Time Series Of Acoustic Data Research Articles

Multiple sampling methods to develop indices of mid‐trophic levels abundance in open ocean ecosystems

AbstractMid‐trophic level (MTL) organisms play a key role in the sub‐Antarctic ecosystem food web, linking primary producers and tertiary consumers transferring energy across trophic levels. In this region, the relative abundance of MTLs has been monitored using single‐frequency acoustic data collected opportunistically in a time series of trawl surveys carried out in the austral late spring/early summer. To advance this approach, we developed a methodology that enabled us to disaggregate multifrequency acoustic data into three different MTL groups to assess their temporal and spatial patterns: mesopelagic fish, gelatinous zooplankton (excluding pyrosomes), and euphausiids. We used acoustic data and biological sampling information collected with two midwater trawl nets from a dedicated voyage, to train a classification tree algorithm. We complemented these data with footage from an in‐trawl camera system that gave us new insights on vertical distribution and presence of pelagic taxa. The testing dataset used to implement our classification algorithm was a time series of acoustic data from the sub‐Antarctic trawl survey. This approach was successful at disaggregating acoustic data and provided useful information to study the relative abundance and temporal and spatial patterns of pelagic taxa. Our analysis revealed the presence of dense shallow layers of gelatinous zooplankton which showed a statistically significant increasing trend over the time series. Although we were unable to detect pyrosomes acoustically, we encountered large biovolumes of these organisms during our biological sampling, which was supported by the in‐trawl camera system.

Estimates of density of mesopelagic fish in the Southern Ocean derived from bulk acoustic data collected by ships of opportunity

The demand for information on mid-trophic level (MTL) organisms in open-ocean marine ecosystems has led to initiatives to collect acoustic data opportunistically in different regions around the world. Although, bulk acoustic data can provide information on the distribution patterns and dynamics of MTL organisms, it is necessary to convert acoustic-derived indices into biologically relevant quantities for parameterising and validating ecosystem and trophic models. A 7-year time series of acoustic data collected by ships of opportunity (SOOP) in the New Zealand sector of the Southern Ocean, information on species’ distribution derived from trawl samples collected in research voyages, and target strength (TS) estimates obtained using a resonance-scattering model and literature TS–length relationships, were used to obtain the first estimates of density of mesopelagic fish in this region. Estimates of mesopelagic fish density decreased from north to south reflecting changes in species composition and scattering properties across three latitudinal regions (Northern, Central and Southern). Density estimates ranged from 16.4–40.1 in the north to 4.4–13.4 g m−2 in the south. Catches revealed that the Northern region was dominated by Lampanyctodes hectoris and Protomyctophum sp. (Myctophidae) and Maurolicus australis (Sternoptychidae); the Central and Southern regions were dominated by the myctophids Electrona antarctica and Protomyctophum sp. and the Antarctic krill, Euphausia superba. Information on species composition was the main source of uncertainty in the density estimates, highlighting the need for more biological sampling. This study demonstrates that it is possible to integrate acoustic data collected opportunistically with auxiliary information from research voyages and literature to provide estimates of mesopelagic fish biomass in remote areas.

Acoustic Assessment of the Micronekton Community on the Chatham Rise, New Zealand, Using a Semi-Automated Approach

Micronekton are a key component of the pelagic food web of the Chatham Rise east of New Zealand. The Chatham Rise is an important fishing area for hoki (Macruronus novaezelandiae), New Zealand’s largest finfish fishery, and a predator on mesopelagic fish. Four fisheries oceanographic voyages provided multi-frequency acoustic data (18, 38, 70 120 and 200 kHz) and midwater trawls, which were used to define a classification tree to separate micronektonic organisms. We carried out validation and sensitivity analyses that showed that we were able to classify pearlside (Maurolicus australis) and euphausiids. Other mesopelagic targets (mainly myctophids) were classified together based on their acoustic frequency response. Using scripting in the open-source software ESP3, we applied our classification tree to an independent time series of acoustic data from trawl surveys on the Chatham Rise between 2009 and 2018, that was not used for model development or validation. Our methodology allowed us to study temporal and spatial patterns of M. australis, euphausiids, and total backscatter in the water column. Total backscatter associated with micronekton has varied over the last ten years, with no clear trend. The abundance of euphausiids showed a significant decreasing trend over the last ten years. Abundance of M. australis also decreased since 2012, though this was not significant. This work contributes to on-going efforts to monitor and detect changes in the pelagic ecosystems.

Temporal and spatial mapping of red grouper Epinephelus morio sound production.

The goals of this project were to determine the daily, seasonal and spatial patterns of red grouper Epinephelus morio sound production on the West Florida Shelf (WFS) using passive acoustics. An 11 month time series of acoustic data from fixed recorders deployed at a known E. morio aggregation site showed that E. morio produce sounds throughout the day and during all months of the year. Increased calling (number of files containing E. morio sound) was correlated to sunrise and sunset, and peaked in late summer (July and August) and early winter (November and December). Due to the ubiquitous production of sound, large-scale spatial mapping across the WFS of E. morio sound production was feasible using recordings from shorter duration-fixed location recorders and autonomous underwater vehicles (AUVs). Epinephelus morio were primarily recorded in waters 15-93 m deep, with increased sound production detected in hard bottom areas and within the Steamboat Lumps Marine Protected Area (Steamboat Lumps). AUV tracks through Steamboat Lumps, an offshore marine reserve where E. morio hole excavations have been previously mapped, showed that hydrophone-integrated AUVs could accurately map the location of soniferous fish over spatial scales of <1 km. The results show that passive acoustics is an effective, non-invasive tool to map the distribution of this species over large spatial scales.

Identification of aero-acoustic scattering matrices from large eddy simulation: Application to whistling orifices in duct

The identification of the aero-acoustic scattering matrix of an orifice in a duct is achieved by computational fluid dynamics. The methodology first consists in performing a large eddy simulation of a turbulent compressible flow, with superimposed broadband acoustic excitations. After extracting time series of acoustic data with a specific filter, system identification techniques are applied. They allow us to determine the components of the acoustic scattering matrix of the orifice. Following the same procedure, a previous paper determines the scattering features of a sudden area expansion. In the present paper, the focus is on whistling orifices. The whistling ability of the tested orifice is evaluated by deriving the acoustic power balance from the scattering matrix. Comparisons with experiments at two different Mach numbers show a good agreement. The potential whistling frequency range is well predicted in terms of frequency and amplitude.

Identification of aero-acoustic scattering matrices from large eddy simulation. Application to a sudden area expansion of a duct

A methodology is presented which allows to determine the coefficients of transmission and reflection of plane acoustic waves at flow discontinuities in piping systems by combining large eddy simulation (LES) of turbulent compressible flows with system identification. The method works as follows. At first, an LES with external, broadband excitation of acoustic waves is carried out. Time series of acoustic data are extracted from the computed flow field and analyzed with system identification techniques in order to determine the acoustic scattering coefficients for a range of frequencies. The combination of broadband excitation with highly parallelized LES makes the overall approach quite efficient, despite the difficulties associated with simulation of low-Mach number compressible flows. The method is very general, here it is applied to study the scattering behavior of acoustic waves at a sudden change in cross-section in a duct system. The complex aero-acoustic interactions between acoustic waves and free shear layers are captured in detail by high resolution compressible LES, such that the scattering coefficients can be determined accurately from first principles. In order to demonstrate the reliability and accuracy of the method, the results for the scattering behavior and the acoustic impedance are presented and physically interpreted in combination with several analytical models and experimental data.