poxton 1987 aquacultural-engineering

7/28/2019 Poxton 1987 Aquacultural-Engineering

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Aquacultural Engineering 6 ( 1 9 8 7) 3 0 1 - 3 2 2

Cy cli ca l F luctuat ions in Am m onia and Ni tr ite -n itrogenR e s u l t in g f r om t h e F e e d i n g o f T u r b ot , Scophthalmusmax imus L.) , in R ec ircu lating Sys tem sM . G . Poxton and S . B . Allouse*

A qua culture G roup , De partm ent of Biological Sciences, Heriot-W att University,Chamber~ Street , Edinburgh E H1 1HX, U K

A B S T R A C TDai ly and week ly cycl ica l f luc tua t ions in am m onia (NH4-N ttg l i t re - l)an d ni tri te-ni trogen (NO 2-N t~g l i tre- l) were show n to occur in marinerecirculating systems containing juvenile turbot, S c o p h t h a lm u s m a x i m u s(L.). A stepwise multivariant regression analysis, com pu ted fo r the pre-feeding morning base levels, showed that ammonia-nitrogen concentra-t ion in the f ish tanks was related to in f low am mo nia-nitrogenconcentration, water flo w rate, water temperature, m ean fish weight an dfish biomass. In f luent am m on ia concentration an d water f lo w rate wereby far the m os t im portant variables accounting fo r 68"0% and 16"7% o fthe variation in the regression respectively.The fluctuations, wh ich occurred at concentrations thou ght to betolerated by the f ish , were at tributed to the am ou nt a nd t iming o f eeding.The significance o f the existence o f such fluctua tions was discussedtogether with the importan ce of their am pli tude and frequency.

INTRODUCTIONT h e g r o w t h o f t u r b o t , S c o p h t h a l m u s m a x i m u s ( L . ) , in rec i rcu la t ings y s t e m s w a s p r e v i o u s l y e v a l u a t e d ( P o x t o n et a l . , 1 9 8 2 ) b y m e a n s o f as e r i e s o f f e e d i n g t r i a l s . T h e s e a l s o a s s e s s e d t h e u s e f u l n e s s o f r e a d i l ya v a i l a b l e i n d u s t r i a l f i s h s u c h a s s p r a t s (Sp ra t tus sp ra t tus L . ) , s a n d e e l s( A m m o d yt es m a r i n u s R a i t t) a n d s m a l l w h i t in g (M er l a n g i u s m er l a n gu s L .)a s f o o d f o r th e in i ti a l o n - g r o w i n g s t a g e o f c u l t i v a ti o n ( 1 0 - 5 0 0 g ). A t th es a m e t i m e t h e d e s i g n a n d p e r f o r m a n c e o f t h e s y s t e m ' s b i o l o g i c a l f i l t e r s*Present address: Insti tute of Agricultural Technology, Depa rtme nt o f An ima l Pro duc -tion, At-Amyeria, Baghdad, Iraq.

301Aquacultural Engineering 0 1 4 4 - 8 6 0 9 / 8 7 / S 0 3 .5 0 - - E l sev ie r A p p l ied S c iencePublishers L td, England, 1987. Printed in G rea t Britain


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302 M. G. Poxton, S. B. Allousew a s a s se s s e d a n d s o m e p r e l i m i n a r y re s u lt s o n t h e i r p e r f o r m a n c e w a sr e p o r t e d b y P o x t o n e t a l . ( 1 9 8 1 ) w h i l st a b e g i n n i n g h a s b e e n m a d e i nunde r s t and i ng , m ode l l i ng and p red i c t i ng t he k i ne t i c s o f n i t r i f i ca t i on byW ea t he r l ey ( 1982 , 1984) .W at e r qua l i t y c r i t e r i a fo r m ar i ne f i sh such a s t he t u rbo t have beenr e v i e w e d b y P o x t o n a n d A l l o u s e ( 1 9 8 2 ) , w h o d r e w a t t e n t i o n t o t h ec o m p l e x i n t e r - r e l a t i o n s h i p s w h i c h e x i s t b e t w e e n s o m e o f t h e m o s ti m por t an t pa ra m e t e r s . A s a r e su l t, m uc h o f t he l it e r a t u re i n t h is a rea w asfe l t t o be o f l i t t l e quan t i t a t i ve va l ue a l t hough t he l eve l s w h i ch a recom pl e t e l y sa fe o r d i r ec t l y le t ha l a re w e l l iden t i fi ed .T h e w a t e r q u a l it y p r e v a i li n g d u r in g t h e 1 3 3 d a y s o f th e t u r b o t f e e d i n ga n d g r o w t h e x p e r i m e n t s w a s s u m m a r i s e d b y P o x t o n e t a l . (1982) t od e m o n s t r a t e t h a t a n a c c e p t a b l e e n v i r o n m e n t w a s m a i n t a i n e d . I t i s v e r yi m p o r t a n t , h o w e v e r , t o a p p r e c i a t e t h e d y n a m i c n a t u r e o f t h e s e w a t e rqua l i t y pa ram e t e r s . In i n t ens i ve sys t em s da i l y and o t he r cyc l ica l va r i a -t io n s i n w a t e r q u a l it y o c c u r d e p e n d i n g o n t h e s t o c k i n g d e n s it y , s e a s o n a le f fec t s such a s day l eng t h , and on t he am oun t and t i m i ng o f f eed i ng .E x a m p l e s o f t h e s e v a r i a t i o n s a r e g i v e n b y R o s e n t h a l e t a l . (1981) fo r aw a r m b r a c k i s h - w a t e r s y s t e m s t o c k e d w i t h T i l a p i a , a n d B r e t t a n d Z a l a( 1 9 7 5 ) f o r a c o l d f r e s h w a t e r s y s t e m s t o c k e d w i t h s o c k e y e s a l m o n ,O n c o r h y n c h u s n e r k a . I n t h is p a p e r a m o r e d e t a il e d c o n s i d e r a t i o n o f th eva r i ab i l it y o f t he w a t e r qu a l i t y da t a fo r t u rb o t i n a m ar i ne sys t em i s g i venw i t h th e a m p l i tu d e a n d f r e q u e n c y o f th e f l u c tu a t io n s b e i n g d e s c r ib e d . A na t t e m p t i s t h e n m a d e t o r e l a t e o b s e r v e d a m m o n i a c o n c e n t r a t i o n s t oope ra t i ng cond i t i ons .

M A T E R I A L S A N D M E T H O D SReeirculating system sT h e s y s t e m d e s i g n h a s b e e n d e s c r i b e d b y P o x t o n e t a l . (1981) bu t i ss h o w n a g a i n h e r e f o r c o n v e n i e n c e ( F ig . 1 ). E a c h s y s t e m c o n t a i n e d t h r e epo l yp r op y l en e f ish tanks , 0 . 37 m 2 i n a rea , w h i ch w ere f i ll ed t o a dep t h o f20 c m w i t h 75 l it re s o f s eaw a t e r . A 5 cm l aye r o f coa r se (20 m m ) g ran i t ec h i p s w a s u s e d t o b u r y w a t e r c o l l e c t i o n p i p e s i n t h e b o t t o m o f t h eb i o f il te r s . F i n e r g rave l (6 m m ) w as t he n use d t o f i ll t he beds t o a dep t h o f3 0 - 3 5 c m w h i l s t b r o k e n s c a ll o p sh e ll s w e r e a d d e d t o th e t o p 5 c m t oi nc rease t he bu f fe r i ng capac i ty . Th ese beds ha d a su r face a rea o f 1 m 2and a t o t a l subs t r a t e a rea ava i l ab le fo r bac t e r i a o f abou t 180 m 2 each .Sm al l bucke t s con t a i n i ng m arb l e ch i ps w ere u sed a s m echan i ca l f i l t e r sand a s a fu r t he r sou rce o f bu f fe r i ng capac it y .


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3 0 * i s o m e t r i c p r o j e c t i o nFig . 1 . Rec ircula tory cul ture uni t .

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Th e b i o f il t e rs w ere ru n fu l ly pon ded , i n o rd e r t o p rov i de a su f f ic i en the ad of wa ter ,for the a i r l i ft s , and as suc h a l so t end ed to ac t as reservoi rs .To t a l vo l um e s w ere 900 l it re s fo r each o f t he sys t em s used (C ,D ,E) , eachof w h i ch had t w o b i o f i l t e r s connec t ed i n se r i e s . F l ow ra t e s t o t he f i sht anks w e re gen e ra l l y be t w een 1 and 5 li tr e s m i n -1 . A l l t he w a t e r w asr e c y c l e d t h r o u g h P V C p i p e b u t l o s s e s f r o m s a m p l i n g , c l e a n i n g a n de v a p o r a t io n a m o u n t e d t o ab o u t 1 % p e r d a y.S o u r c e o f m a t e r i a ls

S e a w a t e rS e a w a t e r w a s o b t a i n e d f r o m t h e F i r t h o f F o r t h a t N o r t h B e r w i c k a n dc o n v e y e d t o E d i n b u r g h b y r o a d t an k e r.E x p e r i m e n t a l f is h0 - g r o u p t u r b o t w e r e s u p p l i e d b y th e W h i t e F i s h A u t h o r i ty , n o w t h e S e aF i s h I n d u s t r y A u t h o r i ty , f r o m t h e i r h a t c h e r y a t A r d t o e o n t h e w e s t co a s to f S c o tl a nd . T h e y w e r e t r a n s p o r t e d t o E d i n b u r g h i n s e a le d p l a st ic b ag sc o n t a i n i n g a n o x y g e n a t m o s p h e r e .A c c l i m a t i o n p e r io dA f t e r a n i n i ti a l 2 - w e e k a c c l i m a t io n p e r i o d , 2 4 0 f is h w e r e w e i g h e d a n dp l aced i n n i ne f i sh t anks o n t h ree (C ,D ,E) co m pl e t e l y sepa ra t e r ec i r cu l a t -


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304 M. G. Poxton, S. B. Allousein g s ys te m s, w i th t hr e e ta n k s ( C / 1 - C / 3 , D / l - D / 3 , E / l - E / 3 ) p e r s y st emas show n i n F i g . 1 . T he i n i ti a l s t ock i ng dens i t ie s r ange d f rom 0 .3 -0 .4 kgm -2 (1 .6 kg m -3 ) , t he i nd i v i dua l popu l a t i ons r ang i ng f rom 25 t o 33 f i shpe r t ank .Food and feedingT h e f is h w e r e f e d t w ic e p e r d a y o n w e e k d a y s a n d o n c e p e r d a y o nw e e k e n d s , t h e a m o u n t a n d t im i n g o f th e f e e d s d i f fe r in g a c c o r d i n g t o t h ee x p e r i m e n t a l de s ig n . S p ra t s a n d w h i t in g w e r e p u r c h a s e d f r o m F i r t h o fFor t h ca t ches and kep t i n a deep f r eeze . P r i o r t o f eed i ng , t he food w ascho pp ed i n t o su i tab l y s i zed and shap ed ( e l onga t ed ) p i eces and a l l ow edt o t h a w , t h e v i s c e r a a n d b a c k b o n e s b e i n g d i s c a r d e d . A f t e r p r e p a r a t i o nt h e f o o d w a s d r i e d o n p a p e r t i s s u e s t o r e m o v e s u r f a c e m o i s t u r e a n dw e i g h e d . U n e a t e n f o o d w a s r e c o v e r e d f r o m t h e f i s h t a n k s , d r i e d a n dr e w e i g h e d t o c a l c u la t e th e a m o u n t o f f o o d e a te n .W e i g h ing the e x pe r i m e n ta l f is hE v e r y 2 w e e k s t h e f is h i n e a c h t a n k w e r e r e m o v e d , s u r fa c e d r i e d o npap e r t is sues , and i nd i v i dua l l y w e i ghed on a t op p an ba l anc e (+ 0 .1 g).T h e f is h w e r e k e p t i n b u c k e t s o f v i g o ro u s l y a e r a t e d s e a w a t e r t a k e n f r o mt he sam e f i sh t anks . A f t e r r em ova l o f t he f ish t he t anks w ere d ra i ned i n t ot he f il te r bed s / r e se rvo i r s , t h o rou gh l y c l eane d a nd r e f il led f rom t heheade r t anks be fo re t he f i sh , and w a t e r , w ere r e t u rned . I t w as no t f e l tneces sa ry t o anaes t h e t i s e the f ish , a l t houg h t hey w ere no t f ed on t he dayso f w e i gh ing .C a l c u la t io n s o f f i s h b i o m a s sT he f i sh b i om ass w as ca l cu l a t ed da i l y , t ak i ng in t o acco un t t he w e i gh t o ffood ea t en and us i ng t he conve r s i on e f f i c i enc i e s p rev i ous l y de t e rm i ned .(Pox t on et aL, 1982) , i n o rd e r t o be ab l e to r e l a t e it t o am m on i a p rod uc -t i o n . A n y n e c e s s a r y c o r r e c t i o n s w e r e m a d e f r o m t h e 2 - w e e k l y c h e c kweighings .W a ter qua l ity c o ntr o l a nd m o ni to r ingSa l i n it y w as m a i n t a i n ed a t 34 _+ 2% o by t he r egu l a r add i t ion o f ae ra t edt a p w a t e r w h i l s t t e m p e r a t u r e c o n t r o l w a s a c h i e v e d b y r e g u l a t i n g t h ea m b i e n t a i r t e m p e r a t u r e t o 1 6 + _ 2 C . A i r a n d w a t e r t e m p e r a t u r e s ,t oge t he r w i t h sa l in i ty , w ere m easu red da i l y u s i ng a m e rcu ry - i n -g l a s st h e r m o m e t e r a n d a h y d r o m e t e r r e sp e c ti v el y . D i s s o l v e d o x y g e n a n d f r e e


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A m m on ia an d n i t r i t e -N f luc tuat ions in rec irculat ing sys tems 3 0 5c a r b o n d i o x i d e w e r e m e a s u r e d l es s f r e q u e n t l y u si n g a n a z i d e m o d i f i c a -t io n o f th e W i n k l e r m e t h o d a n d t h e r e a c ti o n b e t w e e n c a r b o n d i o x i de a n ds o d i u m h y d r o x i d e t o f o r m s o d i u m b i c a r b o n a t e ( A m e r i c a n P u b li c H e a l t hAssoc ia t ion , 1965) .A m m o n i a , n i t r it e a n d p H w e r e d e t e r m i n e d a s re q u i r e d b y t h e e x p er i-m e n t a l d e s i g n . A m m o n i a w a s m e a s u r e d i n t r i p l i c a t e u s i n g t h ep h e n o l - h y p o c h l o r i t e m e t h o d t o p r o d u c e i n d o p h e n o l b l u e a ft er K o r o l e f f( 1 9 7 6 ) . T h e n i t r i t e a n a l y s i s w a s a f t e r B e n d s c h n e i d e r a n d R o b i n s o n(1952) wi th t he m od i f i ca t ion o f u s ing su lph an i l am ide a s t he d i azo t is inga g e n t a n d 1 - n a p h t h y l e t h y l d i a m i n e d i h y d r o c h l o r i d e a s th e c o u p l i n gagen t g iv ing r ap id c o lou r dev e lop m en t wi th a 5% inc rease i n s ens i ti v i ty(Mar t in , 1972) . pH was me asu red us ing a Beck m an se l ec tive i on pHmeter .So ur ces of variabi li ty and m ultivariant regre ss ion an alysisE x c r e t i o n r a te s o f fi sh o b v i o u s l y d e p e n d o n f e e d i n g r a te s a n d w a t e rt empera tu re s . However , o the r f ac to r s such a s t he t o t a l b iomass andave rage w e igh t o f t he f i sh t oge the r wi th t he e f f ec ts o f wa te r f l ow ra t e onb o t h t h e c l e a r a n c e o f m e t a b o l i t e s f r o m t h e f i s h t a n k a n d o n o v e r a l lsys t em pe r fo rmance , by a f f ec t ing ox ida t ion r a t e s , a r e sou rces o f va r i -ab i l it y and mu s t a l so be cons ide red . Becau se o f t he com plex i t ie s o f thes i t ua t ion ( see Pox ton and Al louse , 1982 fo r r ev i ew) i t was dec ided toemploy mul t iva r i an t r eg res s ion t echn iques t o t he ana lys i s o f t hedata .M ul t ip l e r eg res s ion i s an ex t ens ion o f t he use o f t he b iva r i a t e co r r e l a -t i on coe f f i c ie f lt t o m ul t i va r i a t e ana lysi s . W here as t he l i nea r r e l a t i onsh ipb e t w e e n o n e i n d e p e n d e n t v a ri a b l e a n d a d e p e n d e n t v a r ia b l e is m e a s u r e dby a s imple r eg res s ion coe f f ic i en t, m u l t i p l e r eg res s ion a l lows o ne tos t u d y t h e l i n e a r r e l a t io n s h i p b e t w e e n a s e t o f i n d e p e n d e n t v a r ia b l e s a n da n u m b e r o f d e p e n d e n t v a r i a b l e s w h i l e t a k i n g i n t o a c c o u n t t h e i n t e r -r e l a t i o n s h i p s a m o n g t h e i n d e p e n d e n t v a r i a b l e s . T h e b a s i c c o n c e p t o fm u l t i p l e r e g r e s s i o n i s t o p r o d u c e a l i n e a r c o m b i n a t i o n o f i n d e p e n d e n tva r i ab l e s wh ich wi ll no t on ly co r r e l a t e a s h igh ly a s poss ib l e wi th t hed e p e n d e n t v a r i a b le b u t w h i c h c a n t h e n b e u s e d t o p r e d i c t t h e la tt er . T h eres idua l i s t he d i f f e r ence be tween the ac tua l va lue o f t he dependen tv a r i a b l e a n d t h e v a l u e p r e d i c t e d b y t h e l i n e a r c o m b i n a t i o n o f t h ei n d e p e n d e n t v a r ia b l es . T h e r e g r e ss i o n e q u a t i o n i s w r it t e n a s:

Y = b l X l + b 2 X 2 + . . . b s X s + ( c + r)w h e r e Y = t h e d e p e n d e n t v a r i a b le , t h e p r e - f e e d in g m o r n i n g b a s e l e v e la m m o n i a c o n c e n t r a t i o n i n t h e f i sh t a n k s ( k tg N H 4 - N l it re - 1 ), b l _ 5 = th e


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306 M. G. Poxton, S. B. Allouseregress ion coef f ic ien ts , X~_5 = the i nd ep en den t va r i ab le s , X t = in f luen tam m on ia co nc en t r a t i o n (/~g NH 4-N l it r e -~ ) , X2 = w a t e r f low ra t e ( li tr em i n - ~ ) , X 3 f f iw a t e r t e m p e r a t u r e ( C ) , X 4 = a v e r a g e f is h w e ig h t ( g) ,X 5 - - to ta l f i sh b iom ass (g), c = co ns tan t an d r = the res idua l .F u r t h e r u n d e r s t a n d i n g o f t h e r e l a t i o n o f e a c h i n d e p e n d e n t v a r i a b le t ot h e d e p e n d e n t v a r i a b l e c a n b e g a i n e d b y e x a m i n i n g t h e s i g n s o f t h er e g r e ss i o n c o e f fi c ie n ts . T h e c o m p u t e r a u t o m a t i c a l l y p e r f o r m s a n F - te s tt o t e s t t he ove ra l l ' good ness o f fi t' o f t he r eg res s ion equa t ion .A fu r the r d i scuss ion o f t he use o f mu l t i p l e r eg res s ion fo r the t ype o fa p p l i c a t i o n c o n s i d e r e d h e r e i s g i v e n b y A l d e r d i c e ( 1 9 7 2 ) w h i l e f o r am a t h e m a t i c a l a n a l y s i s o f t h e m e t h o d r e f e r e n c e s h o u l d b e m a d e t o as ta t i s t ics tex t which descr ibes leas t - squares f i t t ing techniques . For thep resen t work the r eg res s ions were computed us ing the 'S t a t i s t i ca lPackage fo r t he Soc ia l Sc i ences ' mu l t i p l e r eg res s ion p rogram asd e s c ri b e d b y K i m a n d K o h o u t ( 19 7 5) . T h i s p r o g r a m c o m b i n e s s t a n d a rdmul t ip l e r eg res s ion and s t epwise r eg res s ion in a way wh ich p rov idesc o n s i d e r a b l e c o n t r o l o v e r t h e i n c l u s i o n o f i n d e p e n d e n t v a ri a b le s .Experimental designT h e e x p e r i m e n t s w e r e d e s i g n e d t o i d e n ti f y t h e e x i st e n ce a n d e x t e n t o fany da i ly and o the r va r i a t i ons i n wa te r qua l i t y , pay ing pa r t i cu l a ra t t e n t i o n t o a m m o n i a a n d n i t ri te a n d f l u c tu a t io n s r e su l ti n g f r o m f ee d in g .P r e l i m i n a r y w o r k c o n f i r m e d t h a t t h e w a t e r i n t h e f i s h t a n k s w a s w e l lmixe d an d n o t s ign i fi can t ly d i f f e r en t f rom tha t o f t he ou t l e t s and tha t nof luc tua t ions occur red when the f i sh were no t f ed . Th i s was f e l t t o ben e c e s s a r y d e s p i t e t h e v e r y s h a l l o w ( 2 0 c m ) d e p t h o f w a t e r a s t u r b o ts p e n d , w i t h t h e e x c e p t i o n o f t h e i r b r ie f f ee d i n g r es p o n s e s , m o s t o f t h e irt i m e s t a t io n a r y o n t h e b o t t o m o f th e t a n k s .Experiment 1S e v e n ta n k s o f t u r b o t w e r e f e d a t 0 9 . 3 0 a n d 1 4 .0 0 h w h i l st o n e t a n k( C / 3 ) r e c e i v e d o n l y t h e m o r n i n g f e e d . A m m o n i a w a s m e a s u r e dhou r ly be tw een 09 . 00 an d 17 . 00 h t o d e t ec t da i ly f l uc tua t ions (F ig . 2 ,Ta ble 1 ).Experiment 2I n v e s ti g a te d th e s e c o n d p e a k i n a m m o n i a u s i n g t h r e e t a n k s o f tu r b o t o nsys t em D and ex ten ded sam pl ing un t i l 22 . 00 h (Fig. 3).Experiment 3I n v e s ti g a te d t h e h o u r l y f l u c tu a t io n s i n n i t ri te c o n c e n t r a t i o n i n o n e t a n kof t u rb o t on th ree succes s ive days (F ig . 4).


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Ammonia and nitrite-N luctuations in recirculatingsystems 307T A B L E 1Tem perature (C), pH , W ater Flow Rate (litre m in- l), Bioma ss (g), M ean Fish W eight (g),and W eight of Food Fed (g) to the T urbot in Ea ch o f Three M arine RecirculatingSystems on 30 January 1979 (data relates to F ig. 2)

Conditions Tank numberC/1 C/2 C/3 D/2 D/3 E/1 E/2 E/3

Te m p. (C) 16-0 16.0 16.0 15-8 15.8 15.2 15.2 15.2pH 7.2 7.2 7.2 7.1 7.1 7.3 7-3 7-3Flow (litr e m in - ~) 2-8 2.9 2"9 2.0 2-5 2-0. 2"9 2"9B io m as s(g ) 2 1 0 . 0 4 63 .9 1 8 5 - 0 5 3 0 . 9 5 0 2 . 0 2 9 8 . 0 5 0 2 . 7 1 9 4 .7M ean w t (g) 7-6 13-6 7-8 14.0 16.2 9.4 22.9 9.5Fo od (g /a.m . 3"0 16"5 6"5 18.6 18.6 5"3 19.2 6"0p.m . 5"3 10.5 0"0 10.4 6"3 3"1 12.1 3"2

Experiment 4S a m p l e s f o r b o t h a m m o n i a a n d n i tr it e w e r e t a k e n a t 0 9 .0 0 h , b e f o r ef e e d i n g , f r o m t h e i n f iu e n t s a n d e f f l u e n ts o f t h r e e t a n k s o n s y s t e m C f r o mM o n d a y t o F r i d a y t o d e t e c t w e e k l y f lu c t u a t i o n s i n b a s e l e v el s ( F ig . 5).Experiment 5S a m p l e s f o r b o t h a m m o n i a a n d n i tr it e w e r e t a k e n 4 h a f t er fe e d in g , f r o mt h e i n fl u en t s a n d e ff lu e n ts o f t h r e e t a n k s o n s y s t em D f r o m M o n d a y t oF r i d a y t o d e t e c t w e e k l y f l u c t u a t i o n s i n p o s t - f e e d i n g l e v e l s (F i g . 6 ).Experiment 6.H o u r l y s a m p l e s w e r e t a k e n t h r o u g h o u t t h e d a y f r o m s y s t e m E d u r i n gt h r e e p e r i o d s o f t h e s tu d y i n F e b r u a r y , A p r i l a n d J u n e 1 9 7 9 . T e m p e r a -t u r e a n d w a t e r f l o w r a te s w e r e k e p t c o n s t a n t ( T a b l e 2 ) s o t h a t th is e x p e r i -m e n t r e fl e c ts t h e i n c r e a s i n g f is h b i o m a s s a n d m e a n w e i g h t. S a m p l e s w e r et a k e n f r o m e a c h o f th e t h r e e t a n k s o n t w o s u c c e ss i v e d a y s a n d t h e m e a n sa n d s t a n d a r d d e v i a t i o n s w e r e c a l c u l a t e d ( F ig . 7).Experiment 7F l o w r a t e w a s i n v e s t i g a t e d b y a l te r i n g t h e f lo w s t o t h r e e t a n k s o n s y s t e mD t o 1 .2 , 3"0 a n d 5 .1 l i t re s m i n - 1 , w h i l e o t h e r p a r a m e t e r s w e r e s i m i la r .A f t e r 4 8 h e q u i li b r a t i o n s a m p l e s w e r e t a k e n t h r o u g h o u t t h e th i r d d a y(Fig . 8 ) .Experiment 8S a m p l e s w e r e t a k e n a t 0 9 . 0 0 h f r o m t h e i n fl u e n t s t o s y st e m s C , D a n d Ea t t o t a l f l o w s t h r o u g h t h e b i o f i l t e r s o f 1 3 . 0 , 8-5 a n d 6 . 4 l it r e s m i n - 1


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308 M. G. Poxton, S. B. Allou seTABLE 2Experimental Conditions in System E During Thre e Periods of the S tudy in 1979 (datarelates to Fig. 7)

Condit ions MonthFebruary A pri l June(x +. S.D .) (x +_ S.D .) (x +_ S.D .)

Te m p. (C) 17.0 + 0-3 17.2 + 0.3 17.5 + 0.5pH 7.2 + 0.3 6-8 + 0.2 6-5 + 0-2Flow (litre m in - t) 2.2 +_0.8 2.2 + 0.4 2.2 + 0.5Biom ass (g) 550 + 67 977 + 76 1397 + 62Fo od (g d ay -t ) 27.7 __. 1.1 37.4 + 6.6 53-1 + 10.5

r e s p e c t i v e l y , t o i n v e s t i g a t e t h e e f f e c t o f v a r y i n g t h e h y d r a u l i c l o a d ( F i g .9 ). F i s h b i o m a s s e s w e r e 7 . 1 - 7 . 2 , 3 . 7 - 3 . 9 a n d 2 . 7 - 2 . 9 k g r e s p ec t iv e l y .

R E S U L T SD a i l y fl u c t u a t i o n s d u e t o f e e d i n gE x p e r i m e n t 1A m m o n i a c o n c e n t r a t i o n s i n c r e a s e d i n al l t h e t an k s a s t h e d i r e ct r e su l t o ff e e d i n g t h e f is h ( F ig . 2 ). N o i n c r e a s e w a s d e t e c t e d i n t h e f ir s t 3 0 m i n a f t e rt h e m o r n i n g f e e d b u t s u b s t a n ti a l i n cr e a s e s o f u p t o 1 0 0 % o c c u r r e dw i t h in 9 0 m i n . I n c r e a s e s o f 7 3 - 2 0 5 % ( m e a n 1 4 3 % ) o c c u r r e d a f t er 3-5 h .F e e d i n g o n l y o n c e ( C / 3 ) r e s u l ts i n d e c r e a s i n g c o n c e n t r a t i o n s i n th e a f te r -n o o n , t h e f r e q u e n c y o f t h e a m m o n i a c y c le b ei n g g r e a t e r t h a n 7 .5 h ,w h e r e a s a s e c o n d f e e d s t im u l a t e s a f u r t h e r s u b s t a n ti a l i n c re a s e . T h eo p e r a t i v e c o n d i t i o n s d u r i n g th is e x p e r i m e n t a r e s u m m a r i s e d i n T a b l e 1.E x p e r i m e n t 2T h e d u r a t io n a n d e x te n t o f t h e s e c o n d p e a k i n a m m o n i a c o n c e n t r a t io nc a n b e s e e n i n F i g . 3 w h i c h i n d i c a t e s t h a t i t o c c u r s 4 h a f t er t h e s e c o n df e ed a n d is s o m e 1 0 2 - 1 2 2 % ( m e a n 1 1 2% ) h i g h e r t h a n th e c o n c e n t r a -t i o n s m e a s u r e d a t 0 9 . 0 0 h o n t h e f o l l o w i n g d a y .E x p e r i m e n t 3H o u r l y f l u c t u a t i o n s i n b o t h t h e i n f l u e n t a n d e f f l u e n t n i t r i t e - n i t r o g e nc o n c e n t r a t i o n s w e r e s h o w n t o o c c u r o n t h r e e s u c c e s s i v e d a y s ( F i g . 4 ) .


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A m m o n i a a n d n i t r it e - N f l u c t u a t i o n s i n r e c ir c u la t in g sy s t e m s 309

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poxton 1987 aquacultural-engineering

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