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Not to be cited without prior refecence to the author

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International Council forthe Exploration of the Sea

C.M. 1990/B:28Fish Capture CtteeSession U

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PREL}HINARY TRIALS WITH A SQUARE HESH CODENDIN PELAGIC HERRING TRAWLS

by

Petri Suu~ronen

Finnish Game and Fisheries Research InstituteFisheries Division, P.O.Box 202, SF-00151 Helsinki

Abstract

Comparative fishing trials with a twin codend trawl were used to compare thesize selectivity of a square mesh codend with that of a diamond mesh codend ofthe same nominal mesh size (36 mm). Altogether, 34 hauls were made in thenorthern Baltic (ICES SO 29N) in May-June 1990. The catches in the two sidesof the trawl varied considerably, suggesting a systematic difference incatching efficiency between the sides. Underwater TV observations confirmedthat the separator section of the trawl was not operating properly. Due to thetechnical difficulties, the results must be regarded as preliminary, but theysuggest that a square mesh codend will have a sharper selectivity and willretain fewer juvenile herring than a diamond mesh codend ofequivalent meshsize. Most escapes seemed to occur at the front of the catch bulge, from theupper side of the codend.The problems of fitting aseparator section to anexisting trawl are discussed •

1. Introduction

Juvenile herring form a high proportion of the total herring catch of.Finnishtrawlers. This indicates that the mesh size (20-26 mm stretched whole mesh­length) used in the codends of these trawls is not sufficient to ensure thatyoung herring can escape from the codends. Due to the recent crisis experi­enced by Finnish fur farms the market for small "fodder herring" has de­creased, which has provided an incentive,for the fishermen to discard thesmall herring caught by the trawls. In this new situation they might be readyto accept more selective gears.

The size selection of a 32 mm (whole mesh length) codend in a pelagic herringtrawl was assessed in the Gulf of Finland (SD 32)by JÄRVIK & SUURONEN (1990).Their study shows that a considerable number of juvenile herring will be cap­tured even with a 32 mm diamond mesh codend.

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A square mesh codend is constructed by hanging the netting so that.the meshesof the.codend are not constricted during towing~ Comparisons of the sizeselection of groundfish by square and diamond mesh codends have generallyshown that, for a given mesh, a square meshcodend selects fish of a largersize and retains fewer small fish (e.g. ROBERTSON & STEWART 1988, COOPER & 'HICKEY 1989). Much less material exist for shoaling pelagic fish. SHEVTSOV(1988) reported on Soviet studies made in the easternBaltic, where a squaremesh codend in a herring bottom trawl was found to sift more juveline herringthan the a diamond mesh codend with the same mesh size~ THORSTEINSSON (1989)described Icelandic shrimp trawl experiments in which small, 7-11 ,CIn long,herring were almost completely released by a 36 mm square mesh codend.

The basic requirement for the adoption of;a square mesh codend is a signifi­cant reduction in the numbers of juvenilescaught, together with retention ofcommercially viable quantities of marketable herring. This paper describes afirst set of comparative fishing trials with 36 mm square and diamond meshcodends.

2. Material and methods

Selectivity experiments were carried out in the'period 7May-7June 1990 in theArchipelago Sea (ICES.Sub-division 29N)cn the fishing ground off NE Nauvo. Astandard herring midwater trawl (96x3200 mm) was modified into a trouser trawlby inserting a special separator section behind the belly~ The test codendswere attached to thelegs of the separator,section~ The separator secticn wasconnected,to.the trawl at the point where its cross7sectional diameter wase~al tothat cf the trawl~ A vertical netting panel inside the separatorsecticn cÜvided it into two ."equal" parts (Figure 1).

The two test codends were made of the same knotted PA netting (36 mm nominalmeshlength),' had the same nurnber of meshes in their circumference; and were ofthe same stretched length (FigUra 1). The small mesh (20 mm) codend is astandard codend used by the trawler. In order to stretch the meshes into theeffective "wcrking".shapa, both tbe large mesh codends were used in thecommercial fishery for aperiod of about one month hefore the selectivitytrials started.

During the selectivity trials, the inside mesh opening cf the codends wasmeasured with a flat wedge-shaped gauge, wbich was pushed "gently" by handinto tbe meshes. The mesh opening in the extension piece cf the large meshcodends varied froIn 29.to 32 mm, in,the few mesh rows in,front of the liftingbag from 32 to 35 mIn, and in the lifting bag from 29:to 32 mm. The square meshcodend had on average ca. 1-1.5 mm larger'mesh opening than the diamond meshcodend, the difference being most marked in the extension. It was also notedthat the shape of the square mesbes had slightly changed, aspecially in frontof the lifting bag. The meshes had elongated to a slightly rectangular shape~

The averagemesh opening of the small mesh codend was c~ 15 mm.

Experimental fishing was carried out during the daytime uridernormal commer­cial,fishing conditions aboard achartered side trawler MIA (32.5 m" 850 HP).In Figure 2 two typical echograms of the fishing ground are presented. Thetrawl was towed at the depth of maximum fish concentration. The water tempera­ture at that depth was about 7-9 C. The headline height of the trawl was ca 20

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• ,,-, .-!" •.••• , .• ,t,,,., ',;·.h... ·:f. ,' ...~", .metres. The duration of the,tow was'averaged 1.5 hour. Towing speed measuredby a propeller log, varied from 3.0 tci 3.3 knots. The successive tows weregeneraly performed in opposite directions on the fishing ground.

In order to study the capture process in detail, and especially to find outwhere and how the main selection takes place, the experimental codends weremonitored with an underwater TV-camera during the trials~ The camera wasmounted on a simple self-made foil which was towed by a small boat attached bya rope to the headline of the trawl.

A total of 36 tows were completed, but the data from two tows were rejectedbecause of severe gear problems. The tows were performed using the small meshcodend on one side and an experimentallarge mesh codend on the other side~

The use cif small,mesh/large mesh codends was adopted in order ,to obtain sam­pIes cf the population being fished. The codends were switched at convenientintervals. Fifteen hauls were completed with both test codends by means ofthis arrangement. In addition, four tows were performed using large mesh cod­ends on both sides~ The codends were joined to the legs of. the separatorsection by means of plastic rings, which were joined together by a rope. Thesequence of the codends in the experimental tow~ is presented in Table 1.

The catches consisted almost solely of herring. The average catch per tow(1700 kg) was at the same level as in the commercial fisherY. However, therewas large haul-to-haul variation, and unexpectedly large within-haul variationin the catches (Table 1). The catch was considerably higher in one leg of thetrawl. In five of the first eight hauls, the catch was larger on the port sideof the.trawl. The separator section was then turned around. In.the 26 haulsperformed after that, the catch was bigger on the starboard side in 20 hauls,and in two hauls the catch was the same~ In all the four hauls in which thelarge mesh codends were compared directly, the catch was larger on the star­board side.

These findings strongly suggested that the separator section affected thecatching performance of the two legs unequally. Underwater TV observationsconfirmed that the section was not in fact operating according to expecta­tions. The meshes on orie side of the panel were more distorted than on theether, there was also slight "asymmetry" in the separator legs"and on manyoccasions the fish were seen to swim in much greater numbers inte one of thetwo legs. Occasionally some fish were observed escaping through the "holes" atthe joins. In addition, on every turn cf thetrawler, fish were herded morealong the outer wall into the outer codend. When the trawl caught a largenumber of fish in a short time,some escape through the meshes of the separa­tor section was also observed. Some attempts were made after haul no. 27 toimprove the construction cif the codend attachffient system, but only with minoreffect on the unequal catching performance of the trawl~

The lengths of the herrings were measured on sampIes of on average 462 speci­.mens taken from each codend on each haul (in total 31,447 herring)~ The sizedistributions cf the measured sampIes were then applied to calculate the totalnumbers for the catch of each haul.

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3. Results

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3.1 Selection

The analysis of the results has not yet been completed. At this stage, all thehauls in which there was no gear damage are considered valid. The data of thefour tows in which the large mesh codends were compared directly are not in­cluded in the analysis below.

The combined length distribution diagrams show that above a length of 16 cmthe two large mesh codends caught roughly the same number cf fish (Fig~ 3b).'The almest identical length distribution in,of ,the two small mesh data sets(Fig. 3a)demonstrates that the two test codends were sampling fish of thesamesize composition. Below the critical length of 16 cm, the total numberof fish caught was almest 2.5 times as great in the experimental diamond meshcodend as·in the square mesh codend. .

Both the large mesh codends released the smallest juvenilesvery effectively~In the small mesh codend the proportion of herring below 12 cm was on average28.2 %, while in the 36 mm diamond mesh codend the proportionof herring below12 cm averaged only 4.2 %, and in the 36 mm sqUare mesh codend there wereactually no herring below this size ~. ,

The percentages retained in each length group by the experimentäl largemeshcodends are given in Tables 2 and 3~ In the size classes over 18 cmi' both thelarge codends retained slightly more individuals than the associated smallmesh codend. It.is generally accepted that Unequal sampie sizes of large fishin the codends cf a trouser trawl can invalidate the conventional statisticaltechniqUes describing the shape of the selectivity curves. Nevertheless, toobtain a mathematical description~ the data were preliminarily run through thestatistical model presented by MILLAR & WALSH (1990). This model, which is 'still under development, can handle theuneqUal sampling efficiencY,of largefish in the small and large mesh legs~ The model estimated the split for thediamond mesh codend as,58:42 and for the sqUare mesh codend as 55:45~ showingthat more fish were goinginto both the large mesh codends. Fitted selectivityogives using the 50:50,assUmption are presentedinFigure 4a. The selectivityparameters for the test codends were also calculated by a "conventional"logistic model, using MarqUart's algorithm (non linear least squares)~ Thefitted selectivity curves are shown in FigUra 4b. ~

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•The standard selection parameters (in cm) derived by these two methods are:

L2S.. Lso .. ' L7S .. L7S .. -L2S "

36 mm Diamond: MilIar & Walsh's fit 12.3 14.6 16.8 4.5 ':

Conventional fit 13.7 15.4 17.0 3.3

36 mm Square: MilIar & Walsh's fit 15.0 15.9 ' 16.9 L9Conventional fit 15.1 16~3 17.4 2~3

For the square mesh data, the results revealed by these two methods are mar­ginally different. Por the diamond mesh there appears to be more variationbetween the estimates. However, both methods giveresults which indicate thatthe square mesh codend has a sharper selectivity than the diamond mesh codend

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of the same mesh size. The greatest difference between the diamond and squaremesh is in the lowest selection range, indicating that the square mesh codendreleases the smallest juvelines more effectively than the equivalent diamondmesh codend~

3~2 Escape behaviour

The underwater TV observations revealed some interesting features of thebehaviour of herring in a trawl codend.

The herring generaily swam inside the belly and the extension of the codend inthe tcwing direction,. making short bursts and then "resting". Step by step thefish dropped back towards the after part of the codend, where most of themstill swam actively for a short period.

During tcwing, the extension pieces cf both the large mesh codends were moreor less slack and distorted; compared to the round shape of the extension ofthe small mesh codend. The meshes .of the square mesh extension were generallymore open than the meshes of the diamond mesh extension. On the other hand;most escapes seemed to occur 'at the front of the catch bulge, in the afterpart of the codend, in the area of the lifting hag. There the"shapeof allcodends was round or bulbous, even with notably small catches, and the meshesof the diamond mesh cOdend also remained more or less open. Mostescapesappeared to take place from the upper side of the codend..

The herring swimming inside the large mesh codends generally made more at­tempts to escape" than the herring swimming inside the small mesh codend. Thefish were practically unable to force a passage through the small mesh codend,which indicates that this mesh size was small enough to retain the total sizerange of the population being fished.

The passage of a herring through a mesh occurs at high "speed and with a TVcamera it is difficult to see in detail how it is actually performed. The onlything that can befirmly established is that large numbers of herring were ob-

. served to continue swimming and"to disappear from the view of the camera afterpassage throughthe meshes. Very few damaged herring were observed. As far asthe escape process could be studied with the TV camera, the escapees did notlose scales. Scales seen in the water around the codend were lost mainly bythe fish which were meshed in the belly and the separator panel of the trawl.

No clear difference could be seen in the escape process of fish through theexperimental square and diamond mesh codends.

4. Discussion

In using a trouser trawl approach in seiectivity studies, a basic underlYingassumption is that the same nUinbers cf each size of fish enter the two codendsUnder test. In the present selectivity experiment, this 50:50 probabilityassUmption was not satisfied. Nevertheless, the problems caused by retentionsover 100% may be of secondary importance in this analysis. A more seriousproblem is probably the high variation in the catching performance of the. twosides of the sampling trawl used. The catch data and the underwater TV obser-

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vations clearly indicated that this variation was systematic, and was not theresult of differences in the water, flow caused bythe different mesh sizes,ofthe,codends Under test (as'is usually the case). Problems were caused by thefaulty design and rigging cf the separator section used to splitthe afterpart of the trawl into two equal parts. Further bias in the catching perform­ance may have arisen from the unequal towing tension on the two sides of thetrawl (not measured).

If the flow of water was different in the two sides of the, trawl in front ofthe separator secticn, it is possible that a larger proportion of smaller(weaker) fishwere herded into the side where the fiow was greater, resultingin size-dependent inequality inthe fishing efficiency. The escape through theseparator section arid the joins observed on,some occassioris may also have beensize-dependent. On the other hand; it should be noted that the square meshcodend was mounted eight times and the diamond mesh codend nine times on theside whichtended to catch more effectively. Thus, in that respect the "game"for the coderids under test was almost even~

Some bias may also ,have been caused by the fact that the mesh opening of thesquare meshes tended to be slighly larger than that of the diamond meshes. Inthe main escaping zone, however; the difference was very small.

In the light of all these findings; the data coilected must be consideredinsufficient as a basis for judging whether the square mesh codend is superiorto the diamond mesh codend in releasing juvenile Baltic herring . The resultssuggest that a square mesh codend will retain fewer juveniles than a diamondmesh codend of equivalent mesh size, but whether the difference is also sig­nificant at higher catch rates or with different population structures isstill uncertain. The results indicate that by simply increasing the meshsizeof the existing diamond mesh codends to 36 mm the fishermen could get rid ofmost of the juvenile herring in their trawl catches~

On the basis of the underwater observations made during the tows, riothingexact can be stated about the actual damage done to the fish as they escapethrough the meshes of a large mesh codend. The observations suggest that scaleremoval is not as severe as has beeri thought earlier~ The observations showedthat the fish swim actively through the meshes~.but it isnot possible to as­sess whether herring suffer lessdamage on theirescape through a square meshthan through a equal size diamond mesh. Nor was there äriy direct evidence thätthe fish were trying to escape more actively from the square mesh,codend. Fur­ther experi~ents will hopefully provide answers to these important questions.

5. Some comments on the application of thetrouser trawl and square mesh

These experiments clearly demonstrate that a slight asymmetry in the iegs of .the separator section (one a bit "tighter", than the other), can greatly affectthe catching efficiency ofthetwo sides ofa trouser trawl, probably by'causing unegrial water flow inside and in front of the separator section. Forthe trouser trawl to function even tolerably; the various components of theseparator section must assUme an absolutely symmetrical shape and geometrywhile being towed. As is frequently noted, a full lengthvertical separatorpanel dividing the whole trawl into two eqUal parts would reduce the influenceof unequal water flow er sampling area in the entrarice to the two coderids.

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HOwever~ there is still the possibill.ty'that fish escape "unequally" beforeswimming into the codends. It also seems·to be very difficult to fit a sepa­rater section with a vertical panel to an existing pelagic trawl.and tosatisfy the reqUirement that it divides the trawl inte two equal halves. Somekind of "divided or double trawl" system (e~g. ROBERTSON & SHANKS 1987) inwhich the two test codends are not connected to the same trawl body mightyield better results in future selectivity studies using "one" sampling gear.

Although it was not the primary purpose of this study to assess the practicalaspects of the square mesh codend, some,observations may be considered here~,

There is no doubt that the mesh distortion of the square mesh webbing is adisadvantage~ Use of slightly heavier twines or a large mesh protection netaroUnd the codend may reduce the stretching problems, but will probably alsoimpair the selectivity. Since the main.escape seems to take place from the .upper side of the codend, the lower side of the codend could be censtructed ofa webbing of better strength~On the other side, it was noted that the tenden­cy of herring to become meshed in the netting was significantly smaller in thesquare mesh codend than in the diamond mesh·codend. lf the meshing problemswith a square mesh codend are really smaller in commercial fishing conditionsthis will encourage acceptance of the square mesh.

Acknowledgments: I am most grateful to Mr Steve Walsh, Mr Russel MilIar andDr John Casey for their great help in analysing and interpreting the data.Dr Erdmann Dahm, Dr Peter Stewart, MrDick Ferro, Mr Bjornar lsaksen, Dr AhtoJärvik, Mr. Mikaei Hilden and Mr Sakari Kuikka, among others, read an earlierdraft of this paperand made many useful suggestions for improvements. I alsowould l!ke to record mY,appreciation of the co-operation and willing assis­tance given by Mr. Esa Lehtonen in conducting this experiment~

References:

COOPER, C..& HlCKEY, W. 1989: Selectivityexperiments with square mesh cod­ends of 130~' 140 and 155 mm. - In Proceedings of World SymposiUm on FishingGear andFishing Vessel Design, 1988, Marine Institute, St. Johrl's, New­foundland~ p~ 52-59.JÄRvIK, A. & SUURONEN~ P. 1990: Seiectivity 'of Baltic herring trawls studiedby the covered codend arid twin codend methods~ - lCES Fish Capture Committee,C.M~ 1990/B:27, Sess~U.MILLAR, R. B. & WALSH; S. J. 1990: Analysis of trawl selectivity studies withan application te trouser trawls. - ICES Fish Capture Committee C.M.1990/B:14.ROBERTSON, J~ H. B. & SHANKS~ A. M. 1987: A coritinuous headline whitefishdivided trawl~ Initial fishing trial. - Scottish Fisheries Working Paper No10/87~

ROBERTSON, J. H. B. & STEWART, P. A. M. 1988: A comparison of size seiectioriof haddock andwhiting by square and diamond mesh codends. - J. Cons. int.Explor. Mer, 44: 148-161.SHEVTSOVj S. E. 1988: Selective properties of trawl codends with various meshshapes for Baltic herririg fishery. - lCES Fish Capture Committee, C.M.1988/B:10,SessQ.THORSTEINSSON,.G. 1989: Icelandic experiments with square mesh netting in theshrimp fishery. - lCES Fish Capture Committee, C.M 1989/B:49, Sess P.

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Herring ogives tor 36 mm mesh

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Table 1. The sequence of experimental codends in the port andstarboard legs of the trouser trawl, and the cathes of each haul.(small rnesh codend = 20 mm stretched meshlength).

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Haul Date codend catch (kg)

no. port starboard port starb.

1- 7 May diamond 36 rnrn small mesh 1520 8502. 7 May square 36 rnrn small mesh 790 355

.3. 8 May square 36 rnrn small mesh 1150 6904. 8 May diamond 36 mm small rnesh 800 6905. 9 May diamond 36 mm small mesh 650 940• 6. 9 May small mesh square 36 mm 285 1457. 9 May small mesh square 36 mm 1420 1900 •8. 14 May small mesh square 36 mm 205 370

Separator section was turned around!9. 15 May diamond 36 mm . small mesh 580 1110

10. 15 May diamond 36 mm small mesh 210 55011- 15 May square 36 mm small mesh 520 115012. 16 May small mesh diamond 36 mm 1610 120013. 16 May small mesh square 36 mm 195 49514. 21 May diamond 36 mm small mesh 85 36015. 21 May square 36 mm small mesh 410 102016. 22 May small mesh diamond 36 mm 930 156017. 22 May square 36 mm small mesh 570 145018. 23 May small mesh diamond 36 mm 840 111019. 23 May small mesh square 36 mm 1200 104020. 24 May diamond 36 mm small mesh 490 74021- rejected22. 28 May square 36 mm small mesh 180 59023. 28 May diamond 36 mm small mesh 260 43024. 28 May square 36 mm small mesh 475 157025. 29 May square 36 mm diamond 36 mm 890 1700 •26. 30 May diamond 36 mm square 36 mm 1330 170027. rejected

Adjustment on the rigging of the separator section!28. 4 June diamond 36 mm small mesh 1610 157029. 4 June small mesh square 36 mm 1200 200030. 5 June diamond 36 mm square 36 mm 1100 150031- 5 June square 36 mm diamond 36 mm 1000 150032. 6 June small mesh diamond 36 mm 470 47033. 6 June diamond 36 mm small mesh 90 9034. 6 June diamond 36 mm small mesh 410 910 ;35. 7 June square 36 mm small mesh 260 61036. 7 June small mesh square 18 mm 900 650

In total (kg) 24635 33015

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Table 2. Combined length-frequency distribution for the 36 mmi diamond mesh codend catches, and the percentages retained in

each length group by the codend (n=15 hauls).

Length No. of fish in No. of fish in Percentage(cm) 36 mm codend (diam.) small mesh codend retained

7.5 155 1830 8.58.0 421 7775 5.48.5 1058 18806 5.69.0 2646 30899 8.69.5 3093 26386 11.7

10.0 2361 19106 12.410.5 1326 10224 13.0• 11.0 377 5423 7.011.5 436 5100 8.512.0 492 5043 9.812.5 617 2786 22.113.0 431 2838 15.213.5 787 3805 20.714.0 4225 13974 30.214.5 8152 19678 41.415.0 12661 29669 42.715.5 9086 16134 56.316.0 12716 17897 71.116.5 16376 19645 83.417.0 24381 28247 86.317.5 20438 25879 79.018.0 22550 24316 92.718.5 19888 17316 114.919.0 20923 16955 123.419.5 17617 16603 106.120.0 17005 15288 111.2

• 20.5 11754 9760 120.421.0 13423 12834 104.621.5 9737 8674 112.322.0 8804 6402 137.522.5 4987 5439 91.723.0 4802 4627 103.823.5 3397 2723 124.824.0 2386 2140 111 .524.5 1599 1291 123.925.0 916 838 109.325.5 2470 1901 129.9

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In total 284,493 458,251

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Table 3. Combined length-frequency distribution for the 36 mmsquare mesh codend catches, and the percentages retained in eachlength group by the codend (n=15 hauls).

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Length No. of fish in No. of fish in Percentage(em) 36 mm codend (square) small mesh eodend retained

7.5 0 1274 08.0 66 8485 0.88.5 17 16896 0.19.0 78 35447 0.29.5 62 31589 0.2

10.0 48 26708 0.210.5 77 11669 0.711.0 32 7229 0.4 •11.5 0 5029 012.0 88 5313 1.712.5 125 4754 2.613.0 184 2436 7.613.5 344 4209 8.214.0 1691 12252 13.814.5 3684 21488 17.115.0 8382 38411 21.815.5 5112 26931 19.016.0 11632 19933 58.416.5 16907 20331 83.217 .0 27626 32287 85.617.5 23268 22164 105.018.0 23779 24746 96.118.5 18648 17551 106.319.0 22265 20232 110.0

• 19.5 17792 14815 120.120.0 18667 15347 121.620.5 12612 9825 128.421.0 11091 9978 111.221.5 9593 7138 134.422.0 6932 8307 83.422.5 5813 4262 136.423.0 4017 3481 115.423.5 2422 2703 89.624.0 2681 1646 162.924.5 2109 1172 179.925.0 1166 1390 83.9 .25.5 2145 1807 118.7 •In total 261,155 499,235