Moguedet, Ph. (Philippe)

Pérez-Contreras, M.N. (María Nélida)

English

Moguedet, Philippe

Pérez-Contreras, María Nélida

Digital.CSIC

Estimates of discards from the spanish trawler fleets in the sub-area VII

Repositorio Institucional Digital del IEO

WORKING GROUP ON FISHERJES UNITS IN SUB-AREAS VII AND VIIINANTES. FRANCE, 23 - 30 JUNE 1989.Estimates of discards from the spanish trawler fleets in the sub-area VII.MOGUEDET Ph. and PEREZ N.Instituto Español de OceanografíaCentro costero de VigoCabo Estay. Canido.Apartado 155236080 VIGOSPAINThe assessment of fishery interactions in I.C.E.S sub-areas VII and VIII was untill now realized using only landingdata, or applying discard data from some fleets with knowndiscards. So the 1988's report of this working group pointed outthat" for all species there is a serious shortage ofinformation on discards. The consequence of assumption ondiscarding practises are major in evaluation of technicalmeasures, and to improve the analyses more discard data areessential".Taking into accountthe spanish trawlers discardslaboratory of Vigo (N/W Spain).ship on spanish trawlers.these recommandations a study ofis conducted by the fisheriesSince 1988 two observers monthlyThe objective is to determine the length compositionof discards of target species (megrim, monk, hake and nephrops)and those of a few by-catches species (blue whitting, mackereland horse-mackerel).·"~,-2-Sampling designoThe target population is made up of all the hauls ofthe trawler fleets. The sampling unit is the haul. Thecharacteristic studied is the number of fishes discarded bylength class (centimeter) in the haul, for each species.According with its structure, the target population isstratified by port (Vigo, A Coruña) and quarter. Two phases ofclusters are considered into each stratum :- cluster of "trip".cluster of "haul" belonging to a cluster "trip".The sampling design consist, in a random sample of somehauls of various trips into each of the four annual strata. Apart of the discard is randomly selected from the total in eachsample haul. In this sample all individuals of the target speciesare measured.:..In order to estimate the total number of fishesdiscarded for length classes, the method of the mean by elementpopulation is used (COCHRAN 1977). The estimate of the meannumber of discards by haul is then multiplied by the number ofelements in the target population (number of hauls in the stratum"port-quarter")..r .•••~~•.Fittingtechnique of discard ogives.To fit discarding ogives from length distributions ofdiscards and landings we used the software "TRILOG" written byB.MESNIL (IFREMER NANTES). This program use a simplex algorithmand does not require a log-transformation. This method is notsensitive to the range of length classes extending outside theinterval where actual selection takes place, and gives excellentfit in most cases, particularly when discards length compositionis not plurimodal (Anonymous 1988).• I- 3-Results.Results presented he re are only global ones.Intermediary results, diseards in stratum "port-quarter", revealthat Vigo trawler fleet works most of time in divisions VlIj andVIlh, while A Coruña's one works essentially in divisions VIIeand VIlk.Skippers from Vigo look for megrim in the southern partof the I.C.E.S sub-area VII, on the superior part on thecontinental shelf, at dephs between 150m and 200m. Skippers fromA Coruña look preferently for nephrops and hake in the northernpart of the I.C.E.S sub-area VII, at depths between 300m and500m..- rabies (1) and (2) include the percentage of spanishfleets discards by length class, estimated by the sampling, andfitted by the software, for each specie for 1988.rabIe (3) contains, the parameters of the logisticcurves and their position characteristics, estimated by thefitting model for each specie.Figures (1), (2) and (3) illustrate the curves obtainedfrom the sampling data and from the fitted data, for each specie.rhe analysis of the curves allow the classification ofthe species studied in two categories- speeies whose(L. whiffiagonis)budegassa). Slopediscardandvaluesogives havemonks (L.are respectlystrong slope : megrimpiseatorius and L.0.82,1.03 and 0.69.- speeies whose diseard ogivesmerluecius) and nephrops (N.0.34 for each specie.have a weak slope : hake (M.norvegicus). Slope value isDiscard ogives comparison reveals that studied fishspecies have a L50 very similar (N 20 cm).., ,-4-Discussion.Three factors influence the form of discard ogives- the vulnerability of the species. It is the result of the"in situ" interaction, between the fishes and the trawls.It is directly related with the shape of the fish and themesh of the trawl.- the market conditions.fishes, or for a specie,out by the fishermen.The interest for small lengthinfluence strongly their sorting- the sampling designo If the sample is not a fully random,a bias is introduced and the proportion of smallestindivuals in the discards may be over or under evaluate.Moreover, when sample rate of trip in a stratum "port -quarter" is weak, results would be very affected by the theskippers strategy.Discard ogive slopes show a strong correlation with thethe shape of the fish. Megrim and the monks are the species withthe widest body. The difference noted between the two monks comesfrom the market influence, as L. budegassa individuals are moreappreciated.The weak slopes of hake and nephrops discard ogives isdue both to the high vulnerability of these species and the highmarket interest for them.The present study is a first approach to the estimateof the spanish discards. The whole fleet could not be sampled, asCantabrian trawlers have not be taken into account. Moreover,various bias introduced by the sampling technique and theestimate of the number of hauls in each stratum "port -quarter",might be important. So this has to be considered when using thesedata.- 5-REFERENCES.ANONYMOUS (1988).- Report of the working group on fisheries unitsin sub-areas VII an VIII. Nantes France. 22-28 Juin1988.Int. Coun. exploro of the sea. CM 1988/Assess:25. 127p."..) COCHRAN W.G (1977).- Samplings technics .3rd. Edition. Wiley J. and sons. New York. 413p ..,~ <I ------~----____.---------- -----.-------------------------------..---- 1 L. ~hiffiagonis I M. merluccius I L. piscatorius L. budegassaLength I I I(cm) 1Observed Fitted 1observed I Fitted I Observed 1 Fitted Observed I Fitted----1 1 1 1 1 1----15.5 1 97.55 100.00 1 100.00 100.00 100.00 1 100.00 100.00 1 100.0016.5 1 100.00 100.00 1 100.00 100.00 100.00 I 100.00 100.00 I 100.0017.5 I 96.96 94.64 I 88.42 76.29 100.00 I 100.00 100.00 I 100.0018.5 92.50 88.56 1 83.52 69.53 81.85 I 86.38 100.00 1 100.0019.5 81.84 77.25 I 50.77 61.81 93.69 I 69.34 100.00 1 100.0020.5 58.66 59.83 1 42:85 53.44 44.97 I 44.66 26.41 1 38.87 121.5 40.95 39.51 1 35.42 44.86 22.57 1 22.35 21.72 1 22.63 I22.5 21.23 22.27 1 34.23 36.59 5.17 I 9.31 22.45 I 12.79 II 23.5 10.20 I 11.16 I 18.04 29.04 0.00 I 0.00 7.05 I 6.86 1I 24.5 4.56 5.22 I 24.79 22.49 0.00 I 0.00 2.92 I 3.56 II 25.5 2.40 2.36 1 25.16 17.06 1.66 1 0.00 0.19 I 1.82 I1 26.5 1.04 1.05 I 12.57 12.73 2.15 I 0.00 0.17 I 0.92 II 27.5 0.61 0.46 I 11.52 9.37 0.00 I 0.00 0.36 I 0.46 II 28.5 0.43 0.18 I 6.58 6.83 0.00 I 0.00 0.31 I 0.23 II 29.5 0.27 0.09 I 4.86 4.94 0.00 I 0.00 I 0.00 I 0.00 II 30.5 0.09 0.04 I 4.36 3.56 0.00 I 0.00 I 0.00 I 0.00 II 31.5 0.13 0.07 I 1.96 2.55 I 0.00 I 0.00 I 0.00 I 0.00 II 32.5 0.00 0.00 I 0.37 1.82 I 0.00 I 0.00 I 0.00 I 0.00 II 33.5 0.02 0.00 I 0.52 1.30 I 0.00 I 0.00 I 0.00 I 0.00 II 34.5 0.00 0.00 I 0.12 0.92 1 0.00 I 0.00 I 0.00 I 0.00 II 35.5 0.00 0.00 I 0.00 0.00 I 0.00 I 0.00 I 0.00 I 0.00 II I I I I I Ir ab l e (1) Observed and fitted vaIues (in %) by length class (cm) of the spanish discardsfor 1988 in the sub-area VII.I0'\ITable (2)-7-N. [lorvegicusLength ,(mm) I Observed I FittedI I15.5 I 100.00 I 100.0016. 5 I 100.00 I 100.0017.5 I 100.00 I 100.0018.5 I 100.00 I 100.0019.5 I 78.73 I 100.0020.5 I 100.00 I 100.0021 .5 86.09 69.3322.5 66.86 61 .6723.5 89.35 53.3824.5 90.97 44.9025.5 43.64 36.7126.5 39.81 29.2227.5 28.86 22.7128.5 14.63 17.2929.5 12.60 12.9530.5 7.71 9.5831 .5 3.80 7.0132.5 4.48 5.0933.5 3.09 3.6834.5 1 .55 2.6535.5 0.63 1.9036.5 0.88 1.3637.5 0.59 0.9738.5 0.64 0.6939.5 0.22 0.4940.5 0.45 0.3541.5 0.50 0.2542.5 0.15 O. 1843.5 0.27 O. 1344. 5 0.94 0.0945.5 0.58 0.0646.5 0.06 0.0547.5 0.00 I 0.0048.5 0.00 I 0.0049.5 0.00 I 0.0050.5 0.00 I 0.00Observed and fitted values (in %) by length class (cm)of spanish discards of Nephrops norvegicus for 1988in the sub-area VII.-8-(1) I (2 )Ia I b I LSO I L75-25II I I IL. whiffiagonis I 0.82 I _17.29 I 20.98 * I 2.67 *M. merluccius I 0.34 I - 7. 18 I 20.90 * I 6.39 * IIL. piscatorius I 1 .03 I -20.92 I 20.29 * I 2. 13 * I1L. budegassa I 0.69 I -13.60 I 19.72 * I 3.18 * IIN. norvegicus I 0.34 I - 8. 13 I 23.90 ** I 6.46 ** IJ(1) : logistic curve parameters (a = slope. b = intercept origin)(2) : position characteristics of the discard ogives.* centimeters. ** millimeters.¡. Table (3) Logistic curve parameters and position characteristicsof the discard ogives estimated from the fitting modelo- 9_DISCARD OGIVE L. WHIFFIAGONISICESDMSIOH VII 1988':1~U~~-~---80-1 ~70 -l ,-----,litOe:~litO•605040-130120100+'-1111111-----.-111 1 11::1f=""'IjIIj3Ij1$10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 21 30 31 32 33 34 35LENGTH (cm.)D REAL + F1TI'EDDISCARD OGIVE M. MERLUCCIUSICESDMSIOH VII 1188100~--------------------------90ti)Oe:~litO•8070 JI!60 'l50140-.3O~I20-1" ~O~-I 1 1 1 1 1 -r-r 1 ~~ ,15 16 17 18 11 20 21 22 23 24 25 26 27 28 21 30 31 32 33"~ \LENGTH (cm.)D REAL + f11TEDFigure (l) : Theorical and fitted discard ogivesfor Megrim and Hake.lI1IjII$=-ti!34 35_ 10_DISCARD OGIVE L.PISCATORIUSICESDMSIOHVII 1188100II10 -!¡ao~III70 -1<I~ SO lO11: 50l~O~O• <40-1I!30JI20 -1I':J15 16 17l1\I1t1II111I 11I 11I 132 33 34 35\,III~ 11I 11I 11I 11I18 11 20 21 22 23 24 25 26 27 28 21 30 31LENGTH (cm.)e REAl. + FlTTEDDISCARD OGIVE L.BUDEGASSAICESDIVlSIOHVII 1988100 lB lB lB lB lB I~O11:~~O•90 ...;I80l70...j ,I!I80 -j ~,~50~ 1~ I40~~II1\~30-;! ¡i20 •..• IIrrl~.10 J I:Ok-l ,I•• Ifl 11I 11I 11I 11I 11I 11I---..$15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35LENGTH (cm.)e REAl. + FlTTEDFiqure (2) : Theori~~l ~nn r;++~~ ~~~~--~Ii5•_11-DISCARD OGIVE N. NORVEGICUSICD DIVISIOH vn "18100 lB lB IR IR I 111 i10180-110~50-40:Jo -.j i I i I i I i I i I i I i I i ~ i ~ 11 • " • 11IlfI 11 lfI iI , 11 11I 11 11I •11 17 ,. 21 U 25 Z7 2' 31 31 H !7 •• 41 G 46 47 •••LENcmf (fnm.)e KAL + FlTlEDFigure (3} : Theorica1 and fitted discard ogivesfor Nephrops.A CORUr\JA 19880,90,80,7""'enzO 0,6:J...J~'"" 0,5ena::wal 0,4~::Jz0,30,20,1O101214160,90,80,7""'enzO 0,6:J...J~0,5'""ena::w 0,4al~::Jz0,30,20,1O18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60LENGTH (cm)VIGO 1988--------- ni ~'jl ~r:1..., Ir Irl~I I I I I I I I I I10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60LENGTH (cm)Fig. 3. Length composition of horse mackerel discards estimatedfor Spanish fishing trawlers in ICES Division VII in 1988.

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Estimates of discards from the spanish trawler fleets in the sub-area VII

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