Application of hydroacoustics to investigate the distribution,diel movement,and abundance of fish on Zhubi Reef,Nansha Islands,South China Sea

A combination of traditional fish sampling methods(hand-line and gill net) and modern hydroacoustic techniques were used to study fish community structure,distribution,and diel movements of fish on Zhubi Reef to enhance understanding of the ecosystem.We collected 126 individuals from 29 species,20 genera,17 families,three orders,and two classes using traditional gear.Perciforms were the dominant group in terms of species richness and biomass.The acoustic data indicated that very small(target strength[TS],dB) <-60 dB) and small(-60 dB≤TS<-45 dB) fish contributed the most to abundance and species richness on the coral reef,and that the proportion of medium-sized(-45 dB≤TS<-35 dB) and largesized(-35 dB≤TS) fish increased gradually as depth increased.The single-target detection method revealed two distinct size classes during the day in the 12-16 and 16-20-m layers.One group consisted of very smallsized fish(TS<-60 dB) and the other consisted of medium and large-sized fish(TS>-55 dB).The number of single-target detections was significantly higher during the night than during the day(P<0.05).The singletarget TS frequency distribution during the day was significantly different than during the night at depths of4-8,8-12,12-16,and 16-20 m.Significant differences were observed among the 4-8,8-12,12-16,and16-20-m-depth layers during day and night.Diel vertical movement was evidenced as fish began to spread and move upward just before sunset and began to assemble and descend shortly(15 min) after sunrise.

Hydroacoustic estimates of fish biomass and spatial distributions in shallow lakes

We conducted acoustical surveys with a horizontal beam transducer to detect fish and with a vertical beam transducer to detect depth and macrophytes in two typical shallow lakes along the middle and lower reaches of the Changjiang(Yangtze) River in November 2013. Both lakes are subject to active fish management with annual stocking and removal of large fish. The purpose of the study was to compare hydroacoustic horizontal beam estimates with fish landings. The preliminary results show that the fish distribution patterns dif fered in the two lakes and were af fected by water depth and macrophyte coverage. The hydroacoustically estimated fish biomass matched the commercial catch very well in Niushan Lake, but it was two times higher in Kuilei Lake. However, acoustic estimates included all fish, whereas the catch included only fish >45 cm(smaller ones were released). We were unable to determine the proper regression between acoustic target strength and fish length for the dominant fish species in the two lakes.

Spreading-rate dependence of hydroacoustic and teleseismic seismicity of ridge-transform systems:East Pacific Rise,Galapagos Ridge,and Mid-Atlantic Ridge

Seismicity in ocean ridge-transform systems reveals fundamental processes of mid-ocean ridges,while comparisons of seismicity in different oceans remain rare due to a lack of detection of small events.From 1996 to 2003,the Pacific Marine Environmental Laboratory of the National Oceanic and Atmospheric Administration(NOAA/PMEL)deployed several hydrophones in the eastern Pacific Ocean and the northern Atlantic Ocean.These hydrophones recorded earthquakes with small magnitudes,providing us with opportunities to study the seismic characteristics of ridge-transform systems at different spreading rates and make further comparisons of their differences.This study comparatively analyzed hydroacoustic and teleseismic data recorded on the fast-spreading East Pacific Rise(EPR,10°S to 12°N),intermediate-spreading Galapagos Ridge(103°W to 80°W),and slow-spreading Mid-Atlantic Ridge(MAR,15°N to 37°N).We present a systematic study of the spatial and temporal distribution of events,aftershock seismicity,and possible triggering mechanisms of aftershock sequences.Our analysis yields the following conclusions.(1)From the hydroacoustic data,the EPR transform faults had the highest average seismicity rate among the three regions.(2)Along-ridge event distributions show that a high number of earthquakes were concentrated on the EPR,while they became dispersed on the GR and fewer and more scattered on the MAR,reflecting that the different tectonic origins were closely correlated with the spreading rate.(3)Analysis from mainshock-aftershock sequences shows no significant differences in the aftershock decay rate among the three regions.(4)Multiple types of aftershock triggering models were inferred from Coulomb stress changes:strike-slip mainshocks triggered strike-slip aftershocks and normal faulting aftershocks,and normal faulting mainshocks triggered normal faulting aftershocks.Although these results are case studies,they may be applicable to other ocean ridge-transform systems in future investigations.Our results provide important new insights into the seismicity of global ocean ridge-transform systems.

Effects of oceanic ice cover and density stratification on the propagation of hydroacoustic waves

The propagation characteristic of hydroacoustic waves is studied for an ideal compressible two-layer fluid with different densities covered by an elastic ice sheet.Boundary conditions are simplified by adopting linear assumption and then the dispersion relation is derived.The analysis and visualization of the dispersion relation present the introduction of compressibility leads to the appearance of hydroacoustic wave modes while the density stratification leads to the appearance of interfacial wave mode.Larger ice thickness and the density ratio of the two fluid layers increase the wave number and group speed of hydroacoustic waves at the same frequency while the phase speed decreases.

Numerical Prediction of Marine Propeller Noise in Non-Uniform Inflow

A numerical study on the acoustic radiation of a propeller interacting with non-uniform inflow has been conducted. Real geometry of a marine propeller DTMB 4118 is used in the calculation, and sliding mesh technique is adopted to deal with the rotational motion of the propeller. The performance of the DES （Detached Eddy Simulation） approach at capturing the unsteady forces and moments on the propeller is compared with experiment. Far-field sound radiation is predicted by the formation 1A developed by Farassat, an integral solution of FW-H （Ffowcs Williams-Hawkings） equation in time domain. The sound pressure and directivity patterns of the propeller operating in two specific velocity distributions are discussed.

以波浪滑翔机为移动网关的水下无线传感器网络架构优化

This paper presents an original probabilistic model of a hybrid underwater wireless sensor network(UWSN),which includes a network of stationary sensors placed on the seabed and a mobile gateway.The mobile gateway is a wave glider that collects data from the underwater network segment and retransmits it to the processing center.The authors consider the joint problem of optimal localization of stationary network nodes and the corresponding model for bypassing reference nodes by a wave glider.The optimality of the network is evaluated according to the criteria of energy efficiency and reliability.The influence of various physical and technical parameters of the network on its energy efficiency and on the lifespan of sensor nodes is analyzed.The analysis is carried out for networks of various scales,depending on the localization of stationary nodes and the model of bypassing the network with a wave glider.As a model example,the simulation of the functional characteristics of the network for a given size of the water area is carried out.It is shown that in the case of a medium-sized water area,the model of“bypassing the perimeter”by a wave glider is practically feasible,energy efficient and reliable for hourly data measurements.In the case of a large water area,the cluster bypass model becomes more efficient.

Temporal and spatial characteristics of monopole acoustic energy dominated by unsteady thermodynamic cavitating flow

The acoustic propagation characteristics of the cavitating flow are essential for the noise suppression, but were not well studied. In the current paper, a new technique concerning the propagation path of the monopole acoustic energy is presented and two typical thermodynamic cavitation modes (the inertial and thermal modes) are selected to investigate the effect of the cavity shedding dynamics on the acoustic propagation path. In the inertial mode, the temporal variation and the spatial distributions of the monopole acoustic energy as well as the divergence of the monopole acoustic pressure are both more powerful and concentrated than that in the thermal mode. The acoustic propagation path in the thermal mode strictly satisfies the feature of the convective amplification, while there exists another propagation direction close to the normal direction of the foil surface in the inertial mode. Furthermore, the occurrence of the normal direction propagation will make the path deviate from the convective direction.

Numerical Hydro-Acoustic Prediction of Marine Propeller Noise

A source-to-far-field computation procedure aiming at predicting the noise generated by the underwater propeller was presented. Detached eddy simulation(DES) was used to resolve the unsteady flow field,which was taken as input data as noise propagation. Far-field sound radiation was performed by means of Ffowcs Williams-Hawkings(FW-H) equation. The computation procedure was finally applied to a typical marine propeller,David Taylor Model Basin(DTMB) 4118. The sound pressure and directivity patterns of this propeller were discussed.