Not to be eited without prior reference to the author

International Couneilfor the Exploration ofthe Sea

Fisheries TeehnologyC.M. 1997fFF: 10

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Flow mcasurcmcnts in a cod-cnd

by

Wilfried Thiele1)

Michael Kroeger1)

Steffen Gau2)

Martin HOpp2)Uwe Kliiber2

)

Abstract

The flow through a trawl is very important for the catchability and it is also rcsponsible for theselectivity of the gear. Investigations by other scientists show that the flow through a trawl isdccreasing up to 30 % ofthe trawling speed. Measurements ofthe flow by using the Scanmarspeed sensor were carried out inside and outside of a selectivity eod-end. The ead-end wasequipped with a "hooped" cover and sorting grid in the upper part.

The results show that the speed of the flow inside the cod-end is strongly influeneed by themeshsize and the cover. Additional to the flow measurements with Scanmar a method for theestimation of the flow· by video and computer calculation was introduced. Furtherinvestigations on the behaviour of fish in the eodend show that eod shows no aetiveescapement reaetion. However, herring shows panic-like reactions and goes throug themeshes. The investigation will be continued in the next montlls.

1) Bundesforschungsanstalt rur Fischerei, Institut rur FischereitechnikPalmaille 9, 0-22767 Hamburg

2) Universität Rostock, Institut für Schiffbau und MeerestechnikAlbert-Einstein-Str. 2, 0-18059 Rostock

Introduction

The flow in or beuer through a trawl is very important tor the catchability and catchingefficiency of the gear. Several researchers were dealing with the problem of measurement ofthe flow inside of a trawl. ZIEMBO describes the flow through a trawl as constant in the wingsand the belly. At the begin of the codend the speed of the flow slows down to nearly 30 % ofthe towing speed.

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Figure 1: Water flow in a trawl (by ZIEMBO)

MISJURKIN Cl) finds out similar results. He describes the hydrodynamic field of a trawl andestimates also a decreasing ofthe speed ofthe flow to 30 %.

Figure 2 Hydrodynamic field ofa trawl (by Misjurkin)

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With reference to selectivity other authors (2) describe the codend as "pear-shaped".

1 - trawl bag2 - calch3 - hydrodynamic pressure4 - waler flow leaving lhe bag5 - general Dow enlering the bag6 - escap zone7 - elongaled empty part of the bag wilh

da ed meshes--5

'\-. ---f--

--- ....... --~" "-/ ......

321

•Figure 3: Shape of the codend during catch eollection (by Zafermann and Serebrov, 2)

The flow in the eodend is in this case also respon ible for the results of thc selectivity. It isknown from the eommereial fisheries that the shape and the erosseetion of the entrance of thecodend has a huge influenee on the catchability of the trawl. Research work earried out byREI-IME and SCHEEL (3) in 1989 was dealing with this problems. They investigated in a windtunnel different net eones and estimate the speed and direction of the flow (Figure 4). The aimof this research work was the improvement of the catchability of midwater trawls. Now theobjeetives of the research work have ehanged, the improvement of selectivity is one of themain tasks.

Under the assumption that the fish is influenced by the waterftow through a trawl, theselectivity can be improved by taking into aecount the hydrodynamic aspects

Methods

The experiments were carried out during the 402. research cruise of FRV "Solea" in January1997. The working area was in the Baltie, near the Island Rügen. The objeetives of this cruisewere studies on the behaviollr 01' Baltic eod dllring escapement through a sorting grid. Theu ed trawl was the "eod-hopper" with 105 mm (mesh size) codend and 120 111m codend. The105 mm mesh size codend was equipped with a stainless steel grid with 48 mm bar di, tanee.The dimensions of the grid were 0.7 x 0.5 111. Four 01' this grids were mOlll1ted in line in theupper panel of the codend. The test codends were in both eases (105 mm ami 120 111m)furni hee! with a hooped cover.

Observations of the behaviour and the measurement of the speed were taken with and with utcover The flow was measured by Scanmar speed sensor. The position of the sensor is shownin Figure 5. The observations were carried out with the help of the towed vehic1e.

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Results and discussion

Studies on the behaviour

No aetive reaetion of eod to eseape eould be observed. All of the eod were swimming in thetowing direetion in a distanee from the grid. Is seems that the eod feit weH in the eodendbeeause they were feeding. At the end ofthe bag the eod was pressed against the mcshes, smallfishes were pressed through the meshes without own movement.

The behaviour of herring was eompletely different. Herring showed panic-like reactions anddid goe through the meshes when they were open. The privilcged spot for escapement was infront ofthe grids where the meshes were spreaded.

Measurement ofthe flow

Figure 6 demonstrates the results of the flow measurement in- and outside of the codend. •Inside of the 105 mm-codend with cover is the flow only 28 % of the trawling speed. Outsideofthe codend but inside ofthe cover we have a flow speed 51 % ofthe original. Without coverthe flow speed inside of the codend was 41 % of the trawling speed and outside of the codendwas it 15 %. Similar results show the experiments with the 120 rnrn codend. It should beunderstood that these results not intended to indicate absolute values, but only trends.

Estimation ofthe flow speed by video and computer ca1culation

Charaetesties of flow are gathered from video recordings. For tlie determination of speed anddireetion of a moving objeet video observations have to be carried out simultaneously by twocameras in different positions. Experimental (and financal) problems however allowobservations of a fuH scale trawl only by help of a single video camera. The following examplewill demonstrate how to complete missing informations by sensible assumptions. ..'

In our investigations the speed of flow within thc area of a grid in the codend was of intcrest.From vidco rccordings a scqucnce showing the motion of a dead fish was pieked out. The •assumption was that the speed and direction of the motion of the fish was identical to thewaterflow due to missing an own movement of the fish. The sequence shows that the fish ispassing four different areas of the grid (Figure 7). For thc total sequence every second imagewas grabbed by a digital image processing system. Due to CCIR norm that corresponds to atime interval of 80 msec. Full images were stored during the sequence added by the increasingimage number. When the fish was passing the border of onc of the grid arcas thecorresponding image number was the starting point für the determination of speed. In theexample (Figure 8) the image numbers were

Image # 6Image # 14Image # 22

[mage # 30

beginning ofthe first section ofthe gridbeginning ofthe second section ofthe gridbeginning ofthe third section ofthe grill

end ofthe third section ofthe grid

(FlowOOI)(Flow002)(Flow003)(Flow004)

From image l1umbers thc time intervals for the pathes were calculated. The pathes werc takenfrom thc drawings. Assumption was that the fish moved near to the grid and no error is causedby leaving the central projection of the lens system of thc camera out ofconsideration.

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As a result we got

FlowOOlFlow002Flow003Flow004

t=O.OO sect=0.64 sect=1.28 sect=1.92 m

s=O.OO ms=0.52 ms=1.02 ms=1.51 m

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Thus the speed of flow was

speed path / timepath = 1.51 mtime = 24 images, «24x80)11 000) sec = 1.92 secspeed offish = speed offlow = 1.51 m/1.92 sec = 0.79 m/sec

The result induding the accuracy was

Speed of flow = 1.5 kn +/- 0.2 kn

References

(1) Misjurkin, M. A.: lnfluence of the hydrodynamic field on the behaviour of fish in a trawl.Ekspress Inforrnacija, Ser. Promyslennoe rybolovstvo. Vyp. 7 (1982), in Russian

language)

(2) Zaferman, M. 1.; Serebrov, L. l.: On fish injuring when escaping through the trawl mesh.leES c. M. 19891B:18.

(3) Rehme, W.; Scheel, D.: Experimentelle Untersuchungen zur Durchströmung von Schlepp­netzkegeln. Seewirtschaft 20 (1), 1988.

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Position of the speedsensor inside and outside of the codend1- speedsensor ; 2-sorting grid (120 mm codend without sorting grid);3 - codend ; 4 - plastic hoop ; 5 - cover

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___• 0 _ -----0 ._-----_·_-----_· _

v(st)/v(sch) in % Sununary of the Speed inside/outside of the Codend

105 mrn Codend with Sorting grid 120 mm Codend withoul Sorting grid100 +-----------,----------+----------,------------1

with Cover without Cover with Cöver without Cover

80 -!-----------+------75 % --+-------------1f------------i

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Figo6

I~ inside Codend • outside Codend

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Fig.7The motion of the object over the grid

Fig.8Image nwnbers of tbe videosequence