Distribution and diel vertical migration of mesopelagic fishes in the Southern Sargasso Sea — observations through hydroacoustics and stratified catches

Vertical distribution patterns and relative abundance of mesopelagic fish species and other major taxonomic groups were investigated through vertically stratified trawl sampling and hydroacoustic analyses along the subtropical convergence zone from 52° W to 70° W in the oligotrophic Sargasso Sea. Persistent stationary layers and several migrating components of different scattering characteristics were detected. The results reveal varying vertical migration patterns, including different times of onset of diel vertical migration in different depths and a migrant pathway emerging daily from the lower deep scattering layer (DSL) at dusk and migrating through the upper DSL without affecting its composition. Fish species identification was made based on morphological characteristics and confirmed by genetic barcoding analyses of subsamples. In total, 5022 fish specimens from 27 families, 62 genera and 70 species were caught. In terms of relative abundance (A) and biomass (M), catches were dominated by species of the families Myctophidae (A=59.1%, M=47.4% of total fish catch) and Melamphaidae (A=22.5%, M=17.1%). Myctophidae and Stomiidae were the most species-rich families with 31 and 12 species, respectively. Catches at the two easternmost stations were dominated by Scopelogadus mizolepis and Nannobrachium cuprarium, while Bolinichthys photothorax and Ceratoscopelus warmingii were the most abundant species in catches from the two westernmost stations. This study provides insights into distribution and vertical migration behaviour of mesopelagic fish in the Sargasso Sea and adds to our understanding of the mesopelagic community in this large oceanic area.

Mesopelagic flesh shear viscosity estimation from in situ broadband backscattering measurements by a viscous–elastic model inversion

In fisheries acoustics, target strength (TS) is a key parameter in converting acoustic measurements to biological information such as biomass. Modelling is a versatile tool to estimate TS of marine organisms. For swimbladdered fish, flesh shear viscosity is one of the required parameters to correctly calculate TS around the resonance frequency, where the target scatters most strongly. Resonance of mesopelagic swimbladdered fish can occur over a range of frequencies and can be within commonly used frequencies (e.g. 18, 38, or 70 kHz). Since there is little information on flesh shear viscosity of fish, especially for mesopelagic species, their resonance can bias the biological information extracted from acoustic measurements. Here, first, the applicability of using a spherical model to estimate resonant backscattering of a generic swimbladder is investigated. Subsequently, a viscous–elastic spherical gas backscattering model is used to estimate the flesh shear viscosity of swimbladdered mesopelagic fish (most likely Cyclothone spp., Family: Gonostomatidae) from in situ broadband backscattering measurements. Finally, the effects of flesh shear viscosity on the TS of swimbladdered mesopelagic fish at 18, 38 (a widely used channel to study mesopelagic layers), and 70 kHz are examined.

Shifts in the Assemblage of Summer Mesopelagic Fish Larvae in the Gaoping Waters of Southwestern Taiwan: A Comparison between El Niño Events and Regular Years

We investigated changes in the assemblages of summer mesopelagic fish larvae between El Niño events and regular years in 2014–2018 and evaluated their relationships with the hydrographic conditions of the Gaoping waters off southwestern Taiwan. Seventy-five taxa or morphotypes were identified, with five types of Benthosema pterotum (31.2%), Diaphus slender type (19.9%), Cyclothone alba (7.2%), Diaphus stubby type (5.9%), and Vinciguerria nimbaria (4.4%) being most common during the study period. The hydrographic conditions of the Gaoping waters were likely influenced by large-scale climate change via oceanic physical processes. Apparently higher seawater temperature, mixed layer depth, and lower salinity in the upper 100 m were observed at the end of the strong El Niño events (summer 2016). In addition, the certain dominant taxa exhibited contrasting patterns between El Niño events and regular years. In this study, although the abundance and composition of mesopelagic fish larvae assemblage were not influenced directly by changes in large-scale climatic conditions, the occurrence of mesopelagic fish larvae was related to seawater temperature, salinity, and chlorophyll a concentration. We speculated that despite the abundant food availability and the more mesopelagic fish larvae onto the Gaoping waters transported by the increased inflow of the South China Sea Surface Current during El Niño events, the higher temperature and lower salinity at the inshore upper waters might be unsuitable for mesopelagic fish larvae, possibly resulting in low egg and larval survival.

Global nutrient cycling by commercially-targeted marine fish

Throughout the course of their lives fish ingest food containing essential elements, including nitrogen (N), phosphorus (P) and iron (Fe). Some of these elements are retained in the fish body to build new biomass, which acts as a stored reservoir of nutrients, while the rest is excreted or egested, providing a recycling flux to water. Fishing activity has modified the fish biomass distribution worldwide and consequently may have altered fish-mediated nutrient cycling, but this possibility remains largely unassessed, mainly due to the difficulty of estimating global fish biomass and metabolic rates. Here we quantify the role of commercially-targeted marine fish between 10 g and 100 kg () in the cycling of N, P and Fe in the global ocean, and its change due to fishing activity, by using a global size-spectrum model of marine fish populations calibrated to observations of fish catches. Our results show that the amount of nutrients stored in the global pristine , biomass was generally small compared to the ambient surface nutrient concentrations but significant in the nutrient-poor regions of the world: the North Atlantic for P, the oligotrophic gyres for N and the High Nutrient Low Chlorophyll (HNLC) regions for Fe. Similarly, the rate of nutrient removed from the ocean through fishing is globally small compared to the inputs, but can be important locally especially for Fe in the equatorial Pacific and along the western margin of South America and Africa. This model allowed us to compute the spatial distribution of the cycling of elements by the biomass at pristine and global peak catch state, which is relatively small compared to the estimated primary production demand for nutrients and estimated export production of nutrients. Pristine cycling (excretion + egestion) accounted for less than 2.7 % of the primary productivity demand for N, P and Fe globally. Relative to the export of nutrients, modeled global pristine egestion represents on average 2.3 %, 3.0 % and 1.1–22 % for N, P and Fe (low-high estimates), respectively, with a higher fraction in the low-export oligotrophic tropical gyres. Our study highlights the role of the fraction of the icthyosphere (i.e. does not include non-commercial species such as mesopelagic fish) on nutrient storage and cycling, and the potential role of fishing activities on this cycling, which could be of importance in regions of low nutrient concentration, high fish biomass and/or high productivity demand, and especially at the more local scale for Fe.

Mesopelagic Fish Protein Hydrolysates and Extracts: A Source of Novel Anti-Hypertensive and Anti-Diabetic Peptides

The abundance of fish and zooplankton (1 × 109 tons to 7 × 1010 tons) in the mesopelagic zone of the ocean is a source of novel raw materials that provides opportunities for sustainable new product development. The peculiar conditions of light and pressure in this ecological zone and the position of the specific organisms in the marine food chain results in diversity in their bio-composition. Mesopelagic fish are an underutilized resource, rich in proteins and omega-3 oils, and present opportunities to develop novel feed, food and functional food ingredients and products. However, there is also a need to ensure that this resource is not overfished and is processed to optimize the catch in line with sustainability goals. There is therefore a need to establish sustainable bioprocessing technologies to yield value added products from mesopelagic fish species. In the present study, various protein extracts from the mesopelagic fish Maurolicus muelleri (M) and Meganyctiphanes norvegica (Northern Krill) (K) and combinations of proteins from these species (C) were generated using hydrolysis methods. Protein Hydrolysates were generated using four different enzymes including Alcalase, endocut-01, endogenous M/K enzymes and FoodPro PNL. Hydrolysates were characterized and assessed for their ability to inhibit enzymes important in diseases associated with metabolic syndrome. The ability of generated Hydrolysates to inhibit enzymes including Angiotensin-1-converting enzyme (ACE-1; EC. 3.4.15.1) associated with blood pressure regulation, Acetylcholinesterase (AChE; EC 3.1.1.7) associated with maintenance of the nervous system, and Dipeptidyl peptidase IV (DPP-IV; EC 3.4.14.5) linked with development of type-2-diabetes, was determined. In a separate process, the same mesopelagic fish species were transformed into fishmeal, Hydrolysates, fish-silage, and aqueous extracts (AQ) and screened for bioactivities using the same bioassays. The Hydrolysates contained greater than 60% protein (dry weight basis) when analyzed using the DUMAS method. A hydrolysate from M. muelleri generated using FoodPro PNL (M1) inhibited the ACE-1 enzyme by 89.56% when assayed at a concentration of 1 mg/ml compared to the positive control Captopril©. Aqueous extract two (AQ2) inhibited ACE-1 by 95.28% when assayed at 1 mg/ml compared to the control. Sample M1 inhibited DPP-IV by 100% and aqueous extract one (AQ1) inhibited the same enzyme by 90.08% when assayed at a concentration of 1 mg/ml compared to Sitagliptin used as the positive control. All samples assayed did not significantly inhibit the enzyme AChE–fraction C3 (combined hydrolzate 3: Krill and M. muelleri) inhibited AChE by 27.48% only. Based on these results samples M1, C3, and AQ1 were selected for further characterization and the IC50 values for each were determined in relation to ACE-1 and DPP-IV inhibition as well as their amino acid composition. Glutamate and aspartate were the most abundant amino acids in the selected samples. IC50 values of <0.2 mg/ml and distinct terminal amino acids were identified in each of the three fractions. The study shows that targeting processing of mesopelagic fish have potential to generate Hydrolysates for use in the prevention of type-2-diabetes and hypertension.

Potential for Mesopelagic Fishery Compared to Economy and Fisheries Dynamics in Current Large Scale Danish Pelagic Fishery

Mesopelagic fish species represent a large potentially unexploited resource for the fishing industry and the fish meal, oil, nutraceutical, and pharmaceutical production. However, thorough investigation on ecological sustainability and socio-economic viability are fundamental prerequisites for potential exploitation. The current study explores the economic viability of a potential mesopelagic fishery investigating minimum catch rates, under the assumption of previous assessments of biological sustainability of such exploitation. We analyzed fishery data from the North-East Atlantic fisheries of the Danish large pelagic fleet from 2015 to 2019, by comparing the combined data on fishing dynamics and cost-structures with data from interviews of key pelagic producer organization representatives to develop scenarios of profitability. The results show full year-round fleet occupation with the ongoing fisheries, exposing the need of switching from existing activities, or investing into new vessels for conducting potential mesopelagic fishery. Economic analyses revealed that the minimum revenue to break even (zero profit) by trip varies among métiers between 60,000 and 200,000 euro showing strong positive correlation with vessel sizes. High profitability was discovered for herring, Atlantic mackerel and blue whiting fisheries while low profitability was observed for the Norway pout fishery. Due to the lack of mesopelagic fishery data, different scenarios of profitability were investigated as informed by the pelagic catch sector stakeholder perceptions of prices and costs and compared to current economic dynamics. A high break-even revenue per trip was forecasted given the increased perceived costs for fuel, modifications of gears and on-board processing methods and potential new vessel investments. High profitability may be reached if the catches exceed 220–1,060 tons per trip depending on costs and vessel storage capacity. If the conservation methods are improved from current refrigerated sea water, fishing trips could last longer than 5 days, being the major limiting economic factor for potential mesopelagic fishery. Future investigations on realistic mesopelagic catches, trip durations and spatio-temporal distribution of fisheries in relation to location, resource abundance, fishing rights, storage and conservation methods will be essential to test the robustness of the scenarios proposed in this study, and will in turn benefit of the economic requirements evaluated herein.

Fine-scale structures as spots of increased fish concentration in the open ocean

Oceanic frontal zones have been shown to deeply influence the distribution of primary producers and, at the other extreme of the trophic web, top predators. However, the relationship between these structures and intermediate trophic levels is much more obscure. In this paper we address this knowledge gap by comparing acoustic measurements of mesopelagic fish concentrations to satellite-derived fine-scale Lagrangian Coherent Structures in the Indian sector of the Southern Ocean. First, we demonstrate that higher fish concentrations occur more frequently in correspondence with strong Lagrangian Coherent Structures. Secondly, we illustrate that, while increased fish densities are more likely to be observed over these structures, the presence of a fine-scale feature does not imply a concomitant fish accumulation, as other factors affect fish distribution. Thirdly, we show that, when only chlorophyll-rich waters are considered, front intensity modulates significantly more the local fish concentration. Finally, we discuss a model representing fish movement along Lagrangian features, specifically built for mid-trophic levels. Its results, obtained with realistic parameters, are qualitatively consistent with the observations and the spatio-temporal scales analysed. Overall, these findings may help to integrate intermediate trophic levels in trophic models, which can ultimately support management and conservation policies.