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

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

3. 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

4. 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

5. 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

6. Why statistical inverse processing ?

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

8. 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

9. 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.

10. 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:

11. Kirchhoff-ray mode Backscatter Model results

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

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

14. Backscatter theory (2)

15. Inverse processing

16. Maximum Target Strength TS0

17. 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

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

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

20. 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)

21. 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 

22. Tilt angle statistics (5)

23. 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

24. 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

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

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

27. Inverse processing
[dB]
[m]

28. 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

29. 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 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.

30. Trawl data

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

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

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

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

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

36. Target strength data

37. 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)

38. Results
38kHz
70kHz
120kHz
200kHz
2° ° °

39. Conclusions

40. Thank you very much ... 
Thank you very much