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1 The influence of fish morphological and behavioural parameters on acoustic data in algorithmic reconstruction of fish length distribution Marek Moszynski, Andrzej Stepnowski Gdansk University of Technology Poland ICES ASC 17-21 September 2007, Helsinki, Finland ICES CM 2007/H:08 Effects of environmental changes on the biology, physiology, and behaviour of pelagic fish

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2 The influence of fish morphological and behavioural parameters on acoustic data in algorithmic reconstruction of fish length distribution ICES ASC 17-21 September 2007, Helsinki, Finland ICES CM 2007/H:08 Effects of environmental changes on the biology, physiology, and behaviour of pelagic fish Abstract The paper investigates the algorithm for estimation the fish length distribution from acoustic target strength data. The theory of scattering from a tilted cylinder is used for modelling the fish directivity pattern of swimbladdered fish. The model allows formulating the dependence of target strength on two main components: fish maximum target strength and the fish directivity pattern. As both terms depend on fish length, the inverse technique could be used to reconstruct unknown fish length distribution from acoustic data, when morphological parameters of fish are properly assumed. However, as it is shown, the algorithmic approach is very sensitive to some of behavioural parameters of swimming fish. Thus, although the effect of unknown fish tilt angle could be partially removed by statistical processing, the mean value of fish tilt angle still may produce large errors. The method and its results are verified on actual data acquired during the survey and compared to trawl catches.

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3 Introduction (1) Echo Level E Target Strength TS Fish length L Fish Biomass Q acoustical measuresphysical measures Catch data Fish echo processing chain: regression models measurements: ex situ in situ E i = SL+RS + TS i (l i, i, z i,, f o ) + 2B( i ) - 2TL( R i, α)

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4 Introduction (2) Backscattering model Tilt angle statistics INVERSE PROCESSING Sample catch Regression relation MEAN VALUE PROCESSING p TS Fish length L plpl

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5 Fish length estimation p TS plpl p TS0 -tilt angle statistics -backscattering model backscattering model problems: unknown titl angle during ensonification unknown fish directivity pattern

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6 Fish backscatter models for swimbladdered fish tilted cylinder - Haslett (1962) finite bent cylinder model - Stanton (1989) low resolution acoustic model - Clay (1991) Kirchhoff ray mode model (KRM) - Clay, Horn (1994) boundary element model - Foote, Francis (2002) simple precise

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7 Haslet model for swimbladdered fish Haslett, 1962 swimbladder is approximated by a combination of: a hemisphere, a short cylinder, a cone of fixed dimensions relative to the fish fork length. then this shape is modified to: a cylinder maintaining their geometrical cross section.

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8 Methods

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9 Methods (2)

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10 Backscatter theory (1) + 0 l ecb a ecb k

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11 Backscatter theory (2)

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12 cos = 1 / r 1 : U (0,r) 1 = r u p ( )=sin p ( )=2 / (0, /2)

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13 Tilt angle dependance (1) f = 38kHz 0 =8° l ecb =L/4

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14 Conditional fish beam pattern PDF D f [dB] TS 0 [dB]

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15 random generator p TS TS Statistical processing inversion EMS Inversion

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16 Processing example a) acoustically measured target strength TS at 200kHz b) conditional PDF of the fish directivity pattern assuming swim bladder tilt angle 5 c) estimated maximum target strength PDF d) reconstructed fish length distribution along with the catch histogram (in cm) a) b) c) d)

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17 Case study 1 NOAA/Alaska Fisheries Science Center - summer 2002 - Bering Sea provided by Neal Williamson (PMEL - Seattle)

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18 Survey data Simrad EK500 v.5.30 echosounder 38kHz split beam transducer logged w/ Sonardata's Echolog 500 14-07-2002 8:57 – 11:22 am 6776 pings (540MB) 2002 tracks of walleye pollock (Theragra chalcogramma)

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19 Survey data analysis [dB] [cm]

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20 Survey data analysis (2) [dB] [cm]

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21 Survey data analysis (3) Reconstruction of fish length PDF for different mean swimbladder tilt angle 0 along with estimate from catch data. Upper sequence for 38kHz and lower for 120kHz. X-axis represents fish length in [cm].

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22 Survey data analysis (4) Root mean square error function obtained from 38kHz and 120 kHz estimates versus assumed swimbladder tilt angle

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23 Survey data analysis (6) Estimates of length PDF for mean swimbladder tilt angle 0 =7 along with catch data

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24 Tilt angle dependance (3) Target strengths as a function of tilt angle for a 31.5cm pollock at dorsal aspect at 38kHz and 120kHz Foote (1985) Walleye pollock Theragra chalcogramma (Horne - Radiograph Gallery)

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25 Case study 2 R/V “G. O. Sars” March 17 to April 5, 2004 Lofoten 2004 survey Lofoten islands, from 67 o N to 70 o N, spawning grounds of North East Arctic Cod shelf between 500 m to about 50 meters sea temperature 6.8 – 7.1 o C from 40–300m 5 Simrad EK60 split beam echosounders

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26 Experiment standard sphere calibration methods CU64 (18 kHz), CU60 (38 kHz), WC38.1 (70, 120 and 200 kHz) transducers mounted in one of the instrument keels of the vessel full half-power beam widths 7 o, except for the 18 kHz (11 o ) the transmitted pulse duration was identical on all frequencies - 1.024 ms the Bergen Echo Integrator, BEI. heave, roll, pitch and yaw Seatex MRU 5 -Simrad EM 1002 at 10 Hz CTD observations (Sea-Bird SBE9). trawling partly on fixed locations, mostly on registrations for identification of the targets and for biological sampling. Campelen 1800 bottom survey trawl Åkratrawl, a medium sized midwater trawl Standard biological parameters were measured on all catch samples, individual total length, weight, gonad and liver index, age and stomach content.

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27 Trawl data

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29 38kHz 70 kHz 120kHz200 kHz

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30 Survey data provided by Institute of Marine Research - Bergen Norwegian cod echoes at depth range 100-160m acquired with 18kHz system

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31 Survey data provided by Institute of Marine Research - Bergen Norwegian cod echoes at depth range 100-160m acquired with 38kHz system

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32 Survey data provided by Institute of Marine Research - Bergen Norwegian cod echoes at depth range 100-160m acquired with 70kHz system

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33 Survey data provided by Institute of Marine Research - Bergen Norwegian cod echoes at depth range 100-160m acquired with 120kHz system

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34 Survey data provided by Institute of Marine Research - Bergen Norwegian cod echoes at depth range 100-160m acquired with 200kHz system

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35 Target strength data

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36 Results 38kHz 70kHz 120kHz 200kHz 2° 5° 8°

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37 Tilt angle dependance (3) TS/length relationship on tilt angle for atlantic cod TS = 20log L + B 20, McQuinn, Winger (2002) EK500 38kHz SB 7 B 20 Atlantic cod Gadus morhua (Horne - Radiograph Gallery)

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38 Problems

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39 Conclusions

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