysis. The analysis routine is tested and illustrated with data collected at 38, 70 and 120 kHz in waters affected by contrasting environmental conditions. Results: Acoustic seascape partitioning mirrored the distribution of current systems, fronts and taxonomically based regionalization. The study area was divided between slope‐boundary and open‐ocean waters, and between spring and fall hydrological regimes. Main Conclusions: The acoustic seascape consistency and the spatiotemporal coherence of the regions classified show that the method is efficient at identifying homogeneous and cohesive sound‐scattering communities. Comparisons against hydrological and biological regionalization prove that the method is reliable at delineating distinct pelagic ecosystems in a cost‐efficient and non‐intrusive way. [ABSTRACT FROM AUTHOR]

Mesoscale eddies shape the foraging ecology of predators such as marine mammals and seabirds. A growing number of animal tracking studies show that predators alter their swimming, diving, and foraging behavior within mesoscale eddies. However, little is known about how Southern Ocean eddies influence the distribution of mesopelagic micronekton (fish, squid, and crustaceans), which are major prey items of megafauna. Studies in other oceanic regions have found that eddies can influence the abundance and community composition of micronekton. Here, we analyze acoustic observations from a 14‐day survey of a cyclonic mesoscale eddy, its surrounding waters, and the Polar Frontal Zone (PFZ) waters where the eddy formed. We report and interpret spatial patterns of acoustic backscatter at 18 and 75 kHz, proxies indicating combined changes in species, size, and abundance of micronekton. We find that the vertical distribution of acoustic backscatter matched the underwater light conditions characteristic of the eddy core, periphery, and surrounding waters, at scales smaller than 10 km. The median water‐column integrated acoustic backscatter values in the eddy core were only half of those measured in the Sub‐Antarctic Zone waters surrounding the eddy, but similar to those measured in the PFZ, where the eddy originated 27 days prior. These results suggest that, as for physical and chemical tracers, the eddy maintained its biological characteristics from its source waters creating a unique habitat compared to its surroundings. Plain Language Summary: Mesoscale eddies are rotating currents that are everywhere in the ocean. They are typically tens to one or two hundred km across, with lifetimes spanning weeks to months. Mesoscale eddies have a dramatic impact on the patterns of primary production in the open ocean, the transport of heat and salt across oceanic regions, global biogeochemical cycles, and the feeding behavior of marine predators such as pinnipeds, sharks, billfishes, and seabirds. In this study, we used underwater acoustic measurements to evaluate the effects of a Southern Ocean mesoscale eddy on the distribution of deep water micronekton, a diverse group of small animals including fish, crustacea, and squids. We found that the abundance and vertical distribution of micronekton inside the eddy differed from the surrounding waters. Micronekton distribution was instead more similar to where the eddy had originated a month prior to our sampling. Our results suggest that mesoscale eddies can maintain their biological characteristics from their source waters, creating a unique habitat compared to surrounding waters. Key Points: We observed the distribution of acoustic backscatter in the mesopelagic zone across a Southern Ocean cyclonic eddyThe integrated and vertical distribution of the acoustic backscatter inside the eddy were similar to its origin watersThe eddy presented a unique habitat compared to its surrounding waters, affecting the accessibility of mesopelagic prey to diving predators [ABSTRACT FROM AUTHOR]

Biophysical interactions are influential in determining the scale of key ecological processes within marine ecosystems. For oceanic predators, this means foraging behaviour is influenced by processes shaping the distribution of prey. However, oceanic prey is difficult to observe and its abundance and distribution is regionally generalised. We use a spatiotemporally resolved simulation model to describe mid‐trophic prey distribution within the Southern Ocean and demonstrate insights that this modelled prey field provides into the foraging behaviour of a widely distributed marine predator, the southern elephant seal. From a five‐year simulation of prey biomass, we computed climatologies of mean prey biomass (average prey conditions) and prey biomass variability (meso‐scale variability). We also compiled spatially gridded metrics of seal density and diving behaviour from 13 yr of tracking data. We statistically modelled these metrics as non‐linear functions of prey biomass (both mean and variability) and used these to predict seal distribution and behaviour. Our predictions were consistent with observations (R2adj = 0.23), indicating that seals aggregate in regions of high mesoscale activity where eddies concentrate prey. Here, seals dived deeper (R2marg = 0.12, R2cond = 0.51) and spent less time hunting (R2marg = 0.05, R2cond = 0.56), likely targeting deep but profitable prey patches. Seals generally avoided areas of low eddy activity where prey was likely dispersed. Most seals foraged south of the Subantarctic Front, despite north of the front exhibiting consistently high simulated prey biomasses. This likely reflects seal prey or habitat preferences, but also emphasises the importance of mesoscale prey biomass variability relative to regionally high mean biomass. This work demonstrates the value of coupling mechanistic representations of prey biomass with predator observations to provide insight into how biophysical processes combine to shape species distributions. This will be increasingly important for the robust prediction of species' responses to rapid system change. [ABSTRACT FROM AUTHOR]

Aim: Biological nitrogen fixation supports primary production in oligotrophic water, but its link to higher trophic levels has not been described fully on a biogeographical basis. Here, we determine the regional patterns of the contribution of the combined nitrogen to biological production within the epipelagic layer of the mid‐Pacific Ocean using the isotopic signatures of nitrogen (δ15N) and carbon (δ13C) in the biological components. Location: The mid‐Pacific Ocean along 170° W between the southern subtropical front and the Chukchi Sea. Time period: Northern and austral summer in 2013 and 2014. Major taxa studied: Planktonic and micronektonic biota in the euphotic layer. Methods: We measured the geographical variations in δ15N and δ13C of the suspended particulate organic matter (POM), mesozooplankton assemblage and micronektonic fish. We analysed the relationships among these values and the environmental variables of temperature, nitrate concentration and biological nitrogen fixation activity along a 12,000‐km meridional transect. Results: The POM δ15N at 0 m was negatively correlated with in situ N2 fixation activity in the subtropical region, whereas that in the equatorial and high‐latitude regions was correlated with the nitrate concentration at 0 m. We found that the ratios of the increase in δ15N to δ13C along the grazing food chain were consistent throughout the equatorial and subtropical regions. Cluster analyses based on the stable isotopic signatures in the biotic components revealed that the food chains in the stations within the subtropical mid‐Pacific Ocean were separated into three groups based on the differential contributions of biological nitrogen fixation. Main conclusions: Distinct food chains from primary to tertiary production sustained by different nitrogen sources, nitrate below the euphotic zone, and diazotrophic nitrogen occur within the same biogeographical provinces in the subtropical mid‐Pacific Ocean. The diazotroph‐dominant community contributes substantially to the apex predators in the central areas of the subtropical gyres. [ABSTRACT FROM AUTHOR]

Assessment of open-ocean ecosystems relies on understanding ecosystem dynamics, and development of end-to-end ecosystem models represents an approach that addresses these challenges. These models incorporate the population structure and dynamics of marine organisms at all trophic levels. Satellite remote sensing of ocean colour and direct at-sea measurements provide information on the lower trophic levels of the models, and fisheries studies provide information on top predator species. However, these models suffer from a lack of observations for the so-called mid-trophic levels, which are poorly sampled by conventional methods. This restricts further development, and we argue that acoustic observations from a range of platforms (e.g. buoys, moorings) can be linked to the ecosystem models to provide much-needed information on these trophic levels. To achieve this, the models need to be tailored to incorporate the available acoustic data, and the link from acoustic backscatter to biologically relevant variables (biomass, carbon, etc.) needs attention. Methods to progress this issue are proposed, including the development of observation models and focal areas for ground truthing. To ensure full use of the potential of acoustic techniques, we argue that a systematic and long-term strategy incorporating the following elements is required: development of metadata standards and automated data analysis, inclusion of acoustic sensors in large-scale observatory programmes, improvement of observation-model links, and efficient sampling strategies. Finally, these elements should be tied together in an observation-modelling framework, coordinated by international organizations, to improve our understanding and quantification of open-ocean ecosystem dynamics. [ABSTRACT FROM AUTHOR]

Abstract The biomass distribution, community structure and species composition of the dominant micronektonic animals were determined in the western tropical Pacific, using day/night sampling from upper 200 m depth with a commercial midwater otter trawl in fall-winter of 1994, 1995 and 1997. Night-time micronekton biomass was significantly higher than daytime biomass except for a few cases. All the night-time catches had similar taxonomic composition, dominated by myctophid fishes, small squids, shrimps and euphausiids. Night-time micronekton biomass in the upper 200 m was assumed to be higher in the North Equatorial Counter Current than in the North Equatorial Current, reflecting the zooplankton biomass distribution which micronektons feed on at night. A total of 42 species of myctophid fishes, 34 species of squids, 27 species of shrimps and six species of euphausiids were collected. Community types of each dominant taxonomic group were classified by cluster analysis based on the species composition. A station located north of the subtropical convergence zone was distinct from other tropical stations south of the convergence zone, having significantly different species composition in three of the four dominant taxonomic groups (myctophids, squids and shrimps). The interaction between the larvae of commercially important fishes and micronektonic animals is discussed. [ABSTRACT FROM AUTHOR]