Measurement of sardine-generated turbulence in a large tank

Abstract

While swimming organisms may cause turbulence, it is not clear how strong the turbulence is and if eddies are large enough to mix stratified water columns. We conducted an observational experiment in a large aquarium tank containing several thousand Japanese sardines Sardinops melanostictus. Turbulence data were collected from inside the sardine school using a turbulence microstructure profiler. The averaged turbulent kinetic energy dissipation rate was 2.3 × 10 −4 W kg −1 inside the school, while the averaged background value was 6.7 × 10 −6 W kg −1 . The school displayed fast and non-continuous 'avoidance behavior', or fast and long-lasting 'feeding behavior' during the measurements. A noticeable difference between the 2 behaviors was found in turbulent shear spectra: the avoidance behavior spectra showed a power decline in comparison with the Nasmyth empirical spectrum in the inertial sub-range, but the feeding behavior spectra exhibited no power decline, even in the inertial sub-range. In the latter case, the sardine school imparted kinetic energy into scales larger than the average individual body size of 0.173 m. This result is a counter-example to a general hypothesis that swimming organisms cannot impart kinetic energy at scales larger than their individual body size.

Intense upper ocean mixing due to large aggregations of spawning fish

“…to the point that they become comparable to turbulent eddy sizes, even if these are significantly smaller than the aggregation scale. Our work thus shows that, besides biological factors -the agitated behaviour of the anchovies while spawning may have also played a role 42 -, the mixing efficiency of biophysical turbulence is controlled by the background stratification facilitating the injection of fish-induced TKE at the required scales. This conclusion implies that, while biomixing might be inefficient within the main open-ocean pycnocline (where ≈ 1 m; refs.…”

“…Laboratory experiments suggest that the condition, ∼ , upon which efficient biomixing is contingent, is promoted by aggregations of swimmers, which can produce large, aggregationscale turbulent eddies 15,42 . Conversely, our observations suggest an alternative route toward efficient biomixing via an increase in stratification, which reduces buoyancy length scales (i.e.…”

Effects of abundance, water temperature and light on the turbulence and mixing generated by fish schools of different sizes in quiescent water in a laboratory

“…The mixing efficiency describes the ability of turbulence to mix a stratified water column and is the proportion of kinetic energy that results in potential-energy increase (Visser, 2007). The mixing efficiency can be estimated as G = 0.33(L T /L o ) 0.63 , where, L T is the Thorpe scale (biogenetically-generated overturning scale or integral length scale) and L 0 is the Ozmidov length scale, defined as L o = (ϵ/N 3 ) ½ , where N is a buoyancy frequency ~0.01 1/s for the upper ocean (Visser, 2007;Tanaka et al, 2017). In the present study, L T is assumed to be the integral length scale of the flow generated by the fish school and is estimated as L T = U × T (Moeini et al, 2021).…”

“…In the present study, L T is assumed to be the integral length scale of the flow generated by the fish school and is estimated as L T = U × T (Moeini et al, 2021). The vertical eddy diffusivity can be expressed as K v = Gϵ/N 2 (Katija, 2012;Tanaka et al, 2017;Simoncelli et al, 2018). Note that since the mixing efficiency and eddy diffusivity are obtained using the velocity dissipation rate, they are related to the momentum mixing.…”

“…They argued that although fish could increase the energy of the ocean, they could not play a significant role in mixing the ocean. The dissipation rate within a school of Japanese sardines was measured in the laboratory by Tanaka et al (2017). The dissipation rate was O(10 -4 ) W/kg inside the school, while that of the background level was O(10 -5 ) W/kg.…”