Remote sizing of fish-like targets using broadband acoustics

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CRIMAC cruise report: Development of acoustic and optic methods for underwater target calssification - G.O. Sars 22.11 - 03.12 2022

The overarching objective of the survey is to collect data to support the CRIMAC activities and to collect data for the LoVe observatory. CRIMAC is a center of research-based innovation funded by the research council of Norway through their center for research-based innovation program (SFI). Sustainable, healthy food production and clean energy production for a growing population are important global goals, and CRIMAC will contribute to these by obtaining accurate underwater observations of gas, fish, nekton and other targets. The data will be used in conjunction with CRIMAC data from other surveys to build a reference data set for optical and acoustic target classification. The classification libraries will be used for developing methods and products toward the fishing industry and marine science. The survey was divided into two legs where leg one mainly focused on trawl instrumentation and data collection for behavioural studies on fish-trawl interactions. The main objectives of this part were to test in-trawl camera systems and data processing from such systems, test and develop trawl instrumentation and acoustic and optic monitoring of herring behaviour in relation to the trawl. The second leg of the survey focused mainly on broad band acoustic data, including sizing of fish using broad banded acoustics, noise estimation, calibration, time series consistency when changing to broad band acoustics, gas seep detection as well as performing the standard IMR LoVe transect.CRIMAC cruise report: Development of acoustic and optic methods for underwater target calssification - G.O. Sars 22.11 - 03.12 2022publishedVersio

Report from a krill focused survey with RV Kronprins Haakon and land-based predator work in Antarctica during 2018/2019

The primary objective for this krill research activity was twofold 1) to conduct a survey that provides updated estimates of the biomass and distribution of krill which are used in models to estimate sustainable yield in CCAMLR Area 48 and 2) to develop knowledge on the marine environment essential for the implementation of a Feed-Back Management (FBM) system. The survey follows a similar design as a survey initiated by CCAMLR in year 2000 for comparative purposes, but in addition focuses on high krill-density areas, contains state-of-the art methods and employs modern technology for the research topics currently in focus. In terms of FBM, Marine Protected Area (MPA) development in CCAMLR Planning Domain 1 encompasses the major krill fishing grounds. Thus, data supporting FBM are critical if the fishery is to be managed by an empirical understanding of krill density, distribution, availability and predator needs as opposed to purely conservation-based measures. A future developed FBM system, requires acoustic data to be collected, processed and reported continuously during the fishing season as a measure of the available prey field. This information can be integrated with finer-scale knowledge of krill predator feeding strategies and updated through specific scientific studies at regular (multiyear) intervals. The survey and coupled FBM process studies took place during the Austral summer 2018-2019. The work was coordinated by Norway and involved collaborative international efforts as well as vessels from Norway, Association of Responsible Krill fishing companies (ARK) and the Norwegian fishing company Aker BioMarine AS, China, Korea, Ukraine and United Kingdom. This report presents preliminary results from the survey performed with the Norwegian RV Kronprins Haakon during 08th January – 24th February 2019 and the land-based predator research carried out between 21st November 2018 and 20th February 2019.publishedVersio

Multi-frequency acoustic discrimination between gas bubble plumes and biological targets in the ocean

Seabed-originating gas bubble seeps have been observed worldwide from a variety of sources (e.g. Hovland and Judd, 1988) and are most frequently composed of methane and carbon dioxide. Some seabed gas leaks, such as “melting” methane hydrates, may intensify in the coming decades and are a subject of concern in the context of warming seas (Kvenvolden et al., 1993; Archer, 2007). The subsea gas extraction industry and proposed carbon dioxide storage in geological structures under the seabed are examples of potential manmade sources of gas bubble leaks (IPCC, 2005; DNV, 2010), and require swift and precise leak detection and identification. Active acoustic methods are well suited for rapid and cost effective monitoring of large water volumes. Scientific fisheries echo sounders provide calibrated, quantitative measures and are widely used in fish stock monitoring (Simmonds and MacLennan, 2005). As such, these were chosen within the umbrella R&D projects (AKUGAS and AALDOG), the needs of which shaped the scope and objectives of this doctoral study. Bottom-mounted echo sounders, observing laterally along the seabed are considered suitable for gas leak detection. Gas bubble plumes are easy to detect with echo sounders, but separating them from fish and plankton is not always straight forward, as some required information is lacking both for gas bubble plumes and biological targets. This lack is addressed here via selected case studies. In Paper I, the acoustic backscatter properties and natural body tilt orientation were investigated for a common schooling fish that lacks a swim bladder, lesser sandeel (Ammodytes marinus). Its natural orientation distribution was measured using optical measurement methods and is a crucial parameter affecting the acoustic backscattering from animals that are large enough to be directive targets at commonly used echo sounder frequencies. A more advanced stereo photogrammetric method was adapted and improved to fit the needs of this doctoral study in Paper II. These were implemented to characterize the natural tilt orientation distribution of euphausiids (Euphausia superba and Meganyctiphanes norvegica) in several in situ and ex situ experiments (Paper II). Krill natural tilt orientation was measured to have a rather large variability (SD of up to 30-37°). This suggests, but does not prove, that dorsal and lateral aspect krill acoustic backscatter should not be drastically different due to the variable swimming behaviour and body postures adopted by these animals. Such knowledge will be useful for krill multi-frequency identification and target strength averaging either from models or from empirical data. The stereo photogrammetric measurement method (Paper II) was later applied to support fine scale acoustic backscatter measurements on gas bubble plumes (Paper IV) and saithe (Pollachius virens) (Paper III). In Paper III, the lateral aspect acoustic backscatter of saithe was characterised at 70, 120, 200 and 333 kHz. Saithe is a good representative of large, acoustically directive schooling fish that also possesses a gas-filled swim bladder. These can create strong and similar acoustic targets to plumes of free gas bubbles rising from the seabed. Saithe lateral aspect acoustic frequency response (r(f)) was measured based on both schools, single acoustic targets and single target tracks. It was found to have an opposite trend across the acoustic frequency band compared to dorsal aspect saithe r(f) as reported in the literature. The reasons for such discrepancy are discussed along with the implications for acoustic target identification. Similarly, lateral aspect acoustic backscatter properties were characterised for induced methane, carbon dioxide and air bubble plumes at 70, 120, 200 and 333 kHz (Paper IV). A distinct gas plume frequency response was measured for gas bubbles of non-resonant size and is significantly different from the lateral aspect r(f) of saithe. In synthesis, the similarity in acoustic backscattering between a gas bubble and biological targets possessing gas inclusions is discussed, both from a literature review and the investigations included here (Papers I-IV). The prospects of acoustic-based gas bubble plume detection and identification are discussed in the context of obscuring and confounding biological targets. Acoustic frequency response, routinely used to identify some fish and plankton for species or taxa (e.g. Korneliussen and Ona, 2002; 2003; Anon., 2005), is discussed for laterally observed seabed gas bubble plumes. Lateral aspect gas bubble plumes and swim bladder bearing fish frequency response was not available and hence was measured in Papers III and IV. Based on the available research and that defended here (Papers I-IV), it is suggested that behavioural and acoustic backscattering differences can be used to separate gas bubble plumes from the most common biological targets, plankton and fish. Gas-filled swim bladder bearing fish are the most similar biological acoustic targets to the gas bubble plumes. Schooling and swim bladder bearing fish that are quite directive acoustic targets can be separated using the acoustic frequency response information (indications in Paper III). Smaller, but abundant swim bladder bearing fish, such as members from Myctophidae and Sternoptychidae, can be difficult to separate acoustically from a single gas bubble. However, the behaviour of such fish assemblages is substantially different from the gas bubble plumes. Using both backscattering frequency response and behaviour traits (at one time instance and over time) are likely to give the best chances for acoustic-based detection and identification of seabed gas bubble plumes

Target strength and tilt angle distribution of lesser sandeel (Ammodytes marinus)

Lesser sandeel (Ammodytes marinus) was investigated acoustically as well as using video camera in two field experiments in 2007 and one in 2008, both during North Sea sandeel surveys by the Norwegian research vessel “Johan Hjort”. Specially designed cubic cage was put on the sea bottom trapping sandeels inside in 2007, while in 2008 sandeels were caught by trawl and dredge and investigated acoustically in enclosed cage hanging in mid-water. In total 3 successful experiments conducted. Approximately 22 hours of acoustical and 11 hours of video data was available. Lesser sandeel target strength measurements extracted using LSSS acoustical data post- processing software by manual handpicking. Pictures from video data grabbed with Vegas Pro8.0 computer program and later carefully analyzed using ImageJ photo editing software. The way of calculating sandeel body tilt angle from data, collected with tilted camera, was developed by author. In total 5 mean TS values were calculated and presented separately by discussing strengths and possible error sources in each of them. One of the calculated means was decided to be possibly erroneous and was not used for calculating target strength-length relationship equation. Resulted sandeel target strength-length relationship equation is TS=20*log10(L) – 99.7. Sandeel tilt angle measurements from two experiment video data (2007 and 2008) differ from each other. 1.8º (±3.1º with 95% confidence interval) mean sandeel body tilt angle (as mean of all single tilt angle measurements) was calculated from 2007 video data and 23.3º (±3.0º with 95% conf.int.) from 2008 video data. Strong dispersion of single tilt angle measurements around the means was recorded (standard deviation of 24.1 and 25.4 for 2007 and 2008 data respectively)

Measuring in situ krill tilt orientation by stereo photogrammetry: examples for Euphausia superba and Meganyctiphanes norvegica

The natural body orientation adopted by krill is a crucial parameter for understanding and estimating the acoustic backscattering from these animals. Published data are scarce and are usually acquired with single-camera systems that provide suboptimal control over the measurement accuracy. Here we describe a stereo-photo camera application for accurate krill measurements in situ, based upon several Euphausia superba and Meganyctiphanes norvegica datasets. Body-tilt orientation, body length and school volume density from scattered and schooling krill are presented. Some challenges to the practical implementation of the method are discussed, including practical limits on krill body yaw angles for obtaining useful measurement accuracy and how to account accurately for the true vertical. Calibration and measurement accuracy is discussed together with a practical definition of krill body orientation. Krill sizes determined from stereo-images are compared with those measured from trawl samples. The krill body-tilt measurements yielded mean estimates of positive (head-up) or negative tilt of 9-17° with rather large spread for scattered aggregations of M. norvegica (SD=30-37°) and about half of that for polarized schools of E. superba (SD=14-17°). The measured krill body orientation distributions were also used to calculate krill acoustic target strength as predicted by the stochastic distorted wave Born approximation (SDWBA) model

Remote sizing of fish-like targets using broadband acoustics

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Remote acoustic sizing of tethered fish using broadband acoustics

Remote fish sizing is desirable in fisheries (e.g., pre-catch) and research (e.g., platforms without biological sampling capacity) applications. In those contexts, the high spatial resolution of pulse compressed broadband echoes combined with narrow beamwidth transducers makes it feasible to resolve the scattering from different parts of the fish body and hence can be used to measure the body size. A motorized apparatus was used to suspend individual fish in the acoustic beam of two laterally oriented transducers (45–90 kHz, 160–260 kHz, 12.2 m range) with precise control of rotation angle. Broadband scattering was measured from tethered Atlantic mackerel (Scomber scombrus), saithe (Pollachius virens), and pollack (Pollachius pollachius) ranging in standard length from 239 to 491 mm as a function of orientation angle to validate sizing based on the acoustic resolution of fish body parts. Under these controlled conditions, fish size was underestimated by 11–19 mm, varying with broadband pulse characteristics, orientation angle, species, and fish size. The best remote acoustic sizing results were obtained using 160–260 kHz pulses with a slow rise and fall of pulse amplitude (aka, taper)

The Hermes Lander project - the technology, the data, and the evaluation of concept and results

This report summarizes technology, operations, experience, and scientific results from the Hermes Lander project. The Project established an autonomous, multi-sensor, sub-sea sensor platform powered by batteries. The platform was launched at a coral reef location in Hola off Vesterålen and collected data during seven months in 2010 (March – September). Oceanographic and echo-sounder data were analyzed and compared with similar data from sporadically collected data from research vessels as well as model-produced data. The results demonstrate a partly chaotic variation. The project neither confirmed nor excluded a connection between coral reef and biomass density. The data and experience are crucial for efficient planning, development, and operation of the planned Statoil-financed cable observatory in the same area