Including 38 kHz in the standardization protocol for hydroacoustic fish surveys in temperate lakes

Hydroacoustics has become a requisite method to assess fish populations and allows to describe the relationships of fish with other elements of the aquatic ecosystem. This nonintrusive method is currently an integral part of the sampling procedures recommended for fish stock assessment by the Water Framework Directive and has been standardized by the European Committee for Standardization [CEN (2014) CSN EN 15910 - Water quality - Guidance on the estimation of fish abundance with mobile hydroacoustic methods, Category: 7577 Water quality. Biological.]. In Europe, hydroacoustic surveys are performed in freshwater using different frequencies. Consequently, there is a need to evaluate if survey results can be compared. This study aimed to carry out in situ comparisons at the 38 kHz frequency (noted f) with two other commonly used frequencies, 70 and 200 kHz. The 38 kHz frequency has seldom been compared with other frequencies in freshwater although it is widely used worldwide, especially in the Great Lakes of North America and in Sweden. In 2016, hydroacoustic data were acquired in Lakes Annecy and Bourget using methods validated in previous studies that compared the frequencies 70, 120 and 200 kHz. This study showed similar density and biomass estimations as a function of frequency, density(f) and biomass(f), between the frequencies studied for low to moderate fish densities. For higher fish densities, the results were more variable and need to be verified. Fish density(f) and biomass(f) estimations sometimes exhibit differences between frequencies, which is not fully in agreement with theoretical calculations. The aim of this study was to evaluate frequency comparisons in practise. However, if the differences on acoustic metrics, density(f) or biomass(f) between frequencies were occasionally statistically significant, the differences were small enough to be considered negligible for fish population management. These analyses led to better knowledge of the responses from fish in temperate lakes for the studied frequencies. Our findings should be considered when revising the CEN standard

Plankton community structure in response to hydrothermal iron inputs along the Tonga-Kermadec arc

The Western Tropical South Pacific (WTSP) basin has been identified as a hotspot of atmospheric dinitrogen fixation due to the high dissolved iron ([DFe]) concentrations (up to 66 nM) in the photic layer linked with the release of shallow hydrothermal fluids along the Tonga-Kermadec arc. Yet, the effect of such hydrothermal fluids in structuring the plankton community remains poorly studied. During the TONGA cruise (November-December 2019), we collected micro- (20-200 μm) and meso-plankton (>200 μm) samples in the photic layer (0-200 m) along a west to east zonal transect crossing the Tonga volcanic arc, in particular two volcanoes associated with shallow hydrothermal vents (< 500 m) in the Lau Basin, and both sides of the arc represented by Melanesian waters and the South Pacific Gyre. Samples were analyzed by quantitative imaging (FlowCam and ZooScan) and then coupled with acoustic observations, allowing us to study the potential transfer of phytoplankton blooms to higher planktonic trophic levels. We show that micro- and meso-plankton exhibit high abundances and biomasses in the Lau Basin and, to some extent, in Melanesian waters, suggesting that shallow hydrothermal inputs sustain the planktonic food web, creating productive waters in this otherwise oligotrophic region. In terms of planktonic community structure, we identified major changes with high [DFe] inputs, promoting the development of a low diversity planktonic community dominated by diazotrophic cyanobacteria. Furthermore, in order to quantify the effect of the shallow hydrothermal vents on chlorophyll a concentrations, we used Lagrangian dispersal models. We show that chlorophyll a concentrations were significantly higher inside the Lagrangian plume, which came into contact with the two hydrothermal sites, confirming the profound impact of shallow hydrothermal vents on plankton production

How many fishes in the sea and where ? : active acoustics to assess marine organisms

To establish balanced use between the users of a maritime space, the first step is to know this space: its abiotic characteristics and their dynamics; its biotic components, their distribution and dynamics; vulnerable areas and rich zones; mapping of fish habitats (Le Pape et al., 2014); the human and economic environments, etc. This initial diagnosis requires determining the distribution of living resources, exploited or not. For this purpose, the use of active acoustics was developed in the..

A l'écoute des bancs de poissons

National audienceLes ondes acoustiques sont un moyen efficace pour étudier les poissons. Les sonars sont non invasifs et plus précis que d'autres techniques. - Les chercheurs étudient le comportement des poissons et évaluent l'évolution des stocks. - De nouveaux dispositifs acoustiques répertorient mieux les espèces et reconstruisent les bancs de poissons en trois dimensions. - L'hydroacoustique est un outil polyvalent qui permet une étude globale de l'écosystème

The acoustic multifrequency approach as a help to the description of the ecosystems'components

International audienc

Simultaneous Sv and TS measurements on Young-of-the-Year (YOY) freshwater fish using three frequencies

International audienceIn autumn, the fish population above the thermocline in Lake Annecy mainly comprises young perch (Perca fluviatilis) of the year. The fish are distributed in schools during the day and as scattered individual fish targets at night. Measurements of volume-backscattering strength and target strengths (TS) were carried out in a synchronous way using three echosounders operating at three frequencies (70, 120, and 129 kHz), with different characteristics (split beam, dual beam, beam shape, pulse length, etc.). Target-strength values show variability from one elementary sampling unit to another and from one device to another, but the mean global TS values are similar, independently of the frequency. The volume-backscattering strengths measured on precise schools by the three acoustic devices give significantly similar results. Acoustic measurements on YOY perch are completely independent of the frequency and the characteristics of the echosounders