Aronica, S

Basilone, G

Bonanno, A

Buscaino, G

Caruana, L.

D’Elia, M

Fontana, I

Giacalone, G

Patti, B

SULLI, Attilio

Tranchida, G

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Use of Hydroacoustic Methods for the Identifica-tion of Potential Seabed Habitats for Small PelagicFish Schools in the Strait of SicilyB. Patti, M. D’Elia1, A. Sulli2, G. Tranchida1, A. Bonanno1, G. Basilone1,G. Giacalone1, I. Fontana1, S. Aronica1, G. Buscaino1, L. Caruana11, Institute for Coastal Marine Environment, CNR, Capo Granitola (TP), Italy2, Department of Geology and Geodesy, University of Palermo, Palermo, Italybernardo.patti@cnr.itAbstractContemporary data on both fish distribution and seabed characteristics can beused in ecological studies to investigate the distribution and the behaviour of bothdemersal and pelagic fish schools in relation to the nature of the sea bottom. In thisstudy we describe a method to classify the sea bottom by a single-beam echosounderused during an echosurvey carried out on the north-western part of Southern Sicil-ian continental shelf. The study area was divided in two contiguous regions (la-belled ZONE 1 and ZONE 2), characterized by different dominant texture, ‘sand’for ZONE 1 and ‘clayey-silt’ for ZONE 2. The acoustic classification evidenced dif-ferences in terms of bottom types between the two investigated zones. Though theaverage bottom depth of the investigated transects is higher in ZONE 1, this area isdominated by substrates having greater backscatter, identifying relatively “harder”bottom types. On the contrary, in ZONE 2 “hard” substrate is confined to the shal-lower inter-transects regions, with the bulk of seabed deeper than 50 m classified as“soft” bottom. The acoustic classification of the seabed was already used in the con-text of a recent study, indicating a preference of small pelagic fish schools for softbottom in ZONE 2, where their occurrence was higher and the bulk of total biomasswas concentrated.1 IntroductionA variety of remote hydroacoustic tech-nologies, which employs a single or amulti-echo, are used to analyze and mapseabed characters in term of texture andgrain size [1, 2]. The most widespreadcommercial acoustic bottom classificationsystems are RoxAnn and QTC-View. Theyuse normal incident echosounders and arebased on the measurement, analysis and in-terpretation of the shape and energy fea-tures contained in the bottom acoustic sig-nal [3, 4]. The knowledge of the bottommaterial and topography is very importantin different scientific fields such as geol-ogy, offshore industry and fishery, mainlywhen the activity is focused on demersalfish. Over the last few decades there wasa rapid increase in the use of hydroacoustictechnologies also for schooling pelagic fishand for the biomass evaluation of pelagicfish populations [5, 6]. Echosounders com-monly used to this aim work at fixed fre-Technologiesquencies and are able to acquire informa-tion from both the fish distribution alongthe water column and the bottom seabed.Despite this, information from the sea bot-tom is usually discarded because the acous-tic signal is used exclusively to obtain dataabout pelagic fish biomass and the struc-ture of the fish schools, to the aims ofthe sustainable management of fisheriesresources. However, the consensus thatsingle species stock assessment alone isnot sufficient to manage fisheries sustain-ably has been constantly increasing overthe last two decades. Specifically, it hasbeen argued the importance of the so-calledecosystem approach for the effective andsustainable management of fish popula-tions [7, 8, 9, 10, 11]. For instance, thequality of scientific advice on exploited fishpopulations can be significantly improvedby the knowledge of the effects of environ-mental conditions on the recruitment or fishpopulations. In this context, the contempo-rary collection of information on fish dis-tribution and seabed characteristics couldbe greatly useful in multidisciplinary stud-ies. To this aim, the use of a single beamechosounder was firstly investigated byD’Elia et al. [12], who also demonstratedthe preference of small pelagic fish schoolsfor softer (and finer) subtrates. In thisstudy we describe the rationale of methodtherein used to classify the sea bottom bya single-beam echosounder (SIMRAD EK500), based on the comparison between hy-droacoustic bottom data and granulometricanalysis of sediment samples.2 Materials and methods2.1 Study area and acoustic sur-veyBottom acoustic information were ob-tained from hydro-acoustic survey con-ducted over the continental shelf of theSicily Strait aboard the N/O Salvatore LoBianco in June 1998. The surveyed areawas divided in two sectors, labeled ‘ZONE1’ (north-western sector, over the Adven-ture Bank) and ‘ZONE 2’ (south-easternsector, continental shelf off the central partof the Southern Sicilian coast) (Figure 1).These two sectors are characterized by dif-ferent dominant seabed conditions, harder(and coarser) in ZONE 1, softer (and finer)in ZONE 2.2.2 Backscattering signal by theseabed and bottom samplescollectionThe split-beam echosounder SIMRAD EK500, originally designed for biological re-sources, was used to measure the backscat-tering signal from the seabed, using a trans-ducer operating at the frequency of 38 kHz.The choice of this frequency is linked toits more frequent availability due to itscommon use in acoustic surveys aimed atbiomass evaluations of small pelagic fishspecies. Raw data from the bottom (Sv)were analyzed using the post –processingsoftware Echoview v.3 by Sonar Data andMatlab scripts. A GPS system, interfacedto the echosounder, was used to recordthe position of the bottom signal. Themethod of extracting bottom type informa-tion is based on the following considera-tions. First, there is a direct relationshipbetween the mean diameter of the seabedmaterial and bottom surface (and volume)2364Marine research at CNR Figure 1: Study area, with Zone 1 (western sector) and Zone 2 (eastern sector) evidenced.Along the transects the acoustic classification of seabed based on bottom Sv is shown.“Hard” seabeds (Sv>median value) is displayed in dark grey, “soft” seabeds (Sv≤me-dian value) in light grey. The sediment sample sites are also shown with white. Shepard’sand Folk’s sediment classification labels are also given under the symbols.backscattering value for normal incidentechosounder [13]. It means that to a largergrain size corresponds a greater backscat-tering surface or volume coefficient. Thereason for this is related to the bulk den-sity of the sediment, which mainly de-pends on porosity. Clayey-silt and Silty-clay sediments have a higher porosity val-ues than sand [14], so they bind morewater and have lower density than sandysediments. The bulk density is in rela-tion to the acoustic impedance of a sed-iment: the higher sediment density, thehigher acoustic impedance and the greaterbackscattering coefficient [15, 16]. Theintensity of the energy scattered back tothe instrument is also a function of thebottom roughness, due to the grain sizeof the particles. Gravel is rougher thansand which in turn is rougher than silt andclay [17]. Starting from this considera-tion, for each acoustic transect of Figure1 the Sv volume backscattering values ofthe seabed line were extracted by meansof Echoview software, using the maximumSv algorithm. Then, a moving average to50 terms (= acoustic pings), correspond-ing to about 200–250 m, was calculatedto reduce variability in the Sv bottom val-ues. The median value of these averagedSv bottom data was used to classify the bot-tom type (0 for Sv bottom values ≤ me-dian value, identifying ‘soft’ seabed areas,and 1 for Sv bottom value > median value,2365Technologies Figure 2: Shepard’s (a) and Folk’s (b) diagram, showing the classification into grain sizefor the analyzed samples.identifying ‘hard’ seabed areas). In or-der to verify the results, physical samplingof the bottom was accomplished with graband box-corer, in proximity of the acousticprospected transects and a grain size anal-ysis was conducted on the collected sam-ples. The analysis of the particle size ofsediment samples required a phase of sam-ple pre-treatment, which included diges-tion with hydrogen peroxide, washing, sep-aration of particles with a diameter lowerthan 500 µm and drying and subsequentanalysis by a laser diffraction instrument(Fritsch model Analysette 22). The coarsesand-gravel fraction (> 500 µm) was driedand divided in 2 fractions using a sieve witha 2 mm mesh size. Finally, the sieved parti-cles were weighted by a simple balance. Afinal normalization process of the fraction,analyzed by the laser instrument, was nec-essary in order to obtain the correct com-positional data. The analysis of particlesize distribution were conducted using theWentworth scale, and the classification intograin size class was based on the Shep-ard’s method [18] for samples containingsilt, clay and sand. The Folk’s method [19]was used for the classification into grainsize of sediment samples containing a frac-tion coarser than sand (diameter>2mm, lit-tle pebble or bioclastic granules).3 Results and discussionThe results of the acoustic-based bottomtype classification are displayed in Fig-ure 1, together with information obtainedfrom the analysis of sediment samples col-lected over all the investigated area. Theacoustic classification shows the differ-ences in terms of bottom types between thetwo zones along the investigated transects.ZONE 1 seabeds, at depth less than 100m (the western sector, over the AdventureBank) are dominated by substrates withgreater scattering strength, indicating rela-2366Marine research at CNR Figure 3: Box plots of bottom Sv for the sediment facies named “Silt-clay” (includingsediment samples classified as clayey-silt) and “Sand” (including sediment samples clas-sified as sandy-silt, silty-sand and sand).tively ‘harder’ (and coarser) bottom types.In contrast, in ZONE 2 the ‘hard’ substrateis confined to the shallower inter-transectregions, with the bulk of the seabed deeperthan 50 m classified as ‘soft’ bottom. Thegranulometric analysis of sediment sam-ples is consistent with the results of theacoustic classification of the seabed (Fig-ure 2 and Table 1). Although the sedimentsample sites were available only in proxim-ity of the acoustically prospected transects,it is worth noting that the use of the me-dian value (-17.65 dB re m-1) of bottombackscattering volume strength permittedus to approximately separate finer (andsofter) substrate locations (classified asclayey-silt and silty-clay), correspondingat bottom Sv values lower than the globalmedian value, from coarser (and harder)sediment sites (facies: silty-sand, sandy-silt, and sand) matching with higher bot-tom Sv values (Mann–Whitney U-test, p =0.0055; Figure 3). This paper highlightssome important aspect in the application ofechosounder data for seabed characteriza-tion. If the fisheries oriented instrumentsuch as SIMRAD EK 500 echosounder iscontemporarily used both for fisheries dataand for bottom classification, its use in eco-logical studies could be very efficient andcost saving.2367Technologies Table 1: Compositional data from the granulometric analysis of the collected samples.References[1] R. Freitas, S. Silva, V. Quintino, A.M. Rodrigues, et al. Acoustic seabed classifica-tion of marine habitats: studies in the western coastal-shelf area of Portugal. ICESJournal of Marine Science, 60:599–608, 2003.[2] J.T. Anderson, D.V. Holliday, R. Kloser, D.G. Reid, and Y. Simard. Acoustic seabedclassification: current practice and future directions. ICES Journal of Marine Sci-ence, pages 1004–1011, 2008.[3] B.T. Prager, D.A. Caughey, and R.H. Poeckert. Bottom classification: operationalresults from QTC view. OCEANS ’95 - Challenges of our changing global envi-ronment conference, San Diego, California, 1995.[4] S.P.R. Greenstreet, I.D. Tuck, G.N. Grewar, and E. Armstrong. An assessmentof the acoustic survey technique, RoxAnn, as a means of mapping seabed habitat.ICES J. Mar. Sci., (54):939–959, 1997.[5] R. Muin˜o, P. Carrera, and M. Iglesias. The characterization of sardine (Sardinapilchardus Walbaum) schools off the Spanish-Atlantic coast. ICES Journal of Ma-rine Science, (60):1361–1372, 2003.[6] B. Patti, A. Bonanno, G. Basilone, Goncharov S., S. Mazzola, et al. Interannualfluctuations in acoustic biomass estimates and in landings of small pelagic fish pop-ulations in relation to hydrology in the Strait of Sicily. Chem. Ecol., (20):365–375,2004.[7] P.A Larkin. Concepts and issues in marine ecosystem management. Reviews inFish Biology and Fisheries, (6):139–164, 1996.2368Marine research at CNR[8] S. Jennings, M.J. Kaiser, and J.D. Reynolds. Marine Fisheries Ecology. BlackwellScience, 2001.[9] S.M. Garcia, A. Zerbi, C. Aliaume, T. Do Chi, and G. Lasserre. The ecosystemapproach to fisheries. Issues, terminology, principles, institutional foundations, im-plementation and outlook. FAO Fisheries Technical Paper, (443), 2003.[10] E. Pikitch, C. Santora, E.A. Babcock, A. Bakun, et al. Ecosystem-based fisherymanagement. Science, (305):346–347, 2004.[11] G.H. Kruse, Y. Ishida, and C.I. Zhang. Rebuilding of depleted fish stocks throughan ecosystem approach to fisheries. Fisheries Research, (100):1–5, 2009.[12] M. D’Elia, Patti B., A. Sulli, G. Tranchida, A. Bonanno, et al. Distribution andspatial structure of pelagic fish schools in relation to the nature of the seabed in theSicily Channel (Central Mediterranean). Marine Ecology, (30):151–160, 2009.[13] M. Manik, H.M.and Furusawa and Amakasu K. Measurement of the sea bottomsurface backscattering strength by quantitative echo sounder. Fisheries Science,(72):503–512, 2006.[14] A.W. Nolle, W.A. Hoyer, J.F. Mifsud, W.R. Runyan, and M.B. Ward. Acousti-cal properties of water filled sands. Journal of the Acoustical Society of America,(35):1394–1408, 1963.[15] E.L. Hamilton and T.B. Richard. Sound velocity and related properties of marinesediments. Journal of the Acoustical Society of America, (72):1891–1904, 1982.[16] M.D. Richardson. In-situ, shallow water sediment geoacoustic properties. Proceed-ings International Conference on Shallow-Water Acoustics, Beijing, China, 21–25April. China Ocean Press, page 163–170, 1997.[17] C. Mazel. Side Scan Sonar Record Interpretation. Training Manual. Klein Assoc.,pages 14–16, 1985.[18] F.P. Shepard. Nomenclature based on sand-silt-clay ratios. Journal of SedimentaryPetrology, (24):151–158, 1954.[19] R.L. Folk. The distinction between grain size and mineral composition in sedimen-tary rocks nomenclature. Jour. Geol., 62:344–359, 1954.2369Technologies2370

Use of Hydroacoustic Methods for the Identification of Potential Seabed Habitats for Small Pelagic Fish Schools in the Strait of Sicily

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Use of Hydroacoustic Methods for the Identification of Potential Seabed Habitats for Small Pelagic Fish Schools in the Strait of Sicily

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