Knights 1982 Aquacultural-Engineering

7/28/2019 Knights 1982 Aquacultural-Engineering

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Aquaeultural Engineering 1 (1982) 297-310

B O D Y D I M E N S I O N S O F F A R M E D E E L S ( A N G U I L L A A N G U I L L A L . )I N R E L A T I O N T O C O N D I T I O N F A C T O R , G R A D I N G ,S E X A N D F E E D I N G

B. KNIGHTSSchool o f Engineering and Science, Polytechnic of Central London, London W1M 8JS, UK

ABSTRACTVarious body dimensions were measured in relation to weight in warm-water culturedEuropean eels (0-2- 12 0g ) at 16 and 25C in seawater and at 25C in freshwater toprovide reference standards for use in aquacultural engineering management andresearch. The different conditions had no significant effects on relationships betweenweight and body, head or tail length, or trunk, head or pectoral dimensions. Therewere also no significant differences between small and large eels and no obvious varia-tions were fo un d that could be used to distinguish the sexes. However, the eyes tendto enlarge and the mouth decrease in size relative to weight in larger eels. Possiblerelationships between morphology and feeding are discussed.

The combined data yielded the relationship, log L ( c m )= 1.0306 + 0. 27 log W(g)for eels from elver stage up to about 120 g. A condition factor, CF = L/IO W'2711, isderived and its usefulness in aquaculture and research discussed. Morphometric andgrowth differences between cul tured and wild eels are discussed.

Body girth dimensions are considered and recommendations made for meshapertures and bar spacings fo r basket and grid size-graders.

INTRODUCTIONVar i ous s t ud ie s o f we i gh t / l eng t h r e la t ions h i ps have been ca r r ied ou t on wi l d and pon d-cu l t u r ed Eu r opea n ee l s , e s pec i a ll y wi t h r ega r d t o m e t am o r phos i s and g r ow t h com par i -s ons ( Dec i de r , 1970 ; S i nha and J o nes , 1975 ; Mor i a r t y , 1978) . No ne , how eve r , havei n c lu d e d o t h e r m o r p h o m e t r i c m e a s u r e m e n t s s u c h a s t r u n k g i r th , h e a d a n d m o u t h s iz e,e t c . Such da t a wou l d be u s e f u l f o r A. anguilla und e r i n t ens ive cu l t u r e i n wa r m wa t e r

2 9 7Aquacultural Engineering 0144-8609/82]0001-0153/$02.75 Applied Science Publishers Ltd,England, 1982Printed in Great Britain


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29 8 B. KNIGHTS

and i n r e sea r ch . L eng th /w e igh t r e l a t ionsh ips a r e o f p r ac t ic a l va lue i n g row th s t ud i e sand a s se s smen t o f f i sh cond i t i on , i . e . co rpu l ence r e l a t i ve t o body l eng th . Whi l s t o fgene ra l u se in husb and ry , such know ledge i s a l so o f va lue i n com par ing the con d i t i ono f cu l t u r ed and w i ld - caugh t f i sh , e spec i a l l y when g r ad ing p r i o r t o marke t i ng . Fu r the r -m ore , e el s show c ons ide r ab l e va r i a t i on i n g row th r a t e i n h igh dens i t y cu l t u r e (Usu i ,1 9 7 4 ; F o r r e s t , 1 9 7 6 ; K u h l m a n n a n d K o o p s , 1 9 7 8 ; K u h l m a n n , 1 9 7 9 ; M a t s u i, 1 9 8 0 )wh ich neces s i t a t e s f r equen t s iz e g r ad ing . Ko ops and Kuh lma nn (198 2) r epo r t ed da t afo r t he g r ad ing o f e e l s u s ing va r i ous s i z e s o f squa re mesh , bu t f ew o f t he i r f i sh we rebe low 10 g in we igh t and t he s iz e in t e rva ls f o r t he m eshes we re qu i t e l a rge . I n i n tens ivewarm-wa te r cu l t u r e , f r om e lve r t o marke t s i z e , a ccu ra t e g r ad ing i n to na r row s i zec la sses t o min im i se d i f f e r en t i a l g row th p ro b l em s is des i rab l e f r om the ea r l i es t s t ages .Growth r a t e s i n e e l s be low 10 -15 g a r e po t en t i a l l y ve ry h igh bu t so a r e t he co r r e spond-ing i nh ib i t o ry e f f ec t s o f agg re s s ion by t he f a s t e s t g rowing f i sh (Kn igh t s , unpub l i shedre su l ts ; S eym our , unpub l i shed r e su l ts ) . I t was the r e fo r e an a im o f t h is s t udy t o l o okin more de t a i l a t t he g r ad ing d imens ions o f cu l t u r ed ee l s f r o m e lve r s tage t o 12 0g ,a co m m on m arke t s iz e f o r e e l s f a rm ed in t he UK . In add i t i on , g r ad ing t h rou gh g r idba r s migh t be p r e f e r a b l e i f t he bo dy d imens ions a r e ap p rop r i a t e . Fema le ee l s a r egene ra l l y be li eved t o show f a s t e s t g row th r a t e s and t o con t i nu e g rowing t o a l a rge rs ize . I t i s t he r e fo r e com me rc i a l l y de s i r ab le t o s e l ec t f ema le s a s e a r l y a s pos s ib l e ; how -eve r , t he r e a r e no obv ious s exua l cha r ac t e r i s t i c s t o a l l ow th i s . A de t a i l ed s t udy o fbo dy d imen s iona l r e l a t ionsh ips migh t , i t was hop ed , r evea l d i f f e r ences o f p r ac t i c a ls i gn if i cance . F ina l l y , t he r e migh t a lso be co r r e l a t i ons be twe en t he d im ens ions o f t hef e e d in g a p p a r a t u s a n d s e n s o r y a p p a r a t u s , w h i c h a r e o f i m p o r t a n c e i n f e e d f o r m u l a t i o na n d f e e d i n g t e c h n o l o g y .

MATERIALS AND METHODS

Ee l s we re s t ud i ed i n sp r ing and sum m er 1981 a t Mar ine Fa rm L td , Som er se t , Eng l and ,wh ich is supp l i ed w i th p ow er s t a t i on coo l i ng wa t e r t aken , i n it i a ll y , f r om the Br i s to lChanne l . Sa l i n i ty was abou t 300/0o ; sp r ing and sum m er t emp e ra tu r e s ave r aged 16 and25 C , r e spec t i ve ly . To ch eck t ha t m orp ho m e t r i c r e l a t ionsh ips a r e s imi l a r i n f r e shw a te r ,m e a s u r e m e n t s w e r e a ls o m a d e o n M a r in e F a r m e e ls a f t e r a t le a s t o n e m o n t h ' s a c c l im a -t i on and f eed ing i n f r e shwa te r a t 25 C .

He a l t hy e lve r s and f i sh up t o 120 g si ze we re s e l ec t ed a t r an do m , anaes the t i s ed i nM S 2 2 2 , d r i e d o n a b s o r b e n t p a p e r a n d t h e f o l lo w i n g m e a s u r e m e n t s m a d e .

(1 ) Bo dy we igh t ( t o t he nea r e s t 0 .1 g ).( 2 ) Len g th o f bod y ( t o nea r e s t 0 -1 cm ) , head ( t i p o f nose t o ope rcu l a r s l it ) and

t a il ( anus t o t i p o f ta i l ) .( 3 ) M a x i m u m t r u n k d e p t h a n d w i d t h ( m m ) - m e a s u r e d w i t h ca l li p er s j u s t b e h i n d

t h e v e n t .


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B O D Y D I M E N S IO N S O F F A R M E D E E L S 299(4) Trunk compression depth and width (mm) - eels commonly put their heads

through apertures smaller than their trunk girth during grading but are able towrap their bodies around the basket or grid and pull the trunk through. Con-siderable force can be exerted and the trunk compressed to the extent thatscars are left which remain visible for several days (personal communication,Eels Producers Association, UK). Compression depth and width were measuredjust behind the vent by closing the jaws of callipers as far as possible withstrong finger and thumb pressure.

(5) Pectoral girdle and head dimensions (mm) - callipers were opened and closedon the body and moved back and forth with gentle compression until they justslid over the head and girdle. The width of the head and depth of the girdlewere the larger dimensions in each case as discussed further below.

(6) Horizontal eye diameter (mm).(7) Mouth dimensions (mm) - the length of the upper and lower jaws and un-stretched mouth width were measured with callipers.It was found that only relatively small numbers of fish (25-50) were needed for

each series of measurements because of their uniformity, as shown by the high correla-tion coefficients and narrow confidence limits obtained (Table 1).

T A B L E 1R e l a t i o n s h i p s b e t w e e n b o d y w e i g h t W a n d l e n g t h L f o r c u l t u r e d A. anguilla

Nu mb er log k b b +- 95% Correlationof samples confidence eoeffieient,limits r

L = kW bF a r m e e ls ( 1 6 C ) 5 0 1 - 0 3 6 9 0 - 2 6 0 7 0 - 0 0 6 2 0 . 9 5 2 6F a r m e e ls ( 2 5 C ) 5 0 1 - 0 1 9 5 0 . 2 7 7 9 0 - 0 0 5 8 0 . 9 7 3 2Laboratory eels (25C) 25 1.0231 0-2779 0-0036 0.9941Combined data 125 1-0306 0-2711 0.0051 0-9848

W = k L bC o m b i n e d d a t a 1 2 5 - 3 . 7 3 3 8 3 . 6 3 2 5 0 - 0 1 9 9 0 - 9 8 2 1

R E S U L T S

The results are expressed relative to body weight rather than length, except wherestated, because this is an easier and more meaningful parameter to measure in aqua-culture. Fish, in the present study, had either been starved for 48 h or body weightwas calculated after dissection and removal of gut contents to exclude variations inweight due to recent food intake.


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30 0 B. KNIGHTSGr aphs o f va r i ous li nea r d i m ens i ons aga ins t we i gh t t en ded t o s how p l a t eaux and

gr ea t e r va r ia t i on above ab ou t 10 g (i .e . abou t 20 - 2 2 cm ) , s ugges ting t ha t g r ow t h i sexp on ent ia l wi th progress ive s lowing abov e th i s s ize as sexual d i f fe ren t i a t ion occu rs( S i nha and J ones , 1975 ; B i en i a r z e t a l . , 1981) . Kn i gh t s ( 1983) a l s o f ound changes i nf eed i ng behav i ou r , m o t i va t i on and pa r t i c le s iz e p r e f e r ences above 1 0 - 1 5g . S uchexponen t i a l r e l a t i ons can be s um m ar i s ed by t he equa t i on :

y = kx t~o r , i n loga r i t hm i c f o r m ,

log y = log k + b log xPlo t t ing log x agains t lo g y prod uce s a s t ra ight l ine o f s lope b wi th k as a con s tant an dt he i n t e r cep t on t he y - ax i s . M eas u r em en t s f r om t he p r e s en t s t udy a r e p r e s en t ed i n th i sf o r m . S t a t is t ic a l com par i s ons o f r e l a ti ons h i ps f o r e els f r o m t he d i f f e r en t m ed i a andt em per a t u r e s r evea led no s ign i fi c an t d i f f e r ences , and com p ar i s ons o f e els above andbe l ow 10 g s howed s i gn i fi c ant d i f f e r ences on l y f o r m ou t h d i m en s i on / bo dy we i gh tr e l a ti ons h i ps . A p l o t o f bo dy l eng t h ve r sus bo dy we i gh t f o r ee ls above and be l ow10 g is given in Fig. 1.

Fig. l.

4 0 " o

35 * ~..3 0 " .

. c 1 0 2 0 4 0 7 0 1 2 0~2o y ~ . ~"~ 1 %

15

1 0

6 i i , , r i i l l l E i i t I~ l l l1 2 5 1 2 4 10W e i g h t g

Bo dy length (cm) plo t ted against weight (g) for combined data for eels (a) above and (b)below 10 g.


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BODY DIMENSIONS OF FARMED EELS 301DISCUSSION

We i g h t / l e n g t h r e l a t i o n s h i p s a n d c o n d i t i o n f a c t o r sI f g r o w t h is p e r f e c t l y i s o m e t r i c , v o l u m e ( a n d h e n c e w e i g h t , W ) w o u l d i n c re a s e asa c u b e f u n c t i o n a n d l i n e a r d i m e n s i o n s ( L ) a s a u n i t a r y f u n c t i o n , so t h a t t h e e x p o n e n -

t ia l e q u a t i o n w o u l d r e a d :W = k L 3 o r L = k W '33

Subs t i t u t i ng p r e se n t da t a i n to t he l a t t e r equa t i on y i e ld s b va lue s be tw een 0 .26 and0-28 (Table 1) .

No s i gn if i can t d i f f e r ences we re found be tw een t he d i f f e r en t cond i t i ons and s izes( P < 0 - 0 1 ) , s u gg e st in g t h a t E u r o p e a n e e ls f r o m e lv e r s ta g e u p to 1 2 0 g s h o u l d s ho wc o m p a r a b l e m o r p h o m e t r i c a n d g r o w t h p a t t e r n s i n v a r i o u s c u l t u re s i t u a t io n s i f w e l l f e du n d e r o p t i m u m c o n d i t io n s . R e s u l ts c e r t a in l y a g re e w e l l w i t h t h o s e o f E g u s a ( 1 9 7 0 ) f o rA. anguilla i n f r e shw a te r cu l t u r e i n J apan and t hose o f B i en i a rz et a l . ( 1 9 8 1 ) f o raqua r ium - ra i s ed ee ls . Thus , i t i s j u s t i f i ab l e t o com bine t he p r e sen t r e su l t s to g ive:

L - - 10 .73 W 0"2711Reca l cu l a t i ng t he com bined da t a t o y i e ld t he W = k L b r e l a t i onsh ip g ives b = 3 -6325and l og k = - - 3 . 7338 . I n con t r a s t , S inha and Jo nes (1 967 ) f o und t ha t va lue s o f b va r i edb e t w e e n 3 . 1 9 2 0 a n d 3 . 3 7 6 0 i n v a r io u s W els h p o p u l a t i o n s o f w i ld , u n d i f fe r e n t i a t e d a n dm a l e A. anguilla o f 1 - 8 0 g . F e m a l e s ( 2 5 - 6 5 0 g ) g a v e b v a lu e s b e t w e e n 3 . 0 2 3 8 a n d3 .150 4 ( s ee T ab l e 3 l a t e r) . These r e su l t s suggest t h a t g row th i n Eu rope an ee l s is no tp e r f e c t l y i s o m e t r ic ( e x c e p t p e r h a p s i n w i ld f e m a l e s ) a n d t h a t t h e y t e n d t o i n c re a s e i nw e i g h t p r o p o r t i o n a t e l y f a s t e r t h a n t h e y i n c r e a s e i n l e n g t h . T h i s i s p r e s u m a b l y d u eto p ro po r t i o na t e ly f a s t e r i nc r ea se s i n bo dy g i r t h a s f a t r e se rves a r e l a id dow n du r ingg r o w t h ( M c C a n c e , 1 9 4 4 ; P i a t ek , 1 9 7 9 ) , as b o r n e o u t b y t h e f a c t t h a t t r u n k w i d t hand dep th r e l a ti ve t o bo dy we igh t y i e ld b va lue s o f 0 .4033 and 0 -47 29 , r e spec t i ve ly( see Tab l e 4 l a t e r ) . S t eepe r W /L r e l a t i onsh ips i n cu l t u r ed and aqua r ium ee l s migh tb e e x p e c t e d b e c a u s e o f t h e m o r e c o n s t a n t t e m p e r a t u r e a n d f e e d a v a il a b il it y . C l o s erc o m p a r i s o n w i t h t h e r e s u lt s o f S i n h a a n d J o n e s ( 1 9 6 7 ) s h o w s , h o w e v e r , t h a t t h esma l l e r w i ld ee ls t end t o be heav i e r i n re l a t i on t o t he i r l eng th t han f a rm ed ee ls ; t hed i f f e r ence dec r ea s ing un t i l com para b i l i t y is r e ached a t ab ou t 100 g . Th i s imp l i e st h a t t h e m o r e c o n s t a n t g r o w t h c o n d i t io n s o n f a r m s en c o u r a g e i n c re a s e in l e n g thr a t h e r t h a n w e i g h t u p t o p u b e r t y . W ild ee ls m i g h t b e e x p e c t e d t o a c c u m u l a t e la r g erf a t r e se rves ea r li e r i n l i f e a s an ad ap t a t i o n t o m ore va r i ab le f ood ava i l ab i li t y and ,e s p e c i a l l y , t h e l o w e r t e m p e r a t u r e o v e r - w i n t e r i n g p e r i o d w h e n t h e y d o n o t f e e d(Dee lde r , 1970 ; S inha and Jones , 1975 ; M or i a r t y , 1978 ) . Fa t a c cum ula t i on i n cu l t u r edee ls ma y a l so be d i s cou raged by t he e n fo r c ed ac t i v i t y and s tr e s se s i nhe ren t i n f a rmtanks , b u t i t is m ore l i ke ly t ha t e a r l y g row th i s a cce l e r a t ed and m a jo r f a t r e s e rves a r eno t l a id dow n un t i l s exua l ma tu r i t y i s r e ached . Th i s is im po r t an t because a h igh f a tc o n t e n t is r e q u i re d i n e el s d e st in e d f o r s m o k i n g o n th e C o n t i n e n t , t h e m a i n m a r k e t


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302 B. KNIGHTSfor large British eels (personal communication, Eel Producers Association, UK).Further research is needed before these conclusions can be extrapolated to suggestthat the fat content of diets need not be high in the earlier growth stages.

During stress or starvation, body weight would be expected to decrease morerapidly than length as energy reserves are utilised (Dave e t a l . , 1975). It would beuseful to be able to calculate a 'condition factor' as a measure of corpulence relativeto body size for use in warm water aquaculture and research. The condition factor,k = W I O O O / L 3 , has been commonly used for eels (Koops, 1967; Sinha and Jones,1967; Nose and Arai, 1972; Peters e t a l . , 1980) based on the assumption that growthis perfectly isometric (Gerking, 1978). As pointed out above, this assumption is wrongand k varies from unity, thus making comparisons between studies difficult; similarproblems are experienced when applying such formulae to other species (Weatherley,1972; Gerking, 1978). A more useful but simple condition factor based upon an 'ideal'cultured population can be derived from the present s tudy using the equation:

L = k W bThis converts to:

k = L / W ~and problems of variation in W due to food in the gut can be obviated by short-termstarvation. Substitution of relevant values gives

k = L / W '2 71 1 = log 1-0306= 10.73

for all sizes of eels, where k is now the condition factor. Redesignating the conditionfactor as CF to avoid confusion, dividing by 10 to give values closer to unity andsimplifying b to 0.27 gives:

LC F - - - 1.081014/0.27Values greater than 1.08 indicate a fall in corpulence from the expected ideal in rela-tion to body length, smaller values the converse. Table 2 includes data from variousstudies on A . a n g u i l l a and the derived CF values indicate that this condition factorequation is quite sensitive for use in aquaculture and research: in each case, poorcondition is indicated by CF values of 1-10-1.38. The results for effects of social stressappear equivocal but the initial condition in each experiment was not clearly stated byPeters e t a l . (1980).

Application of the CF equation to wild eel populations is also useful, as shown inTable 3 utilising the data of Sinha and Jones (1967). Again CF values show that thesewild eels are heavier relative to their length than cultured ones until sexual maturi ty.CF values are also useful for interpopulation comparisons; Sinha and Jones (1967)concluded that smaller eels in the River Rhyd-hir grew more slowly than those in the


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BODY DIMENSIONS OF FARMED EELS 30 3TABLE 2Data showing changes in mean weight, length and CF (= L / I O lq '~ 7 ) for A. angui l la under variousstarvation or stress regimes

Souree~f W (g) L (em ) C FCombi ned data, presen t study 4-32 16.0 1-08Elvers: size at capture and critical Originalsize before death in non-feeders 6-14Cafter 3-6 months 25CElvers: size at capture and after Original83 days without food 8-16C15-21C

0.21 7.2 1.100-09 7-1 1.360-09 7-2 1.380-20 7-0 1-080.17 6.8 1-100-14 6-6 1.12

Fingerlings: before and after Before 12.1 21.2 1.082 mont hs without feeding at After 10.0 21-0 1.136-14C

d Fingerlings: recovery of weight of Before 13-6 22-2 1-10poor stocks in ponds over After 19-1 23.7 1.072 months at 9-16CYellow eels: unfed for 4-8 weeksat 22C in social dominance/stress expe riments

Controls 102-9 42-7 1.22Dominant eels 109-4 43-5 1.22Subordinate eels 81.7 39.7 1.21t Sources: a, Strubberg (1913); b and c, Knights (unpublished results); d, Koops (1967); e, Peterse t a l . (1980).

TABLE 3Applicability of the condition factor equation, CF = L / l O W '2 7 to wild A. angui l la using datameans of Sinha and Jones (1967)C F v a lue s a t v a r ious age s ( i n y e ar s ):

M a le s and und i f f e re n t ia t e d Age : 0 1 2 3 4 5 6eels:From the River Ffraw 0.93 0-98 1-00 1.01 1-02 0.99 1-05(W = 5.4 10-4L 3'376)From the River Rhyd-hir 0.85 0.95 0.93 0-96 0-99 1.00 1-02(W = 10.8 10-4L 3"a92)

Fem ale ee ls : Age : 3 4 5 6 7 8 9From the River Ffraw 1.02 1.03 1-05 1.06 1.08 1-09 1.12(W= 13.8X10 4L3"~s4)From the River Rhyd-hir 1-09 1.09 1-06 1.04 1.11 1.13 1.18(W = 18.5 x 10-4L 2"9916)

7

1 - 0 2

101-13 111.13


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304 B . KNmUTSR i v e r F f r a w , a h y p o t h e s i s s u p p o r t e d b y t h e s m a l le r C F v a lu e s f o r t h e f o r m e r u p t oa b o u t y e a r 5 .

W e ig h t / l e n g th r e la t i o n s h ip s a r id s e xThe wi ld s t ocks s t ud i ed by S inha and Jones (1967 ) showed d i f f e r en t r e l a t i onsh ipsf o r f e m a l e a m d u n d i f f e r e n t i a t e d / m a l e o n e s ( T a b le 3 ) . F e m a l e s w e r e g en e r a ll y c lo s e r t othe i dea l i some t r i c and C F r e l a t i onsh ips bu t s ex cou ld no t be de t e rm ined gonada l l yu n t i l m a t u r a t i o n h a d b e g u n . B i e n i a r z e t a l . ( 198 1 ) u sed add i t i ona l ka ryo log i ca l de t e r -min a t i ons on t he i r a qua r ium - rea r ed ee ls f r om 0 -5 g upw ards . Ca l cu l a t i ons o f the i r da t ashow tha t L = 11 .2722W '2603 fo r ma l e s and L = 10 .5463 W 0" 2798 fo r f ema le s ,com pare d w i th L = 10 .7300W '2711 fo r t he p r e sen t com bine d da t a . Th i s suggest s am ore f e m a le - t ype r e l a t i onsh ip i n t he f a rm ee ls wh ich cou ld be due t o a f a s t e r g row thra t e i n f ema le s and subsequ en t d i f f e r en t i a l s e l ec t ion by f a rm g rad ing on the ba s i s o fs i ze a lone . Howeve r , B i en i a r z e t a l . ( 198 0 ) s exed on ly 48 ee l s , g iv ing w ide con f ide ncel im i t s a n d l o w c o r r e la t i o n c o e f f ic i e n t s ( 0 - 2 7 7 9 / 0 . 2 5 7 6 a n d 0 - 8 0 3 2 / 0 - 8 6 3 5 f o r m a l e s/f e m a l e s , r e s p e c t i v e l y ) . G r o w t h a p p e a r e d t o b e r e l a t i v e l y s l o w , a n d m e a n i n g f u l c o m -pa r i son w i th p r e sen t da t a is no t pos s ib le . How eve r , cond i t i on f ac to r s we re nea rop t im a l , the m ean CFs be ing 1 .10 fo r ma l e s and 1 -09 fo r f ema le s . The po s s ib i li t y o fa b i a s t owards s e l ec t i ng f a s t e r -g rowing f ema le s du r ing g r ad ing i s d i s cus sed fu r t he rb e l o w .B o d y d e p t h s a n d w i d t h s i n r e l a ti o n t o s i z e g r a d i n g

Figu re s 2 and 3 show tha t t r unk dep th i s a lways g r ea t e r t han w id th and is t hel a rges t c ro s s - s ec t i ona l d im ens ion i n a l l e e ls . The t r u nk i s com pres s ib l e , how eve r , and

Fig. 2.

z20~-1 5 :2

Eg 5gE 3

7D 2

.6

/A , -

/ / " //

i t I I l ~ , l E t I I I I I I I I II I.5 1 4 10 40 120W e i g h t g .

Bod y de p th d im e ns ions ( m m ) p lo t t e d a ga ins t we igh t t r unk de p th ( so l id l i ne ), t r unkc om pr e ss ion de p th ( da she d l i ne ) a nd pe c to r a l g i r d le de p th ( do t t e d l i ne ).


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B ODY DIMENSIONS OF FA R ME D EELS 305

I:ig. 3.

20 :1 5 :

10 [

g 5

2. . - / /

7"'*" J

/

' " ' ; ' " ~ ' . . . . 1 ' o ' 2 0 . . . .12 0W e i g h t g .Bo dy w idth dim ensions (mm ) plotted against weight; trunk w idt h (solid line), trunkcompression width (dashed line) and head width (dotted line).

e e ls show a ma rked ab i l i t y t o f o r ce t hem se lves t h rou gh sm a l l ape r tu r e s , a ided bycop io us m uco us s ec r e t i on ; la rge ones can exe r t e noug h fo r ce t o b r a i s e t hemse lves andd i s t o r t g r ad ing meshes and ba r s . The re fo r e , t r un k com pres s ion de p th i s t he sma l l e s td imen s ion t ha t c an l im i t pa s sage t h rou gh a me sh ape r tu r e w hen g r ad ing ee l s, pa r t i cu -l a r ly t ho se abov e 2 0 -25 g (F ig . 2 ) . B e low th is we igh t , head w id th i s s l igh t ly la rge r andwi l l be more c r i t i c a l (F ig . 3 ) . Pec to r a l g i rd l e dep th migh t appea r t o be more l im i t i ngin sm a l le r e e l s (F ig . 2 ) bu t t h i s is no t t he ca se because t he g i rd l e p ivo t s back wa rds andu p w a r d s t o a l lo w t h e b o d y t o p a s s f o r w a r d s t h r o u g h a n a p e r t u r e ; t h e d e p t h s s h o w n a r et h o s e f o u n d i m p o r t a n t w h e n t h e b o d y m o v e s in t h e o p p o s i t e d i r ec t i o n a n d t h e g ir dl ep i v o t s f o r w a r d s a n d d o w n w a r d s . T h e s a m e m o v e m e n t o c c u r s d u r i n g o r o b r a n c h i a le x p a n s i o n w h e n s u c t i o n f e e d i n g ( A l e x a n d e r , 1 9 7 0 ; K n i g h t s , 1 9 8 3 ) a n d w h e n t h ecav i t y is fi l led w i th a i r t o supp l em en t r e sp i r a t i on , e spec i a l ly on exp osu re f rom wa te r .Th i s o n ly c r ea t e s p rob l em s , a s i t o f t en does du r ing g r ad ing (pe r so na l obse rva t i on ) , i f anee l f o r ce s i ts head t h rou gh a gap and t he n t r i e s t o pu l l i t s e l f ou t b ackw ards . The ope r -cu lum i s no t a c ause o f e e l s ge t t i ng s t uck because t he w a ll s a r e on ly s t i f f ened by l ongb ranch io s t ega l r ays r a the r t h an cons i s ti ng o f ri g id bon y f l aps .

C u r v e ( a ) in F i g . 4 s h o w s m e s h a p e r t u r e p l o t t e d a g a i n st th e m e a n m i n i m u m s iz e o ff i sh t ha t w i ll be r e t a ined , ba sed o n head w id th and t r un k com pres s ion dep th . C i r cu la rmesh es a r e p rob ab ly be t t e r f o r g r ad ing ee ls be low 3 -5 g whe re head w id th i s r e l a t ive lyl a rge and l im i t ing ; above t h i s s i z e , e e ls c an t oo ea s i ly push t he i r heads t h roug h andthen ge t s t uck a s t he t r u nk t h i ckens beh ind t he ve n t . Us ing squa re meshes a s sugges t edb y K o o p s a n d K u h l m a n n ( 1 9 8 2 ) , e e l s c a n t w i s t t h e i r b o d i e s a n d u t i l i s e t h e l a r g e rc o r n e r - t o - c o r n e r d i m e n s i o n b u t e v e n t h e n m a n y g e t s tu c k a t t h e v e n t , a s w e l l a s a t t h epec to r a l g i rd l e , du r ing g r ad ing (pe r sona l obse rva t i on ) .


10/14

306 B. KNIGHTS2O15 ,(a), ( b )10 , , , " "

E , J5 / / ' "

3 . : ' f / /2 . J /

6 t ~ , L , i L i ~ i i ~ i i i i H1 .5 1 4 10 40 120W e i g h t g

Fig. 4. Critical grading dimensions (ram) plotted against body weight; (a) showing meanminimum size of eel retained by appropriate mesh apertures, (b) size retained by grid bar spacing.

Using grids for grading, the head can be pushed between the bars in the smallerdorso-ventral plane and then the trunk can be twisted to present the smaller of thetrunk compression dimensions, i.e. the width. Curve (b) in Fig. 4 shows bar spacingsplotted against mean minimum size of eel that will be retained, based on trunk com-pression widths from Fig. 3 for all sizes. Grids and meshes need to be rigid, especiallyfor larger eels which are capable of exerting considerable force, to minimise distortionand jamming. The plotted data in Fig. 4 agrees well with that determined empiricallyunder farm conditions for both A. anguilla (personal communication, Eel ProducersAssociation, UK) and A. japonica (Usui, 1974). Similar maximum weights of eels wereheld back by the comparable square mesh graders (10 and 16 mm mesh) used byKoops and Kuhlmann (1982). It must be noted, however, that their eels were some12-17% longer than those of the same weight in the present study, suggesting thatthey were in relatively poorer condition. Koops and Kuhlmann used wild-caught eels,subsequently kept in ponds, but no information on pre-experimental care or feedingis given. Condition factor values for their eels of the relevant size can be calculatedto be about 1-22-1 .32 , confirming that they were relatively emaciated, these valuesbeing similar to the poorest condition eels shown in Table 2.

From studies of agonistic behaviour, density, growth and mortality (Knights,unpublished results), aggressive behaviour is first seen in elvers at about 0.8-1.0 gand growth and survival of smaller fish are inhibited. The initial grading should occuras individuals reach this weight using a 2.5 mm mesh or grid of I-5 mm bar spacingto retain those above approximately 0.7 g. As size hierarchies develop further, growthof fingerlings below 1-0-1-5 g appears to be inhibited by those above 3 g; the latter


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B O D Y D I M E N S I O N S O F F A R M E D E E L S 3 0 7wi ll be r e t a ined by 4 m m m eshes o r 3 m m ba r spac ings. Fu r the r s t ud i e s a r e r equ i r edbe fo re sub sequen t c r i t i c a l g r ad ing s iz e s can be r ecom m end ed .

Head and ta i l l engthTa i l l eng th r educes du r ing m e tam orph os i s f r o m l ep to cepha lus t o e lve r s t age (Fo rd ,1931) bu t i n subsequen t s t ages i n t he p r e sen t s t udy i t r ema ined co ns t an t a t abo u t5 9% o f t o t a l b o d y l e n g th . H e a d l e n g th w a s a ls o a l m o s t c o n s t a n t a t 1 2% o f b o d y l e n g th .L)~e d iam e te r

The eye i s know n to en l a rge and show changes i n r e f r ac t i ve pow er and r e t ina lp igm en t s a t m a tu r a t i on and m e tam orp ho s i s t o t he s ilver m ig ra to ry s tage (S inha andJones , 1975 ) . I t bec om es a m ore e f f i c i en t l igh t co l l e c to r and t he s ens i t i v it y sh i f tst ow ards t he b lue end o f t he spec t ru m a s is app rop r i a t e f o r l if e i n deepe r ocean i cwa te r s du r ing t he spawn ing m ig ra t i on . On ly t h r ee eel s ou t o f 30 exam ined i n t hep r e s e n t s t u d y s h o w e d a n y m a r k e d e n l a r g e m e n t a n d d i v e r g e n c e f r o m t h e a l l o m e t r i cr e l a t ionsh ip g iven i n Tab l e 4 . Kn igh t s ( 198 3 ) f oun d t ha t ye l l ow ee ls show a g r ea t e rr e li a nc e o n v is u al l o c a t i o n o f f o o d a s b o d y w e i g h t e x c e e d s 1 0 - 1 5 g b u t n o i n f o r m a t i o nwas ga ined f rom the p r e sen t s t udy t o he lp exp l a in t h i s .M o u t h d i m e n s i o ns

M outh w id th i s d i f f i cu lt t o m easu re accu ra t e ly w i thou t s t r e tch ing the j aws apa r tb u t t h i s d i m e n s i o n a n d j a w l e n g t h s w e r e t h e o n l y p a r a m e t e r s i n t h e p r e s e n t s t u d y t oshow s ign i fi c an t ly d i f f e r en t r e l a t ionsh ips t o we igh t f o r e el s abov e and be low 10 g(Tab l e 4 and F ig . 5 ) . R e l a t i onsh ips we re a lmos t i some t r i c up t o 10 g , t he l ow er j awa l w a y s b e i n g l o n g e r t h a n t h e u p p e r , a p o s si b le a d a p t a t i o n f o r p i ck i n g u p f o o d f r o m t h eb o t t o m . A b o v e 1 0 g , r e la t io n s h i p s w e r e m u c h m o r e v a ri a b le (a s s h o w n b y l o w e r c o r r e -l a t i on coe f f i c i en t s and w ide r con f idence l im i t s ) w i th a gene ra l t endency fo r a f a l l i nthe r a t e o f i nc rea se in j aw l eng th and m ou th w id th r e l a ti ve t o we igh t .

C o m p a r i s o ns o f m o u t h w i d t h d a t a w i t h th a t o f K n i g h ts ( 1 9 8 3 ) f o r th e m o s tpref er red foo d pa r t ic le s ize in ee l s shows tha t a 1 :1 re la t ionsh ip ex is t s . Ha rd andab ra s ive pa r t i c l e s such a s g r anu l e s o r pe l l e ts appea r t o be m ore d i f f icu l t t o i nges t , how -e v e r, a n d o p t i m u m d i a m e t e r s fo r th e s e a re a b o u t 0 . 4 - 0 - 6 o f m o u t h w i d t h .

J a w l e n g th s s h o w s h a l l o w e r g r a d i e n ts t h a n m o u t h w i d t h i n F i g . 5 , t h o u g h t h i s e f f e c ti s on ly r ea l l y marked fo r e e l s above 10 g . Th i s t ends t o make t he head appea r b roade r ,an e f f ec t more p ronounced i n some ind iv idua l s t han o the r s . These de sc r i p t i ons ag r eewi th t hose o f t e n app l i ed t o w i ld eel s , i .e . so - ca l led 'b roa d -no sed ' and ' na r row -nos ed 'e e ls . Ma t su i ( 198 0 ) c l a ims t h a t b road -no sed ee ls a r e m ore ac t i ve , t end t o ea t f i shr a the r t han i nve r t eb r a t e s , g row f a s t e r and t o a l a rge r s i ze , and , a t l e a st in i t ia l ly , have alower f a t c on t e n t . I n h i s op in ion , such ee l s a r e fem a le s and can be d i s ti ngu i shed a t t hee lve r s t age . These conc lu s ions may app ly t o A. j apon ica b u t T o d d ( 1 9 8 1 ) f o u n d th a tt h e s n o u t o n l y b e c a m e ' c h is e l -s h a p e d ' a t m a t u r i t y a n d m e t a m o r p h o s i s i n A. australisa n d A. d ie f fen bac hi i ; A . anguil la i n t he p r e sen t s t ud y show ed l ow va r i ab i l it y un t i l


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3 0 8 B . K N I G H T ST A B L E 4Re l a t i o n s h i p s b e t w e e n b o d y w e i g h t a n d v a r i o u s l i n e a r d i me n s i o n s f o r c u l t u r e d A. angui l la

L i n e a r b o d y d i m e n s i o n N u m b e r l o g k b b 9 5 % C o r r e la t i on{ m m ) o f c o n f i d e n c e c o e f f i c i en t ,s a m p l e s l i m i t s rT r u n k d e p t h 5 5 0 . 5 8 2 8 0 - 3 8 7 2 0 . 0 0 6 3 0 . 9 5 0 0T r u n k c o m p r e s s i o n d e p t h 2 6 0 . 3 5 4 0 0 - 4 2 5 0 0 . 0 0 2 3 0 - 9 8 0 6Pe c t o r a l d e p t h 2 7 0 - 3 7 7 6 0 - 3 7 4 7 0 - 0 1 2 7 0 . 9 8 8 4T r u n k w i d t h 5 5 0 - 4 5 8 2 0 - 4 1 3 9 0 . 0 1 2 6 0 - 9 5 9 3T r u n k c o m p r e s s i o n w i d t h 2 6 0 - 3 0 0 9 0 - 41 0 1 0 . 0 0 6 9 0 . 9 8 4 9H e a d w i d t h 2 7 0 . 4 7 4 8 0 - 3 2 4 6 0 . 0 0 6 2 0 . 9 8 8 3M o u t h w i d th :F i s h b e l o w 1 0 g 2 7 0 - 2 0 7 1 0 . 3 3 6 5 0 . 0 1 1 3 0 . 8 9 4 3

F i s h a b o v e 1 0 g 3 3 0 . 3 1 8 0 0 - 2 7 0 0 0 . 1 3 3 6 0 . 5 7 1 8U p p e r j a w :F i s h b e l o w 1 0 g 2 7 0 - 5 0 0 7 0 . 3 1 4 0 0 . 0 0 9 9 0 . 9 5 0 8F i s h a b o v e 1 0 g 3 3 0 . 6 6 6 1 0 . 1 9 3 4 0 . 5 9 6 3 0 - 6 3 9 4L o w e r j a w :F i s h b e l o w 1 0 g 2 7 0 . 5 6 6 8 0 - 3 1 4 0 0 . 0 1 0 1 0 . 9 5 0 8F i s h a b o v e 1 0 g 3 3 0 . 6 7 7 5 0 - 2 2 2 5 0 . 5 9 2 5 0 - 6 9 5 8E y e d i a m e t e r 3 0 0 . 4 2 1 0 0 . 2 3 0 7 0 . 0 0 9 6 0 - 9 1 0 0

Eg 5 # / "E 3 ~ , . /

c 1 5

' ' ' ' 5 ' " 1 ' ' , 4 ' ' 1 0 ' 4 ' 0 ' " 1 2 0W e i g h t g

Fi g . 5 . M o u t h d i m e n s i o n s ( mm ) p l o t t e d a g a i n s t b o d y w e i g h t ; m o u t h w i d t h (s o l id l i n e ) , l e n g t h o fl o w e r j a w ( d a s h e d l i n e ) a n d u p p e r j a w ( d o t t e d l i n e ). ( Re l a t i o n s h i p s f o r e e l s b e l o w a n d a b o v e 1 0 ga r e p l o t t e d s e p a r a t e ly a s e x p l a i n e d i n t h e t e x t . )


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B O D Y D I M E N S I O N S O F F A R M E D E E L S 3 0 9g r o w t h w a s w e l l a d v a n c e d . I t i s p o s s i b l e t h a t f a s t- g r o w i n g fe m a l e s h a d b e e n p r e s e l e c t e db y s iz e g r a d i n g . I n t h e f a r m e e l s s t u d i e d , e l v e rs a n d s m a l l fi n g e r l in g s w e r e t a k e n f r o md i f f e r e n t t a n k s b e f o r e a n d a f t e r g ra d i n g a n d s o m e m a l e s s h o u l d h a v e b e e n p re s e n t .M o u t h s h a p e d o e s n o t t h e r e f o r e a p p e a r t o b e a re l i a b le m e a n s o f d i s ti n g u i s h in g s e xin A. anguilla a t a n e a r l y s t a g e e v e n i f i t w e r e p o s s i b l e t o d e v i s e a f e a s i b l e m e t h o d o fs e l e c t i o n b a s e d o n s u c h c h a r a c t e r i s t i c s f o r u s e in l a r g e s c a le c u l t u r e .Sex selection

T h e d a t a f r o m t h e p r e s e n t s t u d y s u p p o r t t h e c o n c l u s i o n s o f D e e l d e r ( 1 9 7 0 ) , S i n h aa n d J o n e s ( 1 9 7 5 ) , M o r i a r t y ( 1 9 7 8 ) a n d o t h e r s t h a t t h e r e a r e n o o b v i o u s e x t e r n a ls e x u a l d i m o r p h i c c h a r a c t e r i s t ic s in A. anguilla. S e x s e l e c t i o n i n a q u a c u l t u r e c a np r o b a b l y o n l y b e r e l i a b l y m a d e o n t h e b a s is o f s iz e ( K o o p s a n d K u h l m a n n , 1 9 8 2 )w h i c h is in t u r n r e l a t e d t o g r o w t h r a t e . M o r e i n f o r m a t i o n o n t h e l a t t e r is n e e d e d t oq u a n t i f y g r o w t h r a t e s o f t h e s e x e s i n w a r m w a t e r a q u a c u l t u r e a n d t o d e v i se g r a d i n gr e g i m e s w h i c h w i ll o p t i m i s e y i e l d s w h i l s t m i n i m i s i n g t h e s tr e ss e s i n h e r e n t i n f r e q u e n tg r a d i n g . I n f o r m a t i o n i s a l so n e e d e d o n p o s s i b le g r o w t h i n h i b i t o r y e f f e c ts a t d i f f e r e n td e n s i t ie s a n d p o p u l a t i o n s i z e - fr e q u e n c y d i s t r i b u t i o n s , a n d o n t h e p o s s ib l e g o n a d a l s e xr e v e r s i o n s n o t e d a t h i g h d e n s i t i e s in t h e w i l d a n d i n e x t e n s i v e c u l t u r e b y P a s s a k a s a n dT e s c h ( 1 9 8 0 ) , R o s s i a n d V i ll a n i ( 1 9 8 0 ) a n d o t h e r w o r k e r s .

A C K N O W L E D G E M E N T S

T h e a u t h o r w o u l d l ik e t o t h a n k M a r i n e F a r m L t d f o r s u p p l y i n g f is h a n d u s e o f o n -s i tef a c i li t ie s a n d m e m b e r s o f t h e E e l P r o d u c e r s A s s o c i a t i o n , U K ( B l u e C i rc l e I n d u s t r ie s ,E a s g a n F i s h e r i e s, M a r in e F a r m , R H M R e s e a r c h a n d S t r a t h d e a r n F i s h e r ie s L t d ) f o rf i n a n c ia l s u p p o r t . T h a n k s a r e d u e t o E . A . S e y m o u r f o r h e l p f u l d is c u s s io n o f w o r k a n dm a n u s c r i p t s .

R E F E R E N C E S

Alexander, R. McN. (1970). Mechanics of the feeding of various teleost f ishes. J. Zool. (Lond.),1 6 2 , 1 4 5 - 5 6 .Bieniarz, K., Epler, P., Malczewski, B. & Passakas, T. (1981). Development of Eu ropea n eel(Anguilla anguilla L.) gonads in ar t i f icial condi t ions . Aquaculture, 22, 53-66.Dave, G., Johansson-Sj6beck, M.-L., Larsson, A ., Lewander, K. & Lid m an , L. (1975). M etaboficand haem atological ef fects of s tarvation in the European eel , Anguilla anguilla L. - I . Carbo -hyd rate, l ipid protein and inorganic ion m etabol ism. Comp. Biochem. Physiol., 52A, 423-30 .Deelder , C. L. (1970). Synopsis of biological data on the eel Anguilla anguilla (Linnaeus) 1758.Fisheries Synopsis No. 80, F A O , R o m e .Egusa, S. (1970). No tes on sex and g row th of Euro pean eels in freshwa ter eel-rearing pond s.Bull. Jap. Soc. Sci. Fish., 36, 1224-5.Ford, E. (1931) . Changes in length dur ing the larval l i fe and metamorphosis of the f reshwatereel (Anguilla vulgarisTurt . ) . J. mar. biol. Assoc. UK, 17, 987-1000.


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Knights 1982 Aquacultural-Engineering

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