A METHOD FOR ESTIMATING FISH SIZE DISTRIBUTION FROM ACOUSTIC DATA   M.Moszynski Gdansk University of Technology Poland

Outline Introduction Fish directivity pattern and tilt angle dependance Inverse techniques in fish length estimation Survey data analysis

Introduction Fish echo processing chain: acoustical measures physical measures Echo Level E Target Strength TS Fish length L Fish Biomass Q regression models measurements: ex situ in situ Catch data

Introduction Fish biomass estimation in fishery acoustics propagation Ei = SL+RS + TSi(li, i , zi,, fo ) + 2B(i ) - 2TL( Ri, α) transducer beam pattern hydroacoustic system TS = 10log BS = 20log lBS < BS > sounding volume V Q – biomass estimation

Introduction pTS pl < l > L Sample catch Regression relation MEAN VALUE PROCESSING < l > Sample catch Regression relation pTS Fish length L INVERSE PROCESSING pl Backscattering model Tilt angle statistics

Why statistical inverse processing ?

Fish length estimation problems: unknown titl angle during ensonification unknown fish directivity pattern pTS pTS0 pl -tilt angle statistics -backscattering model backscattering model

Fish backscatter models for swimbladdered fish simple 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) precise

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.

Kirchhoff-ray mode Backscatter Model (KRM) Clay and Horne, 1994 fish body as a contiguous set of fluid-filled cylinders that surround a set of gas-filled cylinders representing the swimbladder Sockeye salmon (Oncorhynchus nerka) Lateral radiograph: Dorsal radiograph:

Kirchhoff-ray mode Backscatter Model results

Boundary element model Foote, Francis 2002 swimbladder mesh required obtained by X-rays, PCX, CT

Backscatter theory (1)  +0 k aecb lecb

Backscatter theory (2)

Inverse processing

Maximum Target Strength TS0

Mean Target Strength <TS> Reduced scattering length – RSL TS = 20 log L + 20 log (RSL) regression relationship for average target strength ( according to the National Marine Fisheries Service): use lecb = L/4 as in Haslett model for estimate of <lecb> example - fish fork length: L = 31.5 cm - from theoretical equation: TS0( f = 38kHz) = -32dB TS0( f =120kHz) = -27dB - from regression: <TS>= -36dB

Tilt angle dependance (1) f = 38kHz 0=8° lecb=L/4

Tilt angle dependance (2) f = 120kHz 0=8° lecb=L/4

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)

Tilt angle dependance (4) TS/length relationship on tilt angle for atlantic cod TS = 20log L + B20 , McQuinn, Winger (2002) EK500 38kHz SB 7 Atlantic cod Gadus morhua (Horne - Radiograph Gallery) B20 

Tilt angle statistics (5)

Simulation  L TS0 TS pL pTS0 Random generation Statistical processing fish size and orientation generator pTS  L TS0 TS pL pTS0 backscatter model backscatter model inversion backscatter model

Simulation Fish size and orientation - assumptions: backscattering length of fish school between 30cm and 60cm normally distributed random distribution of fish orientation in consecutive fish echoes trace of the fish - straight line, fish tilt angle - normal distribution 8° as mean value for swimbladder tilt angle

Conditional fish beam pattern PDF TS0 [dB] Bf [dB]

Conditional fish beam pattern PDF TS0 [dB] Bf [dB]

Inverse processing [dB] [m]

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

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 7o, except for the 18 kHz (11o) 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.

Trawl data

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

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

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

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

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

Target strength data

Processing example a) b) c) d) 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)

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

Conclusions

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