A fisheries acoustic multi-frequency indicator to inform on large scale spatial patterns of aquatic pelagic ecosystemsby Verena M. Trenkel, Laurent Berger

Ecological Indicators

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Year
2013
DOI
10.1016/j.ecolind.2013.02.006
Subject
Decision Sciences (all) / Ecology, Evolution, Behavior and Systematics / Ecology

Text

Ecological Indicators 30 (2013) 72–79

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Short communication

A fishe to pattern

Verena M a Ifremer, rue d b Ifremer, BP 70 a r t i c l

Article history:

Received 12 M

Received in re

Accepted 4 Fe

Keywords:

Acoustics

Frequency res

Monitoring

EOF

Ecosystem sta e info nd qu sms s forma oposa of th tic ba terpre coust atial rovid the pelagic ecosystem. © 2013 Elsevier Ltd. All rights reserved. 1. Introdu

The stru monitored abundance the species cept of sur can be taxo group bein groups (Ga that is supp

The advent for deriving application

Gould (200 based on sa

Fisheries data source in particula acoustic fre et al., 2011 tion are (i) ∗ Correspon

E-mail add 1470-160X/$ – http://dx.doi.oction cture of exploited ecosystems is generally described and using a suite of indicators such as species richness and based biodiversity indices. However, information on levels is not always available, which is why the conrogate species has been developed. Surrogate species nomic groupings such as genus or families, one species g indicative of the species richness of other species ston, 2000), or any other environmental information osed to be related to species richness (Magurran, 2004). of remote sensing data has provided opportunities surrogate species estimators for terrestrial vegetation s (see review in Rocchini et al., 2010). For example, 0) developed an estimator of plant species richness tellite imagery data. acoustic water-column data are a currently underused for monitoring changes in pelagic ecosystem state, r for deriving indicators making use of the distinctive quency response of different organism groups (Trenkel ). The main principles of fisheries acoustic data collecthe emission of acoustic energy at a given frequency ding author. Tel.: +33 240374157; fax: +33 240374075. ress: verena.trenkel@ifremer.fr (V.M. Trenkel). by an echosounder installed on the hull of a survey vessel and (ii) the registration of the energy reflected (backscattered) by marine organisms encountered by the emitted sound wave on its way from the ship hull to the sea floor. The acoustic backscattered energy is due to the acoustic impedance contrast between the organism and the sea water. The impedance of each organism is a function of the physical properties of its body (density and sound velocity in the organism). Gas containing bodies such as swim bladders of fish are strong scatterers as both density and sound velocity are small in gas compared to sea water (Fig. 1). The dependence of the received acoustic backscattering energy on the acoustic emission frequency is due to the strong dependence of acoustic impedance on frequency (in particular around the resonance frequency) but is also related to the shape and the roughness of the marine organisms, this dependence is commonly referred to as frequency response (Simmonds and MacLennan, 2005). Backscattering data at several echosounder frequencies over many octaves are collected sequentially using separate emission units (so called transducers). The frequency response curve of an organism is obtained by plotting the backscattering energy against the echosounder emission frequency and joining the points. The acoustic frequency response of living and non-living organisms can be measured in situ (Berger et al., 2009) or in the laboratory (Conti and Demer, 2003). Several species groups have distinct frequency response curves in the frequency band from 10 to 200 kHz (Fig. 2a). Making use of this feature, information on species groups with similar scattering properties, so see front matter © 2013 Elsevier Ltd. All rights reserved. rg/10.1016/j.ecolind.2013.02.006ries acoustic multi-frequency indicator s of aquatic pelagic ecosystems . Trenkela,∗, Laurent Bergerb e l’île d’Yeu, BP 21105, 44311 Nantes cedex 3, France , 29280 Plouzané, France e i n f o arch 2012 vised form 6 December 2012 bruary 2013 ponse curve te a b s t r a c t

Fisheries acoustic instruments provid and without swim bladder (tertiary a sumers) and small gas bearing organi producers). We entertain that this in groups in pelagic ecosystems. The pr thesises in a single metric the shape i.e. the dependence of received acous demonstrate the development and in

We then calculate the indicator for a it to create reference maps for the sp scale spatial variability. These maps p/ locate /eco l ind inform on large scale spatial rmation on four major groups in aquatic ecosystems: fish with aternary consumers), fluidlike zooplankton (secondary conuch as larval fish and phytoplankton (predominantly primary tion is useable to describe the spatial structure of organism l we make is based on a multi-frequency indicator that syne acoustic frequency response of different organism groups, ckscattered energy on emitting echosounder frequency. We tation of the multi-frequency indicator using simulated data. ic water-column survey data from the Bay of Biscay and use structure of the four scattering groups as well as their small e baselines for monitoring future changes in the structure of

V.M. Trenkel, L. Berger / Ecological Indicators 30 (2013) 72–79 73

Fig. 1. Schematic view of fisheries acoustics data collection (left) and data visualisation at a single frequency (right). -60 -40 g st re ng th

S v (d

B ) (a) 0.8 1.0 1.2 at te rin g co ef fic ie nt (b) (kHz)

Fig. 2. (a) Sch bles o

Fernandes et a cksca called scatt ter data. Th representin

Multi-fr lected usin been used b species (De

These autho cator, i.e. th to a referen based indic tems, thoug interpretati

Table 1

Simulation bas algorithm (Bal

Functional g

Primary and consumer

Secondary c

Tertiary and quaternary c

Non-living 50 100 150 200 -100 -80

V ol um e ba ck sc at te rin copepods small euphausids mackerel small resonant bubbles large bubbles deep physoclists deep phystostomes 0.0 0.2 0.4 0.6