Multifrequency acoustic target strength of Northern krill

Abstract

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.