Acoustic backscatter from zooplankton and fish explored through an optimized model framework

The purpose of this work has been to test complementary methods in order to classify marine organisms, with particularly attention to zooplankton and fish. Algorithms to separate fish and zooplankton have been developed and implemented at IMR and at IRD. A novel optimised model framework based on known scattering models are used to classify zooplankton and to separate these from fish. Acoustic data from up to 6 frequencies were collected to test the scattering model framework, while concurrent biological samples from multi-net oblique or horizontal MOCNESS tows, WP2 vertical net hauls and pelagic trawl were also obtained and analysed. Great attention are given on one side to the inter calibration and the comparability of all the frequencies, and to the space and time coherence between the samples collected and the acoustical data which are processed. All algorithms involve zooplankton scattering models, the high-pass ones from Stanton, or the more complex ones like the truncated fluid sphere from Holliday or the DWBA set of models from Chu and Stanton. A set of reliable acoustical and biological data has been chosen in order to proceed to comparisons between the results of the acoustic data processing through the classification algorithms and the results of the biological processing

An investigation into the zooplankton composition of a prominent 38-kHz scattering layer in the North Sea

This study aims to determine the contribution made by zooplankton to acoustic scattering layers, which are particularly strong at 38 kHz, in the northern North Sea in summer. It uses a combination of net sampling and forward and inverse acoustic modelling of data collected at 38, 120 and 200 kHz in July 2003. Zooplankton samples were collected from regions of strong acoustic scattering in depths to similar to 50 m, using a undulating towed (U-tow) vehicle. Acoustic data recorded simultaneously were scrutinized to determine actual backscattering, expressed as mean volume backscattering strength (MVBS) (dB). This observed MVBS (MVBSobs) was compared with backscattering predicted by application of appropriate acoustic models (MVBSpred) to sampled densities of zooplankton. In all instances, MVBSobs was greater than MVBSpred, with the difference considerably more pronounced at 38 kHz. There was a weak correlation between MVBSpred and MVBSobs at all three frequencies, with the greatest correlation (r = 0.450, P = 0.545) at 120 kHz. A number of candidate acoustic models were inverted in order to infer the most likely type of scatterer. In most cases, scatterers with a gas inclusion were predicted by this method. Potential sources of inconsistencies between MVBSpred and MVBSobs were identified which, when considered alongside the presented forward and inverse solutions, indicate that 38 kHz scattering in particular must be due to sources other than sampled zooplankton.</p

Biological characteristics of the hydrological landscapes in the Bay of Biscay in spring 2009

In the present study we investigated the biogeography of macrozooplankton and fish biomass in the Bay of Biscay. In this region, we defined six different landscapes based on the hydrogeographical characteristics observed in spring 2009. We then related landscape's characteristics and environmental parameters such as light attenuation depth and chlorophyll-a with macrozooplankton and fish acoustic biomass. Hydrodynamic structures together with coastal influences (river discharges, predation pressure and depth preference) and vertical thermohaline structure/mixing (feeding modes and ability to stay in preferred layers) appeared as the main factors determining the biological distribution. Besides, variance partitioning was used to assess the respective roles played by the hydrological environment, the geographical space and the biological environment alone, and their interactions. Results revealed that: (i) macrozooplankton and fish have a preference for different hydrogeographical landscapes; (ii) the association between hydrological conditions and geographical features, i.e. the spatial structure of the hydrological environment, plays a key role in the distribution of macrozooplankton; and (iii) prey-predator relationships have to be taken into account to provide a comprehensive characterization of habitat suitability

Acoustic reveal the presence of Macrozooplankton biocline in the Bay of Biscay in response to hydrological conditions and predator-prey relationships

Bifrequency acoustic data, hydrological measurements and satellite data were used to study the vertical distribution of macrozooplankton in the Bay of Biscay in relation to the hydrological conditions and fish distribution during spring 2009. The most noticeable result was the observation of a &#8216;biocline&#8217; during the day i.e., the interface where zooplankton biomass changes more rapidly with depth than it does in the layers above or below. The biocline separated the surface layer, almost devoid of macrozooplankton, from the macrozooplankton-rich deeper layers. It is a specific vertical feature which ties in with the classic diel vertical migration pattern. Spatiotemporal correlations between macrozooplankton and environmental variables (photic depth, thermohaline vertical structure, stratification index and chlorophyll-a) indicate that no single factor explains the macrozooplankton vertical distribution. Rather a set of factors, the respective influence of which varies from region to region depending on the habitat characteristics and the progress of the spring stratification, jointly influence the distribution. In this context, the macrozooplankton biocline is potentially a biophysical response to the search for a particular depth range where light attenuation, thermohaline vertical structure and stratification conditions together provide a suitable alternative to the need for expending energy in reaching deeper water without the risk of being eaten

Abundance estimates of micronekton organisms in tropical Pacific Ocean from trawl sampling, acoustic survey and backscatter models [résumé]

ICES. Working Group of Fisheries Acoustics, Science and Technology (WGFAST), Somone, SEN, 25-/04/2022 - 28/04/2022Micronekton, ubiquitous to all oceans plays a pivotal role in the trophic organisation and consti-tutes the food of most top predators. Due to the paucity of sampling, estimates of these organ-isms abundance and specie distribution is largely unknown. Such sampling comes either from trawls or active acoustic. One key question remains on how these two means of observations compare and complement each other. Our study focuses on the analysis of active acoustic data from 8 oceanographic surveys in South Pacific. Two active acoustic methods were used simulta-neously: hull-mounted echo sounders and a wideband profiler. These data are examined to-gether with biological samples obtained by trawling which brings a ground truth to the acoustic measurements. By comparing the in situ acoustic response, the modelled response from biologi-cal sampling and the density of organisms calculated from wideband profiles, we obtain an order of abundance estimates of micronekton in depth layers. This comparison enables us to estimate the observed differences of organisms abundance between the three methods and helps under-standing. Over the entire cruises, the average ratio between abundances derived from acoustic sounders and those obtained from trawled samples is on the order of 10 but varies strongly with depth

International conference ICAWA 2017 and 2018 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters

Matecho is an automated processing method to extract information and perform echo-integration and fish shoal extraction from various scientific echo-sounder sources providing digital acoustic data on fisheries and aquatic ecosystem. The open-source initiative helps foster collaboration and technological transfer. Matecho supports various formats, such as the international standard format for the exchange of fisheries acoustics raw and edited data. The procedure allows the semiautomatic cleaning of echogram data and the application of automatic data filters, i.e., transient noise, attenuated signal and impulsive noise removal and background noise reduction. Echo-integration processing is executed for each depth layer and integrates their characteristics per elementary sampling unit. Sound scattered layers are automatically detected by segmentation from the echo-integrated echogram, and shoals are extracted according to an iterative process of aggregation of filtered echogram echoes that allows, in both cases, the calculation of the ad hoc parameters describing morphological, spatial location and acoustic characteristics of sound scattered layers and shoals. Matecho is open-source software for researchers and provides end users with a user-friendly, free executable program

A bi-frequency discrimination method of copepods in the Senegalese coast [résumé]

ICES. Working Group of Fisheries Acoustics, Science and Technology (WGFAST), Somone, SEN, 25-/04/2022 - 28/04/2022The Canary Current Large Marine Ecosystem (CCLME) is one of the most productive marine ecosystem worldwide and is key for food security for numerous African countries. Nevertheless, its function remains poorly described and ecosystemic data collection are rare. Copepods are the key macrozooplankton group in the CCLME but their dynamic, their distribution and even their abundance remain poorly documented. Multinet net data allowed identifying large Copepod in CCLME. As small pelagic fish assessment acoustics survey were routinely done using 38 and 120 kHz frequencies, we used the same frequencies to propose a bi-frequencies inversion method to discriminate Copepod. We identified copepod backscatter using differences in volume backscat-tering strength (Sv). A close significant relationship were found between the size values of Cope-pod from multinet samples with those calculated by the acoustic highpass model. The correlation between copepod abundance and corresponding Sv were positive. This work showed that 38-120 kHz frequency can be used on Copepod and thus open the way to retrospective analysis in the CCLME. These results were important to better understand marine ecosystem, and constitute a first step for Copepod biomass estimation in the context of ecosystemic approach of small pelagic fish management and climate change