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Aqua cultural Engineering6 (1987) 259-275

A utom at ic C ontro l o f Foo d S upply in the C ul ture o fFi l ter- feeding O rgan ismsW i l l i a m . L . Huggins

Ministry of Agriculture, Fisheries and Food, Directorate of Fisheries Research,Fisheries Laboratory, Lowestoft, Suffolk NR33 OHT, UK

M i c h a e l M . H e l m and Dennis R . W i l l i a m sMinistry of Agriculture, Fisheries and Food, Directorate of Fisheries Research,Fisheries Laboratory, Benarth Road, Conwy, Gwynedd LI_32 8UB, UK

A B S T R A C TThe d esig n, construction an d operation o f a laboratory apparatus isdescribed, wh ich opto-electronically controls fo o d density w ithin th e opti-m um range necessary to pro mo te the m axim um growth o f fi lt er-feedingorganisms. This apparatus, ter m ed an algal cell mo nitor, incorporates aninfra-red transmitter and photo-receiver with circuitry to permit stableoperation over a wide range of cul ture condi tions. Ap plicat ion o f theapparatus in controlling the density o f microalgal fo o d species in the high-density rear ing o f oyster larvae, together w ith cost benefits o f the tech-nique, are discussed.

INTRODUCTIONO f g r e a t i m p o r t a n c e i n th e c u l t u r e o f f il t e r -f e e d i n g o r g a n i s m s i s t h e w a yi n w h i c h t h e i r f o o d r a t i o n i s g i v e n . B i v a l v e m o l l u s c s , w h i c h f i l t e r m i c r o -s c o p i c a l g ae c e l ls f r o m t h e s u r r o u n d i n g w a t e r , a c h i e v e t h e m a x i m u m r a teo f g r o w t h i n c o n d i t i o n s o f c o n t i n u o u s f o o d s u p p l y , b u t w i t h r e l a t i v e l yn a r r o w l im i t s o f f o o d c e ll d e n si ty . W h e n t h e o p t i m u m f o o d d e n s i ty i se x c e e d e d , s o m e o f th e fi lt e r ed c e l ls a r e w a s t e d , b e i n g v o i d e d a s p s e u d o -f a e c e s , a n d t h e c e l l s th a t ar e i n g e s t e d m a y b e d i g e s t e d i n e f f ic i e n tl y .

I n , f o r e x a m p l e , b i v a l v e h a t c h e r i e s w h e r e l a r v a e a n d j u v e n i l e s o fo y s t e r s a n d c l a m s a r e r e a r e d i n t e n s i v e l y , t h e e f f i c i e n t u s e o f c u l t u r e da l g a e , w h i c h a r e c o s t l y t o p r o d u c e , i s v e r y i m p o r t a n t ( H e l m a n d L a i n g ,1 9 8 1 ). M a x i m u m e f fi c ie n c y w o u l d b e a c h i e v e d b y t h e a u t o m a t i c r e p l e n -

2 5 9 Crown Copyright. 1987

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260 W . L . H u g ,g i n s , 3/1 . M . H e l m , D . R . W i l l i a m si s h m e n t o f f o o d g r a z e d b y t h e a n i m a l s s o t h a t t h e o p t i m u m f o o d c e l ldens i ty is m a in t a in ed a t a l l t imes .

T h e u s e o f a u t o m a t i c d e v ic e s t o c o n t r o l n u t r ie n t i n p u t i n th e c u l t u r e o fm i c r o o r g a n i s m s i s d e s c r i b e d b y P o w e l l ( 1 9 6 3 ) a n d M u n s o n ( 1 9 7 0 ) .W i n t e r ( 1 9 7 3 ) d e s c r i b e d t h e u s e o f a p h o t o m e t r i c , a u t o m a t i c f e e d i n gappa ra tus i n c r i t i c a l de t e rmina t ions o f f i l t r a t i on r a t e i n t he musse l ,Myti lus edul i s . This . device , descr ib ed b y H au pt (1979) , u t i l ised a l igh t -de pen de n t r e s i s to r t o de t ec t chang es i n the a t t en ua t io n o f a l igh t bea mc a u s e d b y v a r i a t i o n i n t h e c o n c e n t r a t i o n o f f o o d c e ll s s u s p e n d e d i n t h ec u l tu r e m e d i u m . T h e s ig n a l w a s c o m p a r e d w i t h a r e f e r e n c e vo l ta g e ,w h i c h c o u l d b e v a r i e d t o a l t e r th e r e q u i r e d f o o d c e ll c o n c e n t r a t i o n . A ne lec t rom agne t i c va lve was ac tua t ed w hen the con cen t r a t i on fe ll be lowt h a t c o r r e s p o n d i n g t o t h e r e f e re n c e v o l ta g e a n d f o o d w a s a d d e d t o t h ee x p e r i m e n t a l c o n t a i n e r to r e s t o r e t h e r e q u i r e d c o n c e n t r a t i o n .T h i s r e p o r t d e s c r ib e s t h e d e s ig n a n d c o n s t r u c t i o n o f a s im i l a r a p p a r a -t us , t e r m e d a n a lg a l c e ll m o n i t o r ( A C M ) , d e v e l o p e d i n d e p e n d e n t l y a t th eM A F F F i she r i e s Lab ora to ry , Lowe s to f t , spec if ica lly , bu t no t exc lus ivelyfo r u se i n con t ro l l i ng foo d ce l l dens i ty i n t he i n t ens ive cu l tu re o f l a rvae o ft h e E u r o p e a n f l a t o y s te r , Ostrea edulis, a t t h e M A F F F i sh e ri e s L a b o r a -to ry , Conw y. T he pu rp ose o f th i s dev ice was to eva lua t e t he poss ib i l it y o fr ea r ing l a rvae in cond i t i ons o f con t ro l l ed f eed ing a t g rea t e r dens i t i e st h a n p r e v i o u s ly h a d b e e n p r a c t ic a l w i t h m a n u a l f e e d in g m e t h o d s . T h i sw a s a m o v e t o w a r d s a m o r e e c o n o m i c a l p r o d u c t i o n o f o y s t e r j u v e n i le s( spa t ) . Resu l t s a r e g iven o f a compara t ive se r i e s o f t r i a l s i n wh ich O .edul is l a rvae were r ea red a t a dens i ty o f 1 .4 ml - ~w i t h m a n u a l f o o d a d d i -t i on (Walne , 1974) and a t g rea t e r dens it i e s u s ing the ACM . In fo rm a t ioni s p rov ided on the f eed ing r a t e o f l a rvae , t he i r g rowth and su rv iva l t om e t a m o r p h o s i s a n d t h e p r o d u c t i o n o f s p at w i th th e t w o c u l t u re m e t h o d s .T he cos t bene f i t s o f h igh -dens i ty r ea r ing a re d i scussed .

M A T E R IA L S A N D M E T H O D ST h e a lg a l c e l l m o n i t o r - - d e s i g n c r i t e r i aA n app a ra tus was r e qu i r ed tha t wou ld fu lf il t he fo l lowing spec i fi ca tions :

1 . C on t ro l t he ce l l con cen t r a t i on o f a lgae in cu l tu re ves se ls wi th in t heo p t i m u m r a n g e t o s u p p o r t t h e m a x i m u m g r o w t h ra t e o f l ar v a e.2 . Be c apab le o f r ap id ca l ib ra t ion to d i sco un t t he e f fec ts o f dens i ty o fl a rvae , t he i nc rea se i n si ze o f l a rvae a s t hey deve lop an d changes i nt h e s p e c ie s c o m p o s i t i o n o f t h e f o o d r a ti o n .3 . T h e s e n s o r s h o u l d n o t b e a f fe c t ed b y c h an g e s in a m b i e n t il lu m i n a -t i on in t he cu l tu re a r ea .

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F oo d sup ply in culture o f i l ter-feeding organisms 2614 . T h e c i rc u i t s h o u l d b e t e m p e r a t u r e c o m p e n s a t e d t o p r o v i d e o p e r a -t i ona l s tab i l it y i n cu l tu re s w he re m i nor v a r i a t i ons in t em p era t u r e

c a n b e e x p e c te d .5 . T h e a p p a r a t u s s h o u l d b e p o r t a b l e a n d c a p a b l e o f o p e r a t i o n i n arang e o f cu l t u re v es se ls and s i tua t ions .O perating principleT h e A C M d e s c r i b e d h e r e i s f i t t e d w i t h a n i n f r a - r e d t r a n s m i t t e r a n dpho t o - rece i ve r . I t is in sens i t i ve t o am bi en t l i gh t and use o f t he i n f r a - r edsource d i scou rages se t t l em en t o f a l gae on t he t r ansm i t t e r w i ndow . In t e r -f e r e n c e b y s u b s t a n c e s t h a t d i s c o l o u r t h e c u l t u re m e d i u m i s r e d u c e d .I n o p e r a t i o n , t h e - r e c e iv e r p r o d u c e s a c u r r e n t p r o p o r t i o n a l t o b o t h t h econ cen t ra t i on o f the a l ga l ce l ls u sed t o f eed t he l a rvae and t he opac i t y o ft he sea -w a t e r m e d i um . I f a su i t ab l e r e s i s to r i s i nc l ude d i n se ri e s w i t h t herece i v i ng e l em en t , t he vo l t age p roduced ac ross i t i s p ropor t i ona l t o t hea l ga l concen t ra t i on . Th i s vo l t age i s com pared w i t h a r e fe rence vo l t agean d i f g re a te r ( ind ica t ing a fa l l i n a lgae con cen t ra t ion ) ac t iva tes a per i s t a l -t ic pum p . Eq u i l i b r i um is r eached w hen a su f f ic i en t vo l um e o f a lgae isp u m p e d t o j u s t r e p l e n i s h t h a t g r a z e d b y t h e l a r v a e . W h e n t h e r e c e i v e dvo l t age fa l ls be l ow t he p re - se t r e fe rence l eve l t he c on t ro l c i r cu it sw i tchest he pum p o ff.D e s ig n a nd c o ns t ruc t io n o f the s e ns ing un i tA s c h e m a t ic d i a g r a m o f th e w h o l e s y s te m i s s h o w n i n F i g. 1 a n d s o m e o ft h e c o m p o n e n t p a r t s i n F ig . 2. T h e s e n s o r u n i t w a s d e s i g n e d w i t h w a t e r -t igh t s ea l s t o pe rm i t i m m ers i on i n r ea r i ng t anks con t a i n i ng oys t e r la rvae(F igs 3 and 4 ). T he senso r un i t con t a i n s an i n f ra - r ed t r ansm i t te r , w h i ch isa l ig n e d w i th a s u i ta b l e r e c e iv e r o v e r a p a t h l e n g th o f 2 5 0 m m ( M H S 4 0 0a n d M R H 4 0 0 , r es p ec ti ve ly , m a n u f a c t u r e d b y M C P E l e c tr o n i cs L t d ,M i d d le s e x , U K ) . T h e t r a n s m i t t e r u s e s b e t w e e n 4 a n d 5 m W o f p o w e rf rom t he 20 -V supp l y a t t he no rm a l s e t t i ngs . Each un i t cons i s t s o f ana c t i v e e l e m e n t m o u n t e d i n a b l a c k , a n o d i s e d , m a c h i n e d , a l u m i n i u mb l o c k , w it h a 1 5 .9 m m d i a m e t e r b i c o n v e x l e n s t o p r o d u c e a q u a si -p a r a ll e l b e a m w i t h a d i a m e t e r o f a p p r o x i m a t e l y 1 0 m m a n d a s p r e a d o fa p p r o x i m a t e l y 1 i n w a te r. T h e u n i ts a r e m o u n t e d w i t h in a 1 3 3 m md i a m e t e r s t a i n l e s s s t e e l b u l k h e a d w i t h a g l a s s o r a c r y l i c w i n d o w m a d ew a t e r t i gh t by an ' O ' s ea l . Each un i t i s t o t a l l y enc l osed w i t h t he connec t :i ng l ead sea l ed w i t h epoxy re s i n (Tw o- t ube A ra l d i t e ) . The m a t e r i a l fo rt h e l e a d s m u s t b e n o n - t o x i c : P V C a n d s i l i c o n e r u b b e r s h e a t h e d c a b l e shave p rove d su i tab l e .

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Fo od supp ly in culture o f i l terTfeeding organisms 2 6 3

F i g . 2 . T h e s e n s o r u n i t, c o n t r o l b o x a n d p e r i s ta l t ic p u m p .

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T h e c i rc u i t d i a g r a m o f t h e A C M is s h o w n i n F ig . 5 . T h e i n f r a -r e d tr a n s -m i t t e r is a ga l li um a r sen ide (G aAs) l i gh t - emi t ti ng d iod e (M CP E lec -t r o n ic s L t d ) ( M G A 5 0 0 0 ) , w h i c h e m i ts fi gh t i n t h e n e a r - i n f r a - re d r e g i o na t a wave leng th o f approx ima te ly 900 nm. At t h i s wave leng th , s i l i conp h o t o - d e t e c t o r s o p e r a t e a t t h e i r p e a k e f fi c ie n c y a n d m o s t g l as se s o r c l e a rac ry l ic s can be used in t he op t i ca l pa th .T h e d e t e c t o r i s a n M S P 6 c o m p o s i t e si li c o n p h o t o - r e c e i v e r ( M C PE l e c t r o n i c s L t d ) w i t h p e a k s p e c t ra l r e s p o n s e a t 9 5 0 n m . T h i s i s u s e d i nse r ie s wi th a r e s i s to r and the rm is to r cha in t o p ro v ide the vo l t age s igna l(V~x), w hich i s fed to the inve r t ing inp ut (p in 2) of the op era t ion a l amp l i -f i e r R C 74 1C v ia a 22 k ff2 r e s is to r. I nc lus ion o f t he t he rm is to r , f i tt ed i n tot h e a l u m i n i u m b l o c k h o l d i n g t h e d e t e c t o r , p r o v i d e s s t a b il it y o f + 0 . 0 4 %at t he t em pe ra tu r e used in t he r ea r ing o f l a rvae (24C + I C) . A 4 .7 -/~Fc a p a c i t o r b e t w e e n p i n 2 a n d 0 V h e l p s t o s m o o t h o u t c i r c u i t r e s p o n s ed u e t o t h e p a s s a g e o f a i r b u b b l e s a n d f l u c tu a t i n g c o n c e n t r a t io n s o f l a rv a eac ros s t he op t i ca l pa th .T he r e f e renc e vo l t age (Vree) de r ive d f ro m pa r t o f a r e s i s to r cha in i sapp l i ed to p in 3 o f t he 741 op e ra t io na l ampl i f ie r , i ts va lue be ing ad jus t edby the tw o p o ten t iom e te r s i n t he ch a in , g iv ing ' coa r se ' and ' f ine ' con t ro l .Th i s s e t t i ng o f V r e i s a d j u s t e d t o t h e o p t i m u m c o n c e n t r a t i o n o f a l g a ece ll s i n suspen s ion fo r t he g row th o f the l a rvae ( see ca l ib ra t ion p ro ced uredesc r ibe d l at e r) . Fo r a con cen t r a t i on o f 100 ce ll s /x1-1 o f I s o c h r y s i sg a lb a n a , V ~e ~ s se t b e t w e e n 8 . 5 - 9 . 0 V . E x a c t d e t e r m i n a t i o n d e p e n d s o n

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t he co lou r and c l a r i t y o f t he sea wa te r u sed in r ea r ing , and accoun t i st aken o f these f ac to r s i n the ca l ib ra t ion p roc edu re . A change in food ce l lcon cen t r a t i o n f rom 100 to 75 ce ll s ~1 -~ g ives a vo l t age change o fapp rox im a te ly 2 V fo r V~x. Feed back i s app l i ed b e tw een th e o u tpu t (p in6 ) o f t he o pe ra t iona l ampl i f i e r and p in 3 . T he deg ree o f hys t e re s i s o f t hec o m p a r a t o r c a n b e c o n t r o l l e d b y c h o i c e o f a s u it a b le fe e d b a c k r e si st o r;i n t h i s ca se a 2M2 re s i s to r g ives a hys t e re s i s r ange o f 0 . 2 V abou t t here fe rence vo l t age leve l. T he up pe r hys t e re s i s po in t is 0 . 07 V h ighe r t hanV r e f and the l ow er po in t O. 13 V be low the r e f e rence .O pe ra t io n o f the 741 c i rcu i t i s such tha t i f Vr~ i s l ess than th e low erhys t e re s i s po in t t he ou tpu t i s h igh , app roac h ing the supp ly vo l tage . Fo r ar ece ive r vo l tage g rea t e r t ha n the u pp e r hys t e re s is po in t t he ou tp u t i s l ow,app roac h in g 0 V . W he n Vr~ is g rea t e r t han the h igh e r r e f e rence po in t ,i nd i ca t ing a r ed uce d a lga l ce l l con cen t r a t i on , t he ou tpu t o f t he 741 islow, ho ld ing the BC183LC t r ans i s to r i n an OFF s t a t e . In t h i s s t a t e t her e la y is n o t e n e r g i s e d a n d p o w e r i s a p p l i e d t o t h e p u m p . W i t h t h e p u m pin op e ra t ion the r ece ive r vo l t age f al ls as t he a lgal ce ll con cen t r a t i on r is e stowa rds t he op t imu m . W he n i t f a ll s be low the lowe r hys t e re s i s po in t , theo u t p u t o f t h e 7 4 1 i n c r e as e s a n d t u rn s o n t h e B C 1 8 3 L C , t h u s en e r g is in g

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