Consideration of bottom contact effect on the catch Consideration of bottom contact effect on the catch of demersal species in a trawl survey in Japanof demersal species in a trawl survey in Japan

Yasuzumi Fujimori(Graduate School of Fisheries Sciences, Hokkaido University)Kazushi Miyashita(Field Science Center for North Biosphere, Hokkaido University)Satoshi Honda(Hokkaido National Fisheries Research Institute)

Stock survey for walleye pollock along Stock survey for walleye pollock along the coast of Japanthe coast of Japan

Annual trawl and acoustic surveysare conducted by Fisheries Research Agency and Prefectural Fisheries Experimental Station with relation to the area where pollock is distributed.

The survey area is mainly divided into the Pacific and Japan sea.

North area of Japan

Engås and Godø,1989, Rose and Walters, 1990, McCallum and Walsh, 2002, Weinberg, Somerton, and Munro, 2002 etc.

Main and Sangster, 1981, Godø and Engås, 1989,Walsh, 1992 etc.

Change of catch efficiency with towing conditionChange of catch efficiency with towing condition

Efficiency is changed in different vessel or towing condition even if the same trawl gear is used.

Efficiency and fish size composition is changed with the variation of trawl mouth shape and the condition of bottom contact.

Influence of warp length to catchInfluence of warp length to catch

Measuring the mouth shape and bottom contact in different warp length, scope ratio (warp length / depth)

Comparison of the catch in different scope ratio

InvestigationInvestigation of scope ratio (warp length / depth) of scope ratio (warp length / depth) in the actual trawl survey for walleye pollockin the actual trawl survey for walleye pollock

Depth of these area were shallower than 500 m

Offshore of North-east Japan (Sanriku area)

Survey area:

RV No.3 Kaiyo-Maru(1300PS, 473GT)

Vessel:

Offshore of East Hokkaido

Weight in water: 850kg eachORFH type（Taito Co. Ltd.）

Otter board:

Wing spread: 18 m, Net length: 49.9 m

JAMARC99（Nichimo Co. Ltd.）

Trawl net:

Scope ratio in the past survey for walleye pollockScope ratio in the past survey for walleye pollock

In past survey, scope ratio (W/D) was ranged from 2.4 to 2.9 (Mean=2.54).

Too short ?

W = 2.5483D

0

200

400

600

800

1000

1200

0 100 200 300 400

Bottom depth (m)

War

p le

ngth

(m)

01

23

456

78

910

2.35 2.45 2.55 2.65 2.75 2.85

Scope ratio

Freq

uenc

y

Experimental areaExperimental area

Hokkaido January in 2002Date

JAMARC 99 with ORFH otterboard

Gear

RV No.3 Kaiyo-MaruVessel

Depth: 110 to 130 m

SIMRAD EK60 Echo sounder

Offshore of Kushiro CityArea

Experimental area 4 knotTowing speed

10 min on the bottom.Duration

2.5, 3.0, 3.5Scope ratio

Methods

MeasuringMeasuring devicesdevices

Depth logger

Height sensor

Distance sensor

Methods

GeometrGeometryy

0.18 0.0 0.16 5.1 1.54 18.1 3.5 0.17 0.1 0.15 5.6 1.46 17.4 3.0

0.29 0.1 0.17 7.1 1.69 16.1 2.5 Series 2

0.15 0.0 0.16 5.0 3.78 18.3 3.5 0.12 0.0 0.15 5.1 0.83 17.4 3.0

0.30 0.1 0.19 6.4 1.71 16.1 2.5 Series 1

S.D.Avg.S.D.Avg.S.D.Avg.

Bottom clearanceTrawl heightWing spreadScope

ratio

Results

Mouth shape observationMouth shape observation

The depth of each position were recorded by depth logger with the duration of 10 seconds.

Results

Mouth shape at scope ratio of Mouth shape at scope ratio of 2.52.5

Height was 4.9 to 6.4 m, Wing distance was 16.1 to 16.8 m

0

2

4

6

0246810 2 4 6 8 10Port Starboard

0

2

4

6

0246810 2 4 6 8 10Port Starboard

8

Dis

tanc

efro

mse

abe

d(m

)

Horizontal distance (m)

0

2

4

6

0246810 2 4 6 8 10Port Starboard

0246810 2 4 6 8 10Port Starboard

0

2

4

6

8

Results

Depth of the net and trawl door with elapsed tow timeDepth of the net and trawl door with elapsed tow timewhen the scope ratio was 2.5when the scope ratio was 2.5

0

20

40

60

80

100

1200 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Elapsed time (min)

Dep

th (m

)Trawl door Head line center Fishing line center

Scope ratio: 2.5

Arrival Heaving

Results

Mouth shape at scope ratio of Mouth shape at scope ratio of 3.03.0

0

2

4

6

0246810 2 4 6 8 10Port Starboard

0

2

4

6

0246810 2 4 6 8 10Port Starboard

0

2

4

6

0246810 2 4 6 8 10Port Starboard

0

2

4

6

0246810 2 4 6 8 10Port Starboard

Dis

tanc

efro

mse

abe

d(m

)

Horizontal distance (m)

Height was 4.1 to 5.0 m, Wing distance was 17.1 to 18.2 m

Results

Mouth shape at scope ratio of Mouth shape at scope ratio of 3.53.5

0

2

4

6

0246810 2 4 6 8 10Port Starboard

0

2

4

6

0246810 2 4 6 8 10Port Starboard

0246810 2 4 6 8 10Port Starboard

0

2

4

6

0246810 2 4 6 8 10Port Starboard

0

2

4

6

Dis

tanc

efro

mse

abe

d(m

)

Horizontal distance (m)

Height was 4.0 to 4.3 m, Wing distance was 18.2 to 18.9 m

Results

Catch weight by scope ratioCatch weight by scope ratio

210.7120.784.7Total

14.7-15.9-4.2-Others14.8-5.3-1.7-Flatfishes34.3-11.2-11.2-Sculpins

0-5-0-Pacific cod146.9694583.3393667.63704Walleye pollock

Weight (kg)NumberWeight (kg)NumberWeight (kg)NumberSpecies

3.53.0 2.5

Scope ratio

Results

Variation of catch weight with the scope ratio Variation of catch weight with the scope ratio for sculpins and flatfishesfor sculpins and flatfishes

Series 1Series 2

0.0

10.0

20.0

30.0

2.5 3 3.5

Scope ratio, warp length / depth

Cat

ch w

eigh

t (kg

)

Flatfishes

0.0

20.0

40.0

60.0

80.0

100.0

2.5 3 3.5

Sculpins

Scope ratio, warp length / depth

Results

Length frequency distributions of pollock caughtLength frequency distributions of pollock caught

(N=300)

14.1 3.5

13.7 3.0

13.6 2.5

Series 2

14.3 3.5

13.7 3.0

13.5 2.5

Series 1

Mean length (cm)Scope ratio

Fork length (cm)

Rel

ativ

e fre

quen

cy

Series 1 Series 2

Results

SummarySummary

The change in mouth shape was most notable at scope ratios between 2.5 and 3.0.At a 2.5 scope ratio, it was apparent that the trawl door never touched the bottom. Moreover, the mouth shape and the bottom contact were fluctuated extremely.

1. Relationship of scope ratio to mouth shape

2. Relationship of scope ratio to catchThe catch of each species were increased clearly with scope ratio.The length composition of the catch at a 3.5 scope ratio was different from those at other scope ratios.

ConclusionConclusion

The scope ratio should be set as 3.0,

to prevent the large variation of catchability

to keep the consistency between the echo data and catch data

Thank you for your kind attention Thank you for your kind attention

We also thank the Fisheries Research Agency of Japan supported this study.