Modeling the target strength of Meganyctiphanes norvegica

Abstract only. Journal home page: http://scitation.aip.org/jasa

Evaluation of Antarctic krill biomass and distribution off the South Orkney Islands 2011-2015

-Annual acoustic trawl surveys for krill monitoring have been carried out by the Institute of Marine research, Norway near the South Orkney Islands since 2011. The survey has been conducted early in the fishing season (January/February), using two different krill fishing vessels as platforms. The vessels were equipped with similar Simrad echo sounder systems suitable for quantitative assessments, but the frequencies operated varied between vessels and years. In addition, the survey coverage has varied between years in particular due to ice. In order to allow for comparison of distribution and abundance of krill near the South Orkneys among years, we here attempt to generate a coherent series of krill density estimates from the 5 years of surveys. We follow the CCAMLR protocol for biomass estimation as far as practically possible, given that we work with unconventional sets of frequencies for target strength estimation and target identification. In order to avoid variability due to differences in coverage, we also define a stratum within the survey area on the north side of the islands with full coverage in all years except 2013. The results show that except from the year 2015, krill densities were high, in the range 100-300 g/m2 based on the 120 kHz recordings, and a total estimated biomass of ca. 8 million tons of krill within the stratum in the peak year 2014. There were also consistently higher values of acoustic backscatter on the north side of the islands where the fisheries occur, than on the south side. Values were particularly high in the north-west shelf area associated with underwater canyons. In 2012, and particularly in 2015, low proportions of the acoustic backscatter were allocated to krill. The low proportions are probably caused by shortcomings in the krill identification techniques when other pairs of frequencies than combinations of 38, 120 and 200 kHz are use

Stime della biomassa marina attraverso il metodo acustico: discernimento delle specie e gestione delle risorse ittiche

[EN] Acoustics is the basics of the most important technologies for underwater telecommunication, as well as for target detection and identification in the aquatic media. Multiple frequency measurements are the key for species discrimination and open the door for sustainable fisheries. The development of wider broadband systems and quantitative multi-beam sonars and processing techniques constitute the present challenge for scientists and developers. In parallel, simpler and cost-efficient systems like satellite buoys can offer clue information for marine ecosystem monitoring or target species fisheries.[IT] Acoustics is the basics of the most important technologies for underwater telecommunication, as well as for target detection and identification in the aquatic media. Multiple frequency measurements are the key for species discrimination and open the door for sustainable fisheries. The development of wider broadband systems and quantitative multi-beam sonars and processing techniques constitute the present challenge for scientists and developers. In parallel, simpler and cost-efficient systems like satellite buoys can offer clue information for marine ecosystem monitoring or target species fisheries.We acknowledge the Associazione Italiana di Acustica the invitation and the opportunity of participating in the Symposium The SCIENCE OF ACOUSTICS serving the Food, the Nutrition and the Environment: From protection system to innovative technologies for agriculture and fisheries”, at the EXPO Milan (Italy), in September 2015. E. soliveres acknowledges support of Spanish Government grant AP2009-4459 FPU Subprogram.Espinosa Roselló, V.; Calise, L.; Estruch, VD.; Llorens Escrich, S.; Ordoñez Cebrián, P.; Pérez Arjona, I.; Puig Pons, V.... (2015). Acoustical estimation of fish biomass: species identification and stocks management. Rivista Italiana di Acustica. 39(3):15-21. http://hdl.handle.net/10251/63737S152139

Multifrequency acoustic target strength of Northern krill

Although this thesis is a dissertation based on articles, an exhaustive overview on the main issues involved is also presented *. This is due to the necessity to summarize the numerous aspects of the problem, as well as the recent works on specific topics and the innovative elements presented in this work of thesis. In some of its parts, it may appear repetitive due to the treatment of the specific topic. The contents of the papers included in the thesis are referred when the related issue is treated. With the intent to provide a useful guide to the reader, a large effort has also been undertaken to select and indicate specific bibliographic references. The first chapter describes the background on the acoustic estimation method by using scientific echosounders for zooplankton in general. Then, the specific techniques adopted for identification of krill by using both theoretical and empirical approaches are introduced. The chapter starts with the description of the order Euphausiacea, focusing on its main biological characteristics related to the analysis of the acoustic data. An introduction on the Antarctic krill fishery in Southern Pole areas is also given In Chapter 2, the theoretical approach on target strength of krill and the key parameters for the modelling are resumed. The krill orientation problem is discussed with reference to the main studies in estimating mean angle and standard deviation of a normal distribution of euphausiids orientation. Paragraph 2.3 explores the methods for the evaluation of the key model parameters being the background for the Papers I. The last Section 2.4 describes in details the distorted wave Born approximation model in its various forms. This is recognized as the state-of-the-art in the physics-based models predicting target strength of fluid-like crustacean zooplankton, and deserved a detailed desciption, being also an object of Paper V. Chapter 3 represents a central point of the entire thesis. Ex situ direct acoustic measurements on Northern krill Meganyctiphanes norvegica in a novel set-up consisting in a wide mesocosm venue are described. The practical problems and their consequences from this experience are described as a complement to Paper II. The potential of the mesocosm set-up in identifying the targets is discussed, and some specific experiments are described in detail. Papers IV and V represent part of the conclusions obtained from this experience. As a consequence of the mesocosm practice, the need to evaluate the correctness of the short-range performances of the Simrad EK60 scientific echosounder was addressed. This was done in a controlled way in a tank at Simrad AS in Horten (Norway) and is the subject of Paper III. Chapter 4 justifies the need and presents complementary results to the Paper III. In Chapter 5, a brief overview describing the general issues for a multifrequency echosurvey for krill estimation is presented. Complexity in suggesting a proper procedure for multifrequency echosurvey in the case of Meganyctiphanes norvegica aggregations is emphasized, reflecting Paper VI. Finally, the historical evolution of the scattering models for fluid-like organisms such as euphausiids is presented in Appendix. This is not directly related to the included papers, and some of its sections may appear too detailed. However, the topic is fundamental for the acoustic method for krill biomass estimation, and the presentation in an appendix was somewhat obligatory. The description is organized as a guide for the interested reader with formulations presented in an easy form for numerical implementation. Some of the models are described in more details; this is based on the opinion of the writer on their importance in general applications

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

Modeling the target strength of Calanus finmarchicus

Abstract only. Journal home page: http://scitation.aip.org/jasa

Use of SDWBA predictions for acoustic volume backscattering and the Self-Organizing Map to discern frequencies identifying Meganyctiphanes norvegica from mesopelagic fish species

To acoustically assess the biomass of multiple species or taxa within a survey region, the volume backscatter data should be apportioned to the constituent sound scatterers. Typically, measured backscatter is attributed to certain species using predictions at different frequencies, mostly based on the difference in scattering at the frequencies of 38 and 120 kHz ('dual frequency method'). We used the full version of the stochastic distorted wave Born approximation model (SDWBA) to predict backscatter spectra for Meganyctiphanes norvegica and to explore the sensitivities of ΔMVBS to the model parameters, e.g. acoustic frequency and incidence angle, and animal density and sound speed contrast, length, and shape. The orientation is almost the unique parameter responsible for variation, with fatness affecting longer lengths. We present a summary of ΔMVBS that can serve as the basis for identification algorithms. Next, we simulate the scenario encountered in the Balearic Sea (western Mediterranean) where Northern krill are mixed with mesopelagic fish species (bristlemouths and lanternfishes), which are modeled with a prolate spheroid model. Simulated numerical data are employed to emulate the discrimination process with the most common identification techniques and typical survey frequencies. The importance of using density-independent techniques for acoustic classification is highlighted. Finally, an unsupervised neural network, the Self-Organizing Map (SOM), is used to cluster these theoretical data and identify the frequencies that provide, in this case, the most classification potential. The simulation results confirm that pairs of frequencies spanning the Rayleigh and geometric scattering regimes of the targets are the most useful for clustering; a minimum of four frequencies are necessary to separate the three species, while three frequencies are able to differentiate krill from mesopelagic fish species.En prensa2,421

Evaluation of Antarctic krill biomass and distribution off the South Orkney Islands 2011-2015

Annual acoustic trawl surveys for krill monitoring have been carried out by the Institute of Marine research, Norway near the South Orkney Islands since 2011. The survey has been conducted early in the fishing season (January/February), using two different krill fishing vessels as platforms. The vessels were equipped with similar Simrad echo sounder systems suitable for quantitative assessments, but the frequencies operated varied between vessels and years. In addition, the survey coverage has varied between years in particular due to ice. In order to allow for comparison of distribution and abundance of krill near the South Orkneys among years, we here attempt to generate a coherent series of krill density estimates from the 5 years of surveys. We follow the CCAMLR protocol for biomass estimation as far as practically possible, given that we work with unconventional sets of frequencies for target strength estimation and target identification. In order to avoid variability due to differences in coverage, we also define a stratum within the survey area on the north side of the islands with full coverage in all years except 2013. The results show that except from the year 2015, krill densities were high, in the range 100-300 g/m2 based on the 120 kHz recordings, and a total estimated biomass of ca. 8 million tons of krill within the stratum in the peak year 2014. There were also consistently higher values of acoustic backscatter on the north side of the islands where the fisheries occur, than on the south side. Values were particularly high in the north-west shelf area associated with underwater canyons. In 2012, and particularly in 2015, low proportions of the acoustic backscatter were allocated to krill. The low proportions are probably caused by shortcomings in the krill identification techniques when other pairs of frequencies than combinations of 38, 120 and 200 kHz are use