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Observation of three‐dimensional flow structures and effluent transport around fish cages using a towed ADCP and free‐fall multi‐parameter profiler
Author(s) -
Yamazaki Hidekatsu,
Aoyama Wataru,
Masunaga Eiji,
Tanaka Mamoru
Publication year - 2019
Publication title -
aquaculture research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.646
H-Index - 89
eISSN - 1365-2109
pISSN - 1355-557X
DOI - 10.1111/are.13985
Subject(s) - turbulence kinetic energy , acoustic doppler current profiler , stratification (seeds) , turbulence , log normal distribution , eddy diffusion , hydrography , eddy , diel vertical migration , dissipation , biology , atmospheric sciences , mechanics , physics , oceanography , doppler effect , statistics , geology , mathematics , fishery , seed dormancy , germination , botany , astronomy , dormancy , thermodynamics
Abstract A field campaign was conducted around salmon cages, using a combination of a towed ADCP and a free‐fall multi‐parameter profiler, in order to investigate flow structures and the possible distribution of effluent materials. Two transect observations showed that hydrographic conditions changed dramatically within 5 days, from highly stratified open water conditions to weak stratification. Three‐dimensional observation revealed that flow was blocked behind the cages and that the blockage was reduced as the distance from the cages increased. The flow speed was positively correlated with the intensity of the backscattering signal. The R 2 value was high immediately behind the cages and decreased with distance from the cages. The flow time series behind the cages exhibited a k –2 power law spectrum that was consistent with a typical internal wave spectrum. This suggests that eddies shed from the cages were highly influenced by stratification. We estimated the rate of kinetic energy dissipation, ε , from YODA Profiler data based on the Thorpe scale approach. We also estimated the eddy diffusivity coefficient, K ρ . Both ε and K ρ followed a lognormal probability density function. The mean K ρ was consistent with a one‐dimensional diffusion model assessed from the R 2 values of flow speed and backscatter intensity.