Williamson 1982 Aquacultural-Engineering

7/28/2019 Williamson 1982 Aquacultural-Engineering

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Aquacultural Engineering 1 ( 1 9 8 2 ) 8 1 - 9 1

A N I M P R O V E D H A R V E S T I N G N E T F O R F R E S H W A T E R P R A W N S t

M I C H A E L R . W 1 L L IA M S O N a n d J A W - K A I W A N G

Department of Agricultural Engineering, University of Hawaii at Manoa, 3050 Maile Way,Honolulu, Hawaii 96822, USA

ABSTRACT

In Hawai i, because o f the t rans form at ion f r om a small -farm operat ion in to fa i r ly large

aquaeu l tural en terpr ises , the n eed to deve lop an imp rove d harves t ing ne t fo r fresh-

wa t e r p rawns has been deve l oped . The de s i gn / opera t i on o f a m echan i zed ne t and t he

success o f the fu l l -scale pr oto typ e fo r the ind us t ry i s de ta i led .

I N T R O D U C T I O N

T h e d e v e l o p m e n t o f a q u a c u l t u r e a s a n i n d u s t r y i n H a w a ii is c e n t e r e d a r o u n d t h e f re s h -

w a t e r p r a w n , Macrobrachium rosenbergi i . B e f o r e 1 9 7 5 , p r a w n p r o d u c t i o n i n H a w a i i

came f rom sma l l fa rm s run o n p a r t - t ime ba ses . Recen t l y , how eve r , w i th a m ark e t

po t e n t i a l e s t im a t ed a t 655 000 kg by t he 1980s (S t a t e o f Haw a i i , 1978 ) , s eve ra l l a rge

f a r m s h a v e c o m e i n t o e x i s te n c e .

The t r ans fo rm a t ion o f e s sen t ia l l y sma l l - f a rm ope ra t i on s i n to aquacu l t u r a l en t e r -

p r i s e s t ha t exceed 40 pond-hec t a r e s i n s i z e c r ea t ed many p rob l ems . Mos t o f t he sep r o b l e m s a re r e la t e d t o t h e n e e d f o r c o s t r e d u c t io n a n d p r o d u c t i o n e f f ic i e n c y im p r o v e -

m e n t . T h u s , n e w a p p r o a c h e s to t h e e n t ir e p r o d u c t i o n p r o c e s s, w i t h p a r t ic u l a r e m p h a s i s

on l abo r i n tens ive ope ra t i ons , a r e r equ i r ed t o m ake t he i nd us t ry econ om ica l l y v i ab le .

G ibson and Wang (1977) e s t ima t ed ha rves t i ng cos t t o be 9% o f t he t o t a l cos t a s soc i -

a t ed w i th r a is i ng p r awns . Re cen t e xpe r i ence i nd i ca t e s t ha t t he m a jo r i t y o f t he p rob l em s

in t he i ndus t ry can be d i r ec t l y o r i nd i r ec t l y a t t r i bu t ed t o t he la ck o f an econ om ica l

and e f f i c i en t ha rves t i ng sys t em.

"~ P u b l i s h e d w i t h t h e a p p r o v a l o f t h e D i r e c t o r o f t h e H a w a i i I n s t i t u t e o f T r o p i c a l A g r i c u l t u r e a n d

H u m a n R e s o u r c e s a s a J o u r n a l S e r i es P a p er .

81

Aquacultural Engineering 0 1 4 4 - 8 6 0 9 / 8 2 / 0 0 0 1 - 0 0 8 1 / $ 0 2 . 7 5 © A p p li e d S c ie n ce P u b l is h e r s L t d ,

E n g l a n d , 1 9 8 2P r i n t e d i n G r e a t B r i t a in



8 2 M . R . WILLIAMSON, JAW-KAI W ANG

EXISTING HARVESTING M ETHOD

T r a d i t io n a l l y , a t e a m o f f o u r t o s ix w o r k e r s w i t h a l o n g s e in e n e t e n t e r s t h e p r a w n

p o n d , l a y i n g t h e n e t a c r o s s t h e w i d t h w i t h a m a n o n e a c h e n d . T h e n e t i s p u l l ed s l o w l y

u p t h e p o n d b y t h e w o r k e r s w h o a re s p a c e d a l o n g t h e n e t a n d p u ll w i t h t h e i r f e e t

h o o k e d i n t h e m e s h a r o u n d t h e b o t t o m l i n e t o k e e p t h e b o t t o m l i n e d o w n . A t t h e e n d

o f t h e r u n , t h e n e t is w o r k e d a r o u n d t h e b a n k t o f o r m a c i rc l e w h i c h is s l o w l y c l o s ed .

T h e a n im a l s a r e t h e n g a th e r e d i n to a c o l l e c t i o n b a g . On c e i n t h e b a g , t h e h a r v e s t is

t r a n s f e r re d t o b a s k e t s w i t h s c o o p n e t s a n d h a n d s o r t e d . T h e a n i m a l s a r e t h e n e m p t i e d

i n t o i c e ch e s t s o r w a t e r t a n k s f o r t r a n s f e r t o e i t h e r a p r o c e ss i n g p l a n t o r m a r k e t .

D i s a d v a n t a g e s o f t h is m e t h o d i n c l u d e :

1 . L o w c a p t u r e e f f i c i en c y .

2 . H i g h l a b o r r e q u i r e m e n t s .

3 . H a r d a n d t e d i o u s w o r k .

4 . D a m a g e t o t h e p r o d u c t .

L o w c a p t u r e e f f i c i e n c y i s r e a d i ly o b s e r v e d . W h e n a s e c o n d h a r v e s t is p e r f o r m e d o n

t h e s a m e d a y , t h e s e c o n d y i e l d o f t e n e x c e e d s t h a t o f th e f i rs t h a r v e s t. T h e e x i s t e n c e o f

l a rg e a n im a l s o f 1 2 0 g o r m o r e a l s o i n d i c a t e s t h a t t h e s e p r a w n h a v e e s c a p e d c a p tu r e

d u r i n g p r e v i o u s ha r v es t s. T h e b e s t v a l u e f o r c a p t u r e e f f i c i e n c y e s t im a t e d b y P o l o v i n o

a n d B r o w n ( 1 9 7 8 ) w a s 6 0 % . T h i s l o w e f f i c i e n c y is t h o u g h t t o b e a t t r i b u t a b l e t o a n

i n e f f e c t i v e s e a l b e t w e e n t h e n e t a n d t h e p o n d b o t t o m .

L a b o r u t i l i z a ti o n is p o o r w h e n w o r k e r s ar e u s e d t o p r o v i d e th e m o t i v e p o w e r f o rt h e n e t . A o n e - h e c t a r e p o n d w i ll t y p i c a l l y u s e 1 0 t o 1 2 m a n - h o u r s p e r h a rv e s t . T h e

m i n i m u m c r e w is f o u r w o r k e r s u n l e s s th e p o n d s a r e s m a ll .

A t l ea st t w o w o r k e r s , c al le d th e b a n k m e n , m u s t p r o c e e d a r o u n d t h e p o n d p e r i m e t e r

o n t h e i r h a n d s a n d k n e e s , f l u s h in g a n im a l s o u t o f t h e g r a ss . P u l l i n g a we ig h t e d l i n e

t h r o u g h m u d w h i l e i m m e r s e d i n p o n d w a t e r is d i f f ic u l t w o r k .

As a r e s u l t , t h e r e i s a n i n d u s t r y - w id e f e e l i n g t h a t p r e s e n t h a r v e s t m e th o d s a r e n o t

s a t i s f a c t o r y a n d t h e r e i s a n e e d f o r r e se a r c h a n d d e v e l o p m e n t o f a n e w h a r v e s t i n g

s y s t e m .

OPERATIONAL AND DESIGN REQUIREMENTS

Operational requirements1 . E f f i c i e n t c o l l e c t i o n o f t h e a n im a l s .

2 . R e d u c t i o n i n n u m b e r o f w o r k e rs r e q ui re d .

3 . R e d u c t i o n i n t o t a l h a rv e s t t im e p e r u n i t a r e a.

4 . E l i m i n a t i o n o r r e d u c t i o n o f t i m e w o r k e r s a re im m e r s e d i n p o n d w a t e r .

5 . M i n i m u m d a m a g e to t h e p r o d u c t .

6 . I m p r o v e d l iv e g r a d in g / s i z e s e l e c t i o n in th e p o n d .



AN IMPROVED HARVESTING NET FOR FRESHWATER PRAWNS 83

D esign cr i ter ia

A basic requirement o f a mec hanical ly operated harves t net i s it s abil i ty to m aintain

a t ight sea t betw een the net ' s bo t tom l ine and the surface o f the pon d bank and

bot tom. A force analys i s of a net sys tem has been performed and a s impl i f i ed analys i s

i s sho wn in Figs 1 and 2 .

F = j ( F H + fH )2 + ( 3 W } 2

F1 = v F H2+ ( 3W ) 2

F2 = / FH + (2W ) z

F5 = v 1 FH2+ W 2

H Hitch F4 : v'/ FHFH

gH 4- fH fH I watersurface . C

' 8 t 3 w

F. ~ Fz 2W Net ',

3 W ~ w FH m'"'

2W { FH ~'W B

Fig. l. Vertical c r o s s - s e c t i o n a l f o r c e d i a g r a m .

/•1•ottom Drag in e

LeadWeights

~ . 4 F 4

F i g . 2 . T o p v i e w o f b o t t o m d r a g l in e .

Lead weights can be added to the Bot tom Drag Line (BDL) of the net unt i l i t

provides a s t rong seal between the pond bank and bot tom, and the BDL. The res i s t -

ance force encou ntered by the BD L can be empirically determined and that wi ll be the

value O f F H , w hi ch i s one o f t he hor i zont a l com pon ent s o f t he pulling f orce a t po i n t B .

Another contr ibutor to the horizontal pul l ing force comes f rom the force required to

over com e the res is tance encou ntered by the f loat s and net ting mo ving through the

water , f H . Toget her , f H and F det erm i ne t he hor i zont a l com ponent o f t he pu l l i ng

force .



84 M.R. WILLIAMSON, JAW- KAI WANG

T h e a n g le 0 , w h i c h is d e t e r m i n e d b y t h e v e r ti c a l a n d t h e h o r i z o n t a l c o m p o n e n t s o f

t h e p u l l i n g f o r c e m u s t s a t i s f y t w o c o n d i t i o n s :

1 . A t p o i n t D , t h e v e r ti c a l c o m p o n e n t o f t h e p u l li n g fo r c e m u s t b e e q u a l t o th ee f f e c t i v e l e a d w e ig h t s o t h a t t h e r e i s n o r e s id u a l v e r t i c a l f o r c e a p p l i e d a t B t o

r e d u c e t h e s e al in g b e t w e e n t h e B D L a n d t h e p o n d b o t t o m . I n a c t u a l d e s ig n , i t is

b e t t e r t o l e t th e l e a d w e i g h t a t D e x c e e d t h e v e r ti c a l c o m p o n e n t o f D E . T h is

e x c e s s d o w n w a r d f o r c e w i ll i n c r e a s e t h e h o r i z o n t a l p u l l i n g f o r c e , b u t w i ll i n s u r e

a good sea l .

2 . T h e a n g le 0 m u s t m e e t t h e g e o m e t r i c a l r e q u i r e m e n t t h a t t h e h i t c h h e i g h t m a t c h e s

t h e h i t c h p o i n t o f t h e p u l l in g v e h ic l e.

T o m e e t t h e se t w o c r i te r i a, b o t h t h e l e n g t h O H a n d t h e n u m b e r o f l e ad w e i g ht s

u s e d c a n b e a d ju s t e d .I n a c tu a l d e s ig n , a t h r e e - d im e n s io n a l f o r c e b a l a n c e is r e q u i r e d t o i n su r e a g o o d

s ea l a g ai n st t h e p o n d b a n k a s w e l l, a n d t h e g e n e ra l p r o c e d u r e d e s c r ib e d a b o v e m u s t b e

e x p a n d e d t o m e e t t h is n e e d .

Im pro ved harvesting net design (Fig. 3}

U s i n g t h e f o r c e a n a l ys i s a b o v e , a m e c h a n i z e d h a r v e s t in g n e t h a s b ee n d e s ig n e d a n d

c o n s t r u c t e d . T h e n e t h a s a t h r e e e l e m e n t l i n e s y s t e m t o t r a n s f e r t h e p u l l i n g f o r c e s

a p p l ie d b y t o w i n g v e h ic l e s o n t h e b a n k s t o t h e n e t i n t h e s a m e p l a n e a s t h e p o n d

' ~ o t t o m . T h e t h r e e e l e m e n t s a r e t h e t o w l i n e , t h e i n t e r m e d i a t e l i n e s , a n d t h e b o t t o m

drag l ine .T h e t o w l in e i s m a d e o f 1 2 .7 m m ( ½-in ) b r a i d e d d a c r o n w i t h 4 0 0 0 k g ( 8 8 0 0 l b)

b r e a k i n g s t re n g t h . T h e p u l l in g f o r c e s a re t r a n s m i t t e d f r o m t h e t o w l in e t o t h e n e t b y

p a i rs o f i n t e r m e d ia t e l in e s . E a c h p a i r o f i n t e r m e d ia t e l i n es is m a d e o f a f l o a t i n g li n e o n

th e t o p e d g e o f t h e n e t a n d a s in k in g li n e t h a t i s a t t a c h e d t o t h e w e ig h t e d l i n e , w h ic h i s

w e ig h t e d n e a r i t s l o w e r e n d . T h e s e p a i r s o f i n t e r m e d ia t e l i n e s a r e a t t a c h e d t o a t o w

l in e a t 3 - m ( 1 0 f t ) i n t e r v a l s w i th q u i c k r e l e a s e s n a p c o n n e c to r s .

T h e b o t t o m d r ag l in e o f 1 2 -7 m m (½ i n ) b r a id e d d a c r o n is w e i g h t e d a t i n te r v a ls o f

2 0 - 5 c m ( 8 i n ) b y t u b u l a r l e a d w e ig h t s o f 1 5 0 g ( 5 - 3 3 o z ) . T h e i n t e r m e d ia t e l i n es a r e

a t t a c h e d t o t h e b o t t o m l in e a t 3 . 6 - m ( 1 2 f t ) i n t er v a ls w h i c h is l a rg e r t h a n t h e t o w l in e

s p a c in g o f 3 m ( 1 0 ft ) . T h i s a l lo w s t h e b o t t o m d r a g li n e t o f o r m a s e ri e s o f l o c a l i z e d

c a t e n a r y c u r v e s w h ic h g r e a t l y im p r o v e s i t s v e r t i c a l f l e x ib i l i t y .

A m o n o f i l a m e n t n e t o f 3 . 8 c m ( 1 2 i n ) s t re t c h e d m e s h s iz e is a t t a c h e d b e t w e e n t h e

f l o a t li n e a n d t h e B D L w i t h a w o r k i n g d e p t h o f u p t o 2 m ( 6 -5 ft ) . A s i m p l e r e c t a n g u l a r

f r a m e o f P V C p ip e i s s e w n in to t h e n e t a t t h e c e n t e r c a t e n a r y c u r v e 0 -5 m ( 1.5 f t )

a b o v e t h e b o t t o m t o f o r m t h e t a r g e t e n t r a n c e t o w a r d w h i c h t h e a n i m a l s a r e h e r d e d

b y t h e f o r w a r d m o v e m e n t o f t h e n e t h a rv e s t e r. A t h a r v e st t i m e , a g r a d i n g b o x i s

a t t a c h e d t o t h e r e c t a n g u l a r p i p e f r a m e .

A m e c h a n i z e d n e t f o r u s e i n p o n d s o f 4 5 m ( 1 4 8 f t ) o r l es s i n w i d t h h a s t h e

f o l l o w i n g d i m e n s i o n s :



A N I M P R O V E D H A R V E S T I N G N E T F O R F R E S H W A T E R P R A W N S 8 5

F i g . 3 . Im prov e d ne t sy s te m. ( a ) R e a d y fo r t r a nspo r t , ne t a nd l i nes go i n box ; (b ) d ry s e t up ;(c ) f i rs t s tage se t up in dry po nd ; (d) col lec t io n bo x; (e ) imp rov ed box for l a rge r pon ds ; ( f ) en te r ing

t he pon d ; (g) p rog re ss i ng down t he po nd ; (h ) r e su l t .

(a )

(b )



G'

i

iJaa I



AN IMPROVED HARVEST ING NET FOR FRESHWATER PRAWNS 87

( f )

( g )

( h )



88 M.R. WILLIAMSON, JAW-KAI WANG

T o w l in e 7 7 m ( 2 5 0 f t )

B o t t o m l in e 6 1 m ( 2 0 0 f t )

F l o a t l in e 6 1 m ( 2 0 0 f t )

I n t e r m e d i a t e l in e s r a n g e f r o m 6 .7 m ( 2 2 f t ) a t t h e c e n t e r o f t h e n e t t o 3 - 3 5 m ( 11 ft )

c lo s e t o t h e b a n k s .

OPERATION OF THE MECHANIZED NET

T h e c o m p l e t e m e c h a n i z e d n e t is m a d e i n f o u r s e p a r at e p a r t s f o r e a s y tr a n s p o r t : t o w

l ine , i n t e r m e d ia t e l i n es i n p ai r s , b o t t o m l i n e a n d n e t a s s e m b ly , a n d t h e g r a d in g b o x .

U p o n a rr iv a l a t t h e p o n d t h e t o w l in e i s l a id o u t a n d a t t a c h e d t o t w o t o w i n g v e h ic l es ,

o n e o n e i t h e r s i d e o f t h e p o n d . I n t e r m e d i a t e l in e s a re c o n n e c t e d t o t h e t o w l in e w h i c his m o v e d f o r w a r d t o c l e ar t h e b a n k . T h e n e t i s l a id o u t a l o n g t h e b a n k a n d c o n n e c t e d

t o t h e i n t e r m e d i a t e l in e s. T h e c o l l e c ti o n - g r a d i n g b o x i s a t t a c h e d t o t h e f r a m e i n t h e

c e n t e r o f t h e n e t . T h e u n i t is n o w r e a d y f o r h a rv e s t in g .

T h e t w o v e h i c l e s p r o c e e d s l o w l y a t 1 k m / h a l o n g th e b a n k s p u l l in g f i r st t h e t o w l in e

t h e n t h e n e t a n d f i n al l y t h e g r a d i n g b o x i n t o t h e w a t e r . A p e r s o n a c t i n g a s n e t c o o r d i -

n a t o r e a s es t h e b o x d o w n t h e b a n k a n d w a t c h e s f o r o b s tr u c t i o n s w h i c h m i g h t d am a g e

th e n e t .

T h e b o t t o m d r a g l in e a n d f l o a t l i n e f o r m a s e ri es o f s m a l l c a t e n a r y c u r v e s a s t h e

u n i t p r o c e e d s t o s w e e p t h e p o n d . U p o n a r ri v al a t th e e n d o f t h e r u n , w h e n t h e o u t e r

l e a d in g e d g e s o f t h e n e t h a v e r e a c h e d t h e c o r n e r s o f t h e p o n d , t h e v e h ic l e s s t o p p u l l in ga n d t h e t o w l in e i s d i s c o n n e c t e d . T h e n e t is d r a w n t o w a r d t h e b a n k a n d o u t o f th e

w a t e r , w o r k i n g f r o m t h e o u t s i d e i n t o t h e c e n t e r , r e m o v i n g t h e i n t e r m e d i a t e l i n es a n d

h e r d i n g a n i m a l s t o w a r d t h e g r a d i n g b o x . W h e n a l l t h e a n i m a l s h a v e b e e n s h a k e n i n t o

t h e b o x , a d o o r i s f i t t e d a n d t h e b o x is d i s c o n n e c t e d f r o m t h e n e t f r a m e a n d i s r e le a s ed

o n a t e t h e r i n t o t h e p o n d .

T h e m e c h a n i z e d n e t c o m p o n e n t s a r e e a si ly s t ac k e d o r m o v e d t o t h e n e x t s c h e d u l ed

p o n d w i t h a n e w b o x i n p l a c e. A l l li n es a r e c o l o r c o d e d f o r e a sy i d e n t i f ic a t i o n a n d a ll

c o n n e c t i o n s a r e o f t h e q u i c k a t t a c h / re l e a s e t y p e . T h e m e c h a n i z e d n e t e m p h a s i z e s e a se

o f h a n d l i n g t o s p e e d u p t h e t o t a l h a r v e s t in g p r o c e s s a n d r e d u c e w o r k e r f a t ig u e .

T h e u s e o f t h e g r a d i n g b o x p r o v i d e s t h e p r o d u c t i o n m a n a g e r w i t h se v er al a l t er n a t e

s t r a t e g ie s i n d i s p o s in g o f h i s c r o p .

1 . I m m e d i a t e r e m o v a l o f t h e u n g r a d e d h a r v e s t i n t h e b o x b y m e c h a n i c a l l if t i n t o a

t r u ck m o u n t e d t a nk .

2 . I n - p o n d g r a d in g i n to two o r t h r e e s i z e c l a s s e s w i th s a m e d a y r e m o v a l t o t h e

p r o c e s s in g p l a n t .

3 . H o l d i n g o v e r n i g h t i n th e p o n d f o r r e m o v a l e a r l y t h e n e x t d a y , e i t h e r g r a d e d o r

n o t , t o m e e t a n e a r ly m a r k e t c o m m i t m e n t .

I n - p o n d g r a d i n g s im p l y r e q u ir e s t h a t w h e n t h e a n i m a l s a r e s a f e ly c o n f i n e d in th e b o x

th e f a ls e f l o o r s b e r a i se d . T h e a n im a l s w i ll b e o n t h e t o p f l o o r a n d w i ll b e g in t o se gr e-




g a te b y s iz e b y a t t e m p t i n g t o pa s s d o w n w a r d b e t w e e n t h e b a r s. S m a l l a n d m e d i u m -

s i zed a n ima l s w i ll pa s s t h ro ugh the f i r st f loo r , 2 -54 cm (1 i n ) spacing , wh i l e on ly sm a l l

an ima l s w i ll pa s s t h rou gh the s econd f l oo r , 2 . 23 cm (0 .94 i n ) spac ing , i n to t he b o t t o m

o f t h e b o x . N o n - m a r k e t s i z e a n d m o s t s o f t m o u l t i n g a n i m a l s c a n r e t u r n t o t h e p o n d

th roug h the bo t t o m o r s ides . T he s eg rega t ion i n to s iz e c l as se s imp roves t he l onge r t he

an ima l s a r e l e f t i n t he f l oa t i ng bo x . E xpe r i enc e ha s show n qua l i t y and su rv iva l r em a in

h i g h f o r u p t o 2 4 h , p r o v i d e d t h e b o x i s n o t o v e r l o a d e d w i t h a n im a l s. T h e p r o t o t y p e

box wh ich is 2 . 56 m (10 0 in ) l ong , 1 m (39 i n ) w ide and 1 m deep , ho ld s 100 kg

(220 l b ) o f an ima l s . The ha rves t i ng sy s t em i s env i s ioned t o have seve ra l g r ad ing box es

fo r e ach ne t a s sembly .

RESULTS OF PR ELIMINARY EXPERIMENTS

S e v er al tr ia l r u n s i n a n e x p e r i m e n t a l 1 0 m ( 3 2 . 5 f t ) w i d e p o n d w e r e p e r f o r m e d w i t h a

p r o t o t y p e m e c h a n i z e d n e t . T h e p r o c e d u r e i n v o lv e d r u n n i n g th e u n i t th r o u g h t h e p o n d

twice fo r e ach t r i a l . Af t e r e ach pa s s , t he cap tu red an ima l s we re he ld i n t anks . The

p o n d w a s d r a i n e d a f t e r t h e s e c o n d p a s s a n d a n y r e m a i n i n g a n i m a l s w e r e c o u n t e d .

The cap tu re e f f ic i ency , o r t he nu m ber o f an ima l s c augh t on a si ng le run , was exp re s sed

a s a p e r c e n t a g e o f t h e t o t a l n u m b e r o f m a r k e t s iz e a ni m a l s i n th e p o n d p r i o r t o t h e

run , and was cons i s t en t l y be tween 56 and 60%.

A fu l l - s ize mechanized ne t was bui l t for t r ia l s a t th ree la rge commerc ia l fa rms. S ince

i t is no t pos s ib l e t o d r a in and cou n t an ima l s i n comm erc i a l pond s , r e su lt s a r e ba sed onp rev ious ha rves ts p lu s t he f a rm er s expec t e d y i e ld s u s ing hand ha rves t i ng ne t m e tho ds .

In a l l cases the ac tua l harves t exceeded the an t ic ipa ted y ie ld (Table 1) , ind ica t ing

t h is m e t h o d m a y b e m o r e e f f ic i e n t a t c a p t u r in g t h e a n i m a l s t h a n t h e c u r r e n t m e t h o d s .

Add i t i ona l t r a in ing o f t he ha rves t c r ews i n t he new t echn iques shou ld r educe work ing

t imes b y a t l e a s t 30%.

TABLE 1

Results

A t e a Hand harv es t Ex p e c t e d T i me s ince Ac t u a l Harv e s te r(acres) t im e required yie ld last harvest y ie M t im e required

(h) (kg) (kg) (h)

1-0 1-5 50 New pon d 91 0.752.0 2.0 25 5 day s 90 1.251.8 3.0 114 23 day s 272 1.25

ADVANTAGES OF THE MECHANIZED HARVESTING NET

Direc t bene f i t s :

1 . Re duc ed l abo r cos t s .



9 0 M . R . WILLIAMSON,JAW-KAIWANG

2. Reduced t ime r equ i r ed t o ha rves t a un i t a r ea .

3 . R e d u c e d w o r k e r fa t ig u e a n d d i s c o m f o r t .

I nd i r ec t bene f i t s i nc lude t he ab i l i t y o f t he manage r t o :

1 . M ee t sudden i nc r ea se s i n dem and by ha rves t i ng ex t r a po nds a t sho r t no t i c e .

2 . Ho ld an im a l s ove rn igh t f o r e a r ly de l i ve ry t o a i rpo r t s , e t c . , w i th l i t t l e mor t a l i t y .

3 . I n v e n t o r y t h e p o p u l a t i o n i n a g i ve n p o n d a n d t r a n s fe r s to c k b e t w e e n p o n d s in

o rde r t o o p t im ize t he en t i r e s e t o f pon ds a s a g row -ou t sy s t em.

4 . Ma in t a in op t im a l b iomas s wh en e f f i c i en t cu l li ng is u sed wi th m u l t i p l e r e s tock ing .

S U M M A R Y " A N D R E C O M M E N D A T I O N S

A m echa n ized ha rves t i ng ne t ha s been de s igned and a fu ll -s ca le p ro to t yp e cons t ru c t ed

f o r t h e H a w a i ia n p r a w n i n d u s t r y .

T e s t r u n s i n c o m m e r c i a l p o n d s h a v e s h o w n c o n s i d e r a b l e p r o m i s e . T h e m e c h a n i z e d

n e t i s c u r r e n t ly b e i n g u t il iz e d b y f a r m e r s an d d e m o n s t r a t e s s ig n i fi c an t i m p r o v e m e n t

o v e r t h e e x i st in g h a r v e s ti n g m e t h o d s i n l a b o r s a v ed an d t i m e t a k e n t o c o m p l e t e a

ha rves t .

More s i gn if i can t l y , the new ha rves t equ ipm en t w i l l m ake a l t e rna t e p rod uc t io n

m a n a g e m e n t s t ra t eg i e s a v a il ab l e to t h e p r o d u c t i o n m a n a g e r so t h a t h e c an b e t t e r c o p e

w i t h t h e u n c e r t a i n ti e s o f m a r k e t d e m a n d , u n p r e d i c t a b le w e a t h e r c o n d i ti o n s , av a il ab l e

supp ly o f s t ock ing ma te r i a l s , e t c . The ab i l i t y t o ha rves t qu ick ly , e f fi c i en t l y and a tm i n i m u m c o s t c a n b e a n i m p o r t a n t e c o n o m i c a d v a n t a g e .

The fo l lowing r eco m m end a t ion s a r e sugges t ed t o ensu re the be s t r e su l ts o f t he

m e c h a n i z e d n e t .

1 . The ponds shou ld be de s igned and cons t ruc t ed t o su i t mechan i zed ha rves t i ng .

2 . A l l pond s shou ld be o f s imi l ar s iz e and shape wi th in t he l im i t s o f t opo g rap hy ,

e tc .

3 . P o n d s s h o u l d b e k e p t w e l l m a i n t a i n e d w i t h t r i m m e d b a n k s a n d m i n i m u m s ilt

bu i l dup .

4 . The ha rves t c r ew shou ld be we l l t r a ined and mo t iva t ed t o u se t he ha rdwareco r r ec t l y .

5 . T h e h a r v e s t e r s h o u l d b e i n t e g r a t e d i n t o t h e p r o d u c t i o n s y s t e m b y m a n a g e m e n t

in s t ead o f s imp ly r ep l ac ing t r ad i t i ona l ha rves t i ng me thods .

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 s a re g r a t e f u l t o M r J o h n C o r b i n o f t h e A q u a c u l t u r e D e v e l o p m e n t P r o g ra m ,

D e p a r t m e n t o f P l a n n in g a n d E c o n o m i c D e v e l o p m e n t , S ta t e o f H a w a i i ( G r a n t N o .




10998) and to the Hawaii Office of the National Seagrant Office for financial support

(Gra nt No. 04-8-M01-178).

The pra wn farmers of Hawaii have cont rib ute d generously of their time and facilities

to promote this work.

REFERENCES

Gibson, Richard T. & Wang, Jaw-Kai (1977). An alternative prawn production systems design inHawaii. Sea Grant TechnicaI R eport UNIHI-SEA GRANT- TR- 77-05 , May 1977.

Polovina, J. & Brown, H. (1978). Population dynamics and production economics. In: Proceedingsof the Ninth A nnual Meeting o f the World Mariculture Society, pp. 393-404.

State of Hawaii, State Center for Science Policy and Technology Assessment (1978). Aquaculturedevelopment for Hawaii: Assessments and recommendations. Report by the Aquaculture Plan-

ning Program, Center for Science Policy and Technology Assessment. Honolulu: Dept. ofPlanning and Economic Development.

WiUiamson, Michael R. & Wang, Jaw-Kai (1978). Mechanical separation of fresh water prawns.American Society of Agricultural Engineers, Paper No. 78-5035.

Williamson 1982 Aquacultural-Engineering

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