koops 1982 aquacultural-engineering

7/28/2019 Koops 1982 Aquacultural-Engineering

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Aquaeultural Engineering 1 (1982) 11-21

G R A D I N G O F F A R M E D E E L S B Y M E A N S O F M E S H W 1 R E B O X E S

H. KOOPS and H. KUHLMANNInstitut fiir Kiisten. und Binnenfischerei Bundesforschungsanstalt fiir Fischerei, Ham burg,West Germany

ABSTRA CTTh i s paper de sc r ibe s t he succes s fu l g rad ing o f f a rm ed ee ls u s ing square m esh ed bo xscreens . The g rad ing curve s f o r y e l l ow and s il v e r e e l s a re eva lua t ed f o r t he square meshs i z e s: 10 , 16 , 20 and 25 m m wh i l e some ind i ca ti on i s g i ven t o t he l oca t ion o f t he s exe sw i th in these gradings .

INTRODUCTIONWi th f i nanci a l suppor t f r om t he Fede r a l M i n is t ry f o r Resea r ch and Techno l ogy , t heI ns t i t u t e f o r Coas t a l and I n l and F i she r ie s o f the Fede r a l F i she r ie s Resea r ch Boa r dca r ri e s ou t e e l f a rm i ng exp e r i m en t s i n Em den , u s ing t he r m a l e f f l uen t w a t e r f r oma c onve n t i ona l pow er p l an t . Ee l s a r e kep t i n c i rcu l a r t anks . The t anks a r e s t ocked w i t hs m all e e l s - a b o u t 4 0 g a ve ra ge w e i g h t - c a u g h t b y t h e s t o w - n et -f is h e ry i n th eL o w e r E r o s.

More evident than for o ther cu l tured f i shes i s an ext reme indiv idua l growth var ia -t ion as descr ibed by severa l inves t iga tors (Ko op s , 1965; Meske, 1 969; Ko ops , 1971 ;R e h b r o n n e t a l . , 1 9 7 2 ; K u h l m a n n , 1 9 7 4 ; K u h l m a n n a n d K o o p s , 1 9 7 8) . A c o n s e q u e n c eof the in tens ive cul ture of ee l s i s tha t in ungraded mater ia l , smal l ee l s fa l l behind wi thi nc rea s ing g r ow t h va r i a ti on . Th ese r un t s a r e p r even t ed f r om f eed i ng by t he la r gerdom i nan t e els i n t he g r oup and t hey t he r e f o r e su f f e r, l o se w e i gh t and bec om e suscep t-ible to disease. C on seq ue nt ly mo rtal i ty, weight- losses, disease-risks an d feed conv ersio nshould imp rove , if ee ls co uld be graded in to s ize-groups several t ime s dur ing thecul ture per iod , as i t i s done in Japanese ee l -cul ture (Matsui , 1952, 1954, 1964) .So r t i ng b y han d , a s is u sua l ly p r ac t i s ed , how eve r , i s ex t r e m e l y t i m e - consum i ng and11Aquacultural Engineering 0144-8609/82/0001-0011/$02.75 Applied Science Publishers Ltd,England, 1982Printed in Great Britain


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12 H. KOO PS, H . KUHLMANN

p u t s a s tr e ss o n t h e f i s h , w h i c h m a y c a u s e o u t b r e a k s o f d i s e as e s h o r t l y a f te r w a r d s .M e s h w i r e b o x e s f o r s e l f - g r a d i n g o f t h e e e l s h a v e b e e n u s e d s u c c e s s f u l l y i n p r e v i o u sp o n d f e e d i n g e x p e r i m e n t s o f t h e i n s t i t u t e a t t h e ri v er M o s e l . T h i s m e t h o d s h o u l d n o wb e t e s te d u n d e r t h e E m d e n c o n d i t i o n s .

M A T E R I A L S A N D M E T H O D S

I n t h e a u t u m n o f 1 9 7 8 , t w o c i r c u l a r t a n k s o f 6 a n d 8 m d i a m e t e r r e s p e c t iv e l y w e r es t o c k e d w i t h e e l s w h i c h h a d b e e n c o l l e c t e d in t h e s p r i n g o f t h e p r e v i o u s y e a r , 1 9 7 7 .T h r e e m o r e t a n k s w e r e s t o c k e d w i t h e e l s w h i c h h a d b e e n c o l l e c t e d i n t h e s p ri n g o f1 9 7 8 . I n N o v e m b e r 1 9 7 8 t h e r e w e r e 1 8 7 0 0 e e ls i n t h e s y s t e m t o t a l l i n g a b o u t 2 1 0 0 k ga n d t h e s e w e r e g ra d e d b y m e a n s o f m e s h w i r e b o x e s .T h e s e b o x e s a r e c o n s t r u c t e d f r o m s t r o n g w o o d e n f r a m e s f i t te d w i t h s q u a r e m e s h -w i r e . L i g h t g a u g e h e x a g o n a l m e s h , e . g . c h i c k e n m e s h , i s u n s u i t a b l e s i n c e it ha s b e e nf o u n d f r o m p r e v i o u s t e st s t h a t e e l s e s c a p i n g ta i l f ir s t t h r o u g h t h i s t y p e o f m e s h h a v es e v e r e ly d a m a g e d t h e i r s k i n w i t h t h e r e s u l t a n t r i s k o f in f e c t i o n / f u n g u s . O t h e r n e t t i n gm a t e r i a l h a s a ls o p r o v e d u n s u i t a b l e , e . g . s t o w n e t , t r a w l a n d fy k e - n e t s a n d t h e r e f o r es t r o n g g a u g e , s q u a r e m e s h e s a r e r e c o m m e n d e d a s g i vi ng t h e l e a st d is t u r b a n c e t o t h eg r a d i n g o f t h e e e l s.

F o r b o t h g r a d i n g s , i n t h e a u t u m n o f 1 9 7 8 a n d t h e s p r i n g c a t c h o f 1 9 7 7 g r a d in gb o x e s t o t h e f o l l o w i n g s p e c i f ic a t i o n w e r e u s e d :

M e s h s iz e : 1 0 m m 1 6 m m 2 0 m m 2 5 m m 3 0 m mL e n g t h : 2 m 2 m 1 m 1 m 0 . 7 mW i d t h : 1 m 1 m 0 . 5 m 0 . 5 m 0 . 7 mH e i g h t : 0 - 7 5 m 0 . 7 5 m 0 - 6 m 0 . 6 m 0 - 6 m

T h e g r a d i n g t a n k w a s r e c t a n g u l a r ( 6 2 1 .2 m ) a n d t h e b o x e s w e r e p l a c e d in it i np a i r s , th e l a rg e r o n e , s a y 1 6 m m m e s h s i ze , b e i n g k e p t a t l e a s t 8 c m c l e a r o f t h eb o t t o m , w h i l e t h e l a rg e m e s h e d s m a l l e r b o x , s a y 2 0 m m m e s h s i z e , f l o a t e d i ns i d e i t.I n t h is w a y t h e w a t e r l e ve l a b o v e t h e m e s h b a s e o f e a c h b o x w a s r o u g h l y 7 0 c ma n d 4 0 c m r e s p e c t i v e ly .

T h e e e ls w e r e t a k e n o u t o f th e f e e d i n g t a n k s i n p o r t i o n s o f a b o u t 2 0 k g a n d p u ti n t o t h e i n n e r g r a d i n g b o x w i t h t h e l a r g e m e s h . I n t h is w a y o n e g e t s t h r e e si ze g r o u p si n o n e g r a d i ng p r o c e d u r e , f o r e x a m p l e :

1 . e e ls h e l d b a c k b y 2 0 m m m e s h ( ' c a u g h t ' ) ,2 . e e ls p a s si n g 2 0 m m b u t h e l d b a c k b y 1 6 m m m e s h a n d ,3 . s m a l l e e l s p a s s i n g b o t h m e s h - s i z e s .' S t i c k i n g ' o f e e ls i n t h e s m a l l e r b o x e s o c c u r r e d i f t o o m a n y e e ls w e r e u s e d , i f t h e

s o r t i n g o u t w a s s t a r t e d t o o e a r l y o r i f t h e b o x e s w e r e l if t e d . A b o u t 3 m i n s e e m s t ob e a s u i t a b l e t i m e f o r t h e s e l f -g r a d in g . T h e e x t r e m e l y l a rg e g r o w t h v a r i a t i o n m a d e i t


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GRADING OF FARMED EELS BY MEANS OF MESHWIRE BOXES 13n e c e s s a r y t h a t s o m e o f t h e e e ls h a d t o b e g r a d e d s e ve r al t i m e s w i t h d i f f e r e n t m e s hs iz e s. F u r t h e r m o r e i n t h e s p r in g o f 1 9 7 9 t h e s m a l l e el s w h i c h h a d p a s s e d t h e 1 6 m mm e s h t h e p r e v io u s a u t u m n w e r e g r a d e d t h r o u g h 1 0 m m m e s h . T h e g r ad i n g w a s a l w a y sd o n e a t d a y t i m e . W i t h t h e E m d e n w a t e r s h a v i n g a h i g h t u r b i d i t y t y p i c a l o f b r a c k i s ht i d a l z o n e s o f r iv e r e s t u a ri e s i t w a s f el t b y t h e a u t h o r s t h a t t h i s p r o b a b l y h e l p e d i n t h eg r a d in g o f t h e e e l s . I t wa s f e l t t h a t i n a c l e a r wa t e r s i t u a t i o n t h e b e h a v io u r o f t h e e el sc o u l d c a u s e c o n c e r n o n s t i c k i n g i n t h e m e s h e s .

F r o m t h e g r a d e d s iz e g r o u p s ( d e p e n d i n g o n t h e n u m b e r o f e e ls ) a ll f is h o r a s m a n ya s p o s s ib l e w e r e m e a s u r e d ( i n c m - b e l o w ) a n d w e i g h e d ( in g ) i n d i v i d u al l y . F o r e a c hl e n g t h a n d w e i g h t g r o u p t h e p e r c e n t a g e o f e e ls ' c a u g h t ' w a s c a l c u l a t e d f o r g ra p h i ce v a l u a t i o n .

R E S U L T S

T h e g r a d in g e f f e c t o f t h e s i n g l e m e s h s i z e s r e s u l t s i n s p e c i f i c s i g m o id c u r v e s ( F ig s1 - 4 ) , w h i l e s o m e s e l e c t e d v a lu e s a r e l i s te d i n T a b l e s 1 a n d 2 . ( M o r e e x t e n s iv e r e s u l t sa r e s h o w n in T a b l e s 3 - 7 . ) T a b l e 1 g iv es , f o r t h e d i f f e r e n t m e s h s iz e s, t h o s e l e n g th -v a l ue s a t w h i c h c e r t a i n p e r c e n t a g e s o f y e l l o w o r si lv e r e e ls w e re r e t a i n e d ( ' c a u g h t ' ) .F o r t h e 3 0 m m m e s h t h e n u m b e r o f l ar g e e e ls c a u g h t w a s t o o s m a l l to r e p o r t .

F o r t h e s a m e m e s h s i z e s t h e g r a d in g c u r v e s f o r y e l l o w e e l s a r e s h i f t e d t o l a r g e rl e n g th s c o m p a r e d w i th t h e c u r v e s f o r si lv e r e e ls b e c a u s e t h e g r a d in g e f f e c t i s r e l a t e dto t h e c i r c u m f e r e n c e o f t h e f i s h r a th e r t h a n t o i t s l e n g th a n d s il v er e e ls a r e t h i c k e rt h a n y e l l o w e e l s o f e q u a l l e n g t h .

T h e s i l v e r e e l s r e t a in e d h a v e a l o we r we ig h t t h a n y e l l o w e e l s o f t h e s a m e l e n g th .

/o l O m m 1 6 m m 2 0 r n m 2 5 r n m

fo . + ; , - ' ~ " + ~ +8 0 . + / / 7 0 - /

I6 0 - / ~5 0 . + /4 o , /30. I2 0 - i1 0 . I

iJ i , k~ 2 'o 2 ~ 3 0 3 ~ 4 0 4 5 5 o ~ ~ O : r ~

F i g . 1 . L e n g t h - g r a d i n g c u r v e s , y e l l o w e e l s . x - a x i s : l e n g t h o f e e l s ( o n ) , y - a x i s : % o f e e l s ' c a u g h t 'per cm group. Grading curves are drawn fo r 10, 16, 20 an d 25 mm-meshes.


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14 H. KOOPS, H. KUHLMANN% 2 0 m m ' 2 5 m m

I O 09 0 + +8 07 0So5 0 .,- .,'- 4 03 0 e2 01 0 i i2 ' 0 ' ,5 2 5 3 0 3 5 4 0 4 5 5 0 5 5 G O c mFig. 2. Len gth-grading curves, silve r eels. For further explanation see Fig. 1. N o silver-eels are'caught ' by 10 and 16 mm -meshe s .

% l O m m 1 6 m m 2 0 r a m 2 5 r n m

1 0 0 -0 -0 -0 -0 -0 -0 - + , 'I / * " + ~ + - ', .- ++ + 4 + + y

2 0 - I1 0 - /

0 50 lo o 150 2oo 250 30o sso gFig. 3. Weight -grading curves , ye l low ee l s . x -ax i s : we ight o f ee l s ( 10 g - resp. 5 g group), y - a x i s :% of ee l s ' caught ' per 10 g group . Grading curves are dra wn for 10, 16, 20 and 25 m m - m e s h e s .

T h e n u m b e r o f s m a l l f is h m a y n o t s t r i c tl y r e p r es e n t t h e m e s h - s e l e c t i o n b e c au s e o ft h e ' b y - c h a n c e ' d i s t r i b u t i o n o f t h e s e s m a l l e e l s i n t h e t a n k .

D I S C U S S I O N

S q u a r e m e s h w i r e b o x e s a l l o w a s i mp l e a n d q u i c k g r a d i n g o f e e l s i n t o s i z e g r o u p s a n dp u t s l it tl e t r e ss o n . t h e f i sh . I t i s i m p o r t a n t , h o w e v e r , t o c h o o s e a m e s h s i z e w h i c h f it st h e p r a c t ic a l s i tu a ti o n. T h e 2 5 m m m e s h w i l l r e s tr a i n e e l s g r e a t e r t h a n 2 5 0 g w h i c h


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GRADING OF FARMED EELS BY MEANS OF MESHWIRE BOXES 15

% 2 0 ra m 2 5 m m100-90_ ~. + # ;

8 0 -7Q -,50-5 0 - + 4 0 -3 0 -2 0 -1 0 .

r1 0 5 '0 1 0 0 1 ;0 2 0 0 2 5 0 3 0 0 3 % 0 g

Fig. 4. Weight-grading curves, silver eels. For fur the r explanati on see Figs 1 and 2.TABLE 1Selected values for the length-grading curves (lengths in cm)

Percentage of 10 mm 16 mm 20 mm mesh 25 mm mesheels 'caught' mesh meshYellow Yellow Yellow Silver Yellow Silvereels eels eels eels eels eels

10 26.0 34.9 44-2 40-1 50.3 48. 417 26.2 35-4 45.0 41-3 51.4 49.433 26.4 36-5 46-3 43-4 53.0 50.550 27-5 37.6 47.3 45-0 54-5 51. 467 28-9 38.6 48-9 46. 4 55.9 52-483 30.1 39-8 50.6 48-0 57.3 53.690 31-5 40.5 51.8 49.2 58.2 54.8

TABLE 2Selected values of the weight-grading curves (weight in g)Percentage of 10 mm 16 mm 20 mm mesh 25 mm mesheels 'caught' mesh mesh

Yello w Yellow Yellow Silver Yellow Silvereels eels eels eels eels eels

10 5 63 157 138 246 24317 6 68 163 146 256 25333 9 '72 172 157 268 26550 11 74 178 163 279 27267 14 75 188 170 293 28183 20 81 198 179 309 39190 25 84 204 189 317 298


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~

Q

~

O

5

~

N


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TABLE 4Weight-grading of yellow eels stocked in 1977/7 8 and graded in the au tu mn of 1978g 25 ~ % of eels 'caught '

mm mm mm mm mesh size16 mm 20 mm 25 mm0-9 ~ 31 3110-19 733 73320-29 878 87830-3 9 1 249 20 1 269 240--49 1 301 39 1 340 350- 59 1 084 20 1 104 260-6 9 1 374 39 1 413 370- 79 836 429 2 1 267 34 080-8 9 238 926 1 164 8090-9 9 21 926 947 98100- 109 770 7 777 100 1110- 119 722 2 724 100 0

120-1 29 634 2 636 0130-1 39 478 2 480 0140- 149 390 12 402 3150-1 59 497 7 504 1160-1 69 224 27 1 252 11 0170-1 79 224 95 1 320 30 0180- 189 88 150 0 238 63 0190- 199 49 150 4 203 76 2200- 209 49 170 4 223 78 2210 -21 9 29 179 7 215 87 3220 -22 9 0 150 5 155 100 3230- 239 0 103 5 108 100 5240 -24 9 0 91 5 96 100 5250-2 59 10 107 16 133 92 12260- 269 72 20 92 100 22270- 279 43 15 58 26280- 289 38 38 76 50290- 299 17 25 42 60300- 309 14 42 56 75310- 319 5 36 41 88320-3 29 5 29 34 85330-3 39 27 27 100340- 349 23 23350- 359 19 19360- 369 17 17370-379 26 26380- 389 15 15390-399 5 5400-409 8 8410-419 9 9420-429 7 7430-439 1 1440-44 9 2 2450-459 2 2460-469 2 2470-479 2 2480-489 1 1490-499 1 1500-509 1 1510-51 9 0 0520-529 1 1Total 7 745 6 563 1 450 422 16 180Sample 750 673 842 422 2 687


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1 8 H . K O O P S , H . K U H L M A N N

T A B L E 5L e n g t h - g r a d m g o f si N e r e e ls s t o c k e d m 1 9 7 7 / 7 8 a n d ~ a d e d m t h e a u t u m n o f 1 9 7 8a m < 1 6 1 6 - 2 0 2 0 - 2 5 > 2 5 ~ % o f e e ls ' c a u gh t '

m m m m m m m m m e s h s ~ e1 6 m m 2 0 m m 2 5 m m

33 1 1 10034 6 635 12 1236 34 34 037 66 1 67 138 106 4 110 439 132 7 1 140 6 140 132 22 0 154 14 041 149 29 0 178 16 042 123 43 0 166 26 043 109 47 0 156 30 044 87 44 0 131 34 045 69 65 0 134 49 046 50 62 3 115 57 347 35 84 2 121 71 248 15 100 6 121 88 549 9 69 11 89 90 1250 3 64 31 98 97 325 1 3 5 0 4 7 1 0 0 9 7 4 75 2 1 5 4 5 4 1 0 9 9 9 5 053 21 56 77 100 7354 11 62 73 8555 13 59 72 8956 7 57 64 9857 1 44 45 9758 1 38 39 1005 9 2 9 2 9 1 0 060 15 15 10061 11 11 10062 11 11 10063 1 1 10064 2 2 10065 2 2 10066 3 3 10067 2 2 10068 2 2 100

T o t a l 1 1 4 2 7 9 9 5 4 9 2 4 9 0S a m ple 1 104 799 549 2 452a re c o n s i d e r e d t h e m o s t p r o f i t a b l e s iz e g r o u p . A t t h e g r a d i n g i n th e a u t u m n o f 1 9 7 8 ,o f si lv e r e el s s t o c k e d i n 1 9 7 7 / 7 8 , n o n e p a s s ed t h r o u g h t h e 1 6 m m m e s h .

A s s h o w n b y K u h l m a n n a n d K o o p s ( 1 9 7 8 ) t h e w e i g h t d i s t r i b u t i o n o f s il ve r e el ss h o w s t w o p e a k s ( s ee T a b l e 6 ) . T h e f i r s t w e i g h t g r o u p b e g i n n i n g a t a b o u t 8 0 g w i t ha m a x i m u m a t a b o u t 1 5 0 g a n d a f t e r a m i n i m u m a t 1 9 0 g a s e c o n d p e a k a p p e a r sw i t h i ts m a x i m u m a t 3 8 0 g.

T h e f i rs t g r o u p c o n t a i n s t h e m a t u r i n g m a l e e e ls a n d w i l l b e m a i n l y i n t h e s e l e c t e dg r o u p b e t w e e n 1 6 a n d 2 0 m e s h s iz e . T h e se h a v e t o b e s o r t e d b y h a n d . O n l y a f e w


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TABLE 6Weight-grading of silver eels stocked in 1977/78 and graded in the autumn of 1978g 2 5 ~ % of eels 'caught"mm mm mm mm mesh size

16 mm 20 mm 25 mm60-69 3 3 070-79 15 15 080-89 52 52 090-99 81 81 0 0100-109 124 124 0 0110-119 143 143 0 0120-129 175 5 1 181 3 1130-139 176 5 181 3 0140-149 143 30 173 17 0150-159 112 44 156 28 0160-169 63 71 134 53 0170-179 32 70 102 69 0

180-189 11 65 76 86 0190-199 4 80 84 95 0200-209 2 67 1 70 97 1210-219 2 58 60 97 0220-229 1 57 1 59 98 2230-239 2 52 3 57 96 5240-249 1 44 5 50 98 10250-259 59 9 68 100 13260-269 36 15 51 100 29270-279 31 28 59 100 47280-289 14 30 44 68290-299 6 21 27 78300-309 2 44 46 96310-319 1 54 55 98320-329 32 32 100330-339 35 35 100340-349 37 37350-359 26 26360-369 25 25370-379 1 30 31380-389 26 26390-399 12 12400 -409 21 21410-419 17 17420 -429 17 17430-439 11 11440-449 7 7450-459 4 4460-469 4 4470-479 6 6480-489 5 5490-499 3 3500-509 7 7510-519 2 2520-529 4 4530-539 1 1540-549 1 1550-559 0 0560-569 2 2570-579 0 0580-590 4 4Total 1 142 798 551 2 491Sample 551 2 453


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TABLE7Le ng ~ and weight gradmg of smaH yeUow eels in the springof 1979A : L e n g t h - g r a d i n g % o f e e l s ' c a u g h t '

N o . o f e e l s p e r c m - g r o u pm m < l O m m > l O m m T ot a l19 1 1 020 0 0 021 0 0 022 0 0 023 0 0 024 3 3 025 3 3 026 10 10 027 15 15 30 5028 30 45 75 6029 37 134 171 7830 27 179 206 8731 31 283 314 9032 12 432 444 9733 7 417 424 9834 5 476 481 9935 1 313 314 10036 1 387 388 9637 253 253 10038 194 19439 89 89

Total 183 3 217 3 400Sample 183 216 399

B : W e i g h t -g r a d in g % o e e l s ' c a u g h t 'g < l O m m > l O m m T o ta l

5- 9 6 15 21 7110- 14 22 15 37 4115-19 47 104 151 6920-24 70 179 249 7225-29 31 298 329 9130-34 7 417 424 9835-39 417 417 10040- 44 491 49145-49 417 41750-54 402 40255-5 9 209 20960-6 4 149 14965-69 74 7470-74 0 075- 79 15 1580-84 0 085-89 0 090- 94 15 1595-99

Total 183 3 217 3 400Sample 183 216 399


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G R A D I N G O F F A R M E D E E L S B Y M E A N S O F M E S H W I R E B O XE S 2 1

f e m a l e s i lv e r e e ls a p p e a r e d i n t h e 2 0 - 2 5 m m m e s h s i ze g r o u p ; b y f ar th e m o s t f e m a l e s ,b o t h s i lv e r a n d b r o w n e e l s , a p p e a r e d i n th e + 2 5 m m m e s h s e l e c t i o n .

I t is m u c h e a s ie r t o s o r t o u t t h e m a l e a n d t h e f e w s m a l l f e m a l e s il v er e e ls o n c eg r a d e d t h a n t o a t t e m p t t h e o p e r a t i o n o n u n g r a d e d e el s.

F i n a l ly , i n l i n e w i t h g o o d h u s b a n d r y p r a c t i c e a ' f r o m t i m e t o t i m e ' g r a d i n g i n t o s iz eg r o u p s s h o u l d p o s i t i v e l y i n f l u e n c e s u c h th i n g s a s m o r t a l i t y , f e e d c o n v e r s i o n a n dg r o w t h r a t e .

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