cain jr. 1983 aquacultural engineering

7/28/2019 Cain Jr. 1983 Aquacultural Engineering

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Aquacu l tu ra l Eng ineer ing 2 (1983) 135-152

E v a l u a t i o n o f a B o a t - M o u n t e d E l e c t r o - T r a w l a s aC o m m e r c i a l H a r v e s t i n g S y s t e m f o r C r a w f i s h *

C . D e a n C a i n , J r . a n d J a m e s W . A v a u l t , J r .

School of Forestry and Wildlife Management, Louisiana Agricultural ExperimentStation, Louisiana State University Agricultural Center, Baton Rouge,Louisiana 70803, USA

A B S T R A C TFie ld e xpe r im en t s wer e cond uc t e d t o a sce rta in i f t rawl ing u t i l iz i ng e lec t ri -c a l c u r r e n t s w a s a n e f f e c t i v e m e t h o d o f h a r v es t in g c r a w f is h .

Resu l t s sugges t t ha t ca t ch e f f i c i ency r e la t i v e t o vege ta t i on dens i t y i sm o r e d e p e n d e n t u p o n t r a w l m e c h a n ic a l a b i li t y th a n o n r e l at iv e c r a w f is habundance . Ca tch i s a lso dep end en t upo n t rawl ing speed , wa te r dep th andt im e o f t rawl ing . Trawl ing cons i s t en t l y y i e ld ed h igher ca t ches pe r a reathan d id conv en t ion a l t raps. Unm arke tab l e i nd i v idua l s and m or ta l i t y weren o d i f f e r e n t f o r t h e t w o g e ar ty p e s .

T r a w l i n g c o u l d b e a n e f f e c t i v e s u p p l e m e n t a l h a r v e s t i n g m e t h o d , b u tadd i t i ona l w or k n eeds t o b e pe r fo rm ed to a l le v ia te e lec t r ica l and mechan i -ca l p ro b l em s be fo re eco nom ic e f f i c i en cy can be r ea lis ti c.

INTRODUCTIONPresent methods of harvesting crawfish P r o c a r n b a r u s spp. with passivegear involving traps and bait are inefficient. Traps and bait are expen-sive, and bait requires freezer space for storage and labor to cut it up,and preferred baits are not always available during the season. Normally,a professional trapper is hired for up to a 50% share of the gross* Funded by the office of Sea Grant NOAA Department of Commerce and theLouisiana Agricultural Experiment Station.135Aquacu l tu ra l Eng ineer ing 0144-8609/83/$03.00 Applied Science Publishers Ltd,England, 1983. Printed in Great Britain


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136 C. O . Ca in , J r . , J . W. A va u l t , J r .revenue. Moreover, there is often a shortage of trappers who will staywith one farmer during the season after the wild crop in the AtchafalayaBasin in Louisiana becomes available.Active methods of crawfish harvesting with trawls would require notraps or bait and therefore would alleviate associated costs. Varioustrawls and push-nets have been designed for sampling and commercialpurposes. Massman e t a l . (1952) described a modified shrimp net whichrequired two boats to pull it. Daswell (1975) described a small ottertrawl which effectively collected benthic organisms in lakes at depthsbetween 5 and 100 m, but it required two operators . Surface trawlswhich could be pulled by one boat were described by Chapoton (1964)and Trend (1967). However, none of these trawls are efficient for use inheavily vegetated waters less than 1 m deep, and they cannot bemaneuvered easily.

Ellertsen (1978) described a modif ied plankton trawl that was pushedin front of a small boat. This allowed for an undisturbed sample sinceorganisms were not affected b y the m otor 's propellor. However, thisnet could not be used in shallow waters less than 90 cm deep.

Push-nets used for shrimp and small fishes were described by deSylva (1954), Strawn (1954), Allen and Inglis (1958) and Tabb andKenny (1968). These nets were used in areas of moderate to heavyvegetation. Herke (1969) devised a push-trawl which proved effectivein estuarine sampling. When pushed by an airboat it could be easilymaneuvered and used in areas having less than 30 cm of water. Experi-ments on harvesting crawfish with a push-trawl have been conductedat Louisiana State University (by L. Buchart, July 1979).

Since shallow, heavily-vegetated ponds are not suited for conventionaltrawling methods, harvesting methods must be altered to suit conditions.Previous workers have found that in many cases electrical stimulationincorporated into trawling systems greatly increases catch efficiency.Kuroki (1959) recommended it be used only for underutilized fisheriesand not for those in danger of over-exploitat ion. However , problemsinvolving a large power supply, complex equipment design and highcosts have kept the application of electric trawling at a modest leveldespite increased fishing efficiency.

In crustacean fisheries, electric trawling systems for penaeid shrimphave been most extensively used experimentally ill the US (Pease, 1967;Pease and Seidel, 1967) and commercially in Japan (Ko and Kim, 1970;


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Commercial electro-trawl Jbr crawfish 137Ko et al., 1972). Catch efficiency of lobsters, which react similarly asdo crawfish to electrical currents, was enhanced when an energized elec-trode array was used in conjunction with a trawl net (Williams, 1971:Saila and Williams, 1972). Stewart (1975) was successful in increasingthe capture rate of the mud-dwelling Norway lobster Nephrops norvegi-cus by incorporating an electrode system anterior to the groundrope ofa beam trawl.

This paper reports the second part of a study conducted at LouisianaState University to observe the behavior of crawfish in a pulsed DCcurrent field and an evaluation of a boat-mounted electro-trawl as acrawfish harvesting system. The study objectives were to: (1) determinewhat c ombi nati on of boat speed and vegetation density maximizedcatch of marketable crawfish, but minimized mort alit y: and (2) com-pare the crawfish harvesting efficiency of push-trawl to traps.

MATERIALS AND METHODS

F i e ld f a c il it ie s a n d e q u i p m e n tTrawling experiments were conducted at two locations from 29 Marchto 26 May 1981. Two ponds were located at Ben Hur Farm near BatonRouge, Louisiana. The ponds (B-1 and B-3) had surface areas of 0.97and 0.84 ha, respectively, and an average depth of 0.8 m. Vegetation inpond B-1 was 75% rice Oryza sativa which had been planted in June asa forage, and 20% three-cornered sedge Cyperus spp. SmartweedPolygonurn spp. comprise d most of the remainder. Pond B-3 vegeta tionconsisted of equal amounts of rice, vetch Vic& spp., sugar caneSaccharurn officinarum, soybean Soja max, and sorghum Holcussorghum with sparse smartweed throughout.

A 22 ha commercial crawfish pond (pond A-l) with an average depthof 0.7 m was located in Hend erso n, Louisiana. White crawfish Pro-cambarus acutus acutus was the predominant crawfish species. Vegeta-tion was over 90% alligator weed Alternanthera philoxeroides withmixed smartweed and waterprimrose Ludwig& peploides comprisingmost of the remainder.


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138 C. D . Ca in , J r . , J . W. A va u l t , J r .Trawling system descriptionMechanical configurationA standard 4.3 m aluminum boat (Ouachita model 140W40) poweredby an 8-hp vertical shaft, weedless propellor engine (Go Devil model 8)was used. Figure 1 shows the basic design of the boat and trawl design.

The bow-mounted push-trawl frame, electrode extenders and braceswere constru cted from 2.54 cm 2 stock steel. The trawl m ou th measured2.4 by 1.2 m. Rec tangul ar 25 by 15 cm skid shoes welded on to thebottom of the trawl provided support in case the trawl contacted thepond bottom during operation. A 30-cm stock steel arm was attachedvertically to each side of the boat by welding the arm to a 15 by 10 by0-6 cm steel plate. The plate was bolted in four corners to a plate of thesame dimensions inside the boat with 8 by 1 cm mach ine bolts. Themain trawl frame was attached to each vertical arm and could pivotfrom this point.

A two-panel balloon-type net with 10 mm diameter headline andfootrope was tied to the frame mouth at each corner of the frame. Thefront body was constr ucted from 5 cm stretched mesh size 9 nylonnetting, and the bag was constructed from 2.5 cm stretched mesh size

W I N C HP O L EELECTRODEWNCH

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Fig. 1. Boat and trawling system used in the field study. Electrical components(except anode array) have been omitted.


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Co mmercial electro-trawl for crawfish 1395 n y l o n n e t ti n g . A 6 1 c m h e a v y d u t y z i p p e r s e w n t o th e c o d e n da l l o w e d t h e c a t c h t o b e r e m o v e d e a si ly . T h e n e t w a s h e ld o p e n d u r in gt ra w l i n g b y s ix f o a m p o l y e t h y l e n e f l o a ts 1 0 c m in d i a m e t e r t ie d o n t h et o p p a n e l a n d s ix 2 8 g c y l in d r i c a l s p l it l e a d s c r i m p e d o n t o t h e f o o t -r o p e . T h e n e t w a s t r e a t e d w i t h a s y n t h e t i c - b a s e p l a s ti c n e t c o a t i n g .Electrical syste mF i g u r e 2 s h o w s t h e l o c a t i o n s s e l e c t e d f o r t h e v a r i o u s e l e c t r ic a l c o m -p o n e n t s o f t h e s y s t e m . E l e c t ri c a l p o w e r w a s s u p p l ie d b y a r e v ol v in gf i e ld , s i n gl e p h a s e , 4 0 0 0 W a l t e r n a t o r w i t h a n a i r- c o o l e d g a s o l i n e e n g in e( H o m e l i t e m o d e l E - 4 0 0 0 - 1 A ) .

A v a r ia b l e v o l ta g e p u l s a t o r ( C o f f e l t E l e c t r o n i c s C o m p a n y , I n c .,E n g l e w o o d , C o l o r a d o , m o d e l V V P - 3 2 ) w a s u s e d to p r o d u c e a lo w f re -q u e n c y , D C p u l s e d v o l ta g e v a ri a b le f r o m 7 0 t o 2 7 0 V . T h e p u l s e r u n i tw a s s e t a t 4 - 5 t o t a l a m p s , 4 p u l s e s p e r s, a n d 4 1 . 5 m s . T h e s e p a r a m e t e r sw e r e p r e d e t e r m i n e d f r o m p r e v i o u s l a b o r a t o r y e x p e r im e n t s . E l e ct ri ca ld i sc h a r g es w e r e c o m p o s e d o f a s er ie s o f s p ik e s r e f e rr e d t o a s a t y p e o f' d u a l f r e q u e n c y p u l se ' . A v e r a g e c u r r e n t w a s m o n i t o r e d o n t h e m u l t i-t e s t e r p l a c e d in l in e w i t h t h e p u l s e r u n i t . A r e m o t e s a f e t y f o o t s w i t c hw h i c h c l o s e d th e c i r c u it w h e n d e p r e s s e d w a s p l a c e d t o w a r d t h e b o w .

T h e n e g a t i v e l e a d w a s c o n n e c t e d t o a s in g le c a t h o d e b y e l e c tr i c a lc o p p e r c o n n e c t i o n s . T h e c a t h o d e c o n s i s te d o f tw o , 3 m l e n g th s o f 1 2 md i a m e t e r w o u n d s te e l c a b le . T h e c a t h o d e w a s d r a g g e d b e h i n d t h e b o a td u r i n g t r a w l i n g .

T h e p o s i ti v e l e a d w a s c o n n e c t e d t o a n a n o d e a r r a y b y s iz e 1 2 t e f lo n -c o a t e d e l e c t ri c a l w i re a r r a n g e d in p a r al le l . E a c h w i r e w a s w r a p p e da r o u n d t h e e l e c t r o d e a n d t h e n w a t e r p r o o f e d w i t h e l e c t r i c a l t a p e .

T w o e l e c t r o d e e x t e n d e r s w e r e h in g e d an d c o u l d p i v o t f r o m t h e c ro s sb e a m o f t h e t ra w l . A b r a c e c o n s t r u c t e d f r o m 1 c m 2 s t o c k s t e e l h e l d th ee x t e n d e r s in p la c e . A n e l e c t r o d e f r a m e 2 . 4 m i n l e n g t h c o n s t r u c t e df r o m 2 .5 c m d i a m e t e r , s c h e d u l e 4 0 p o l y v i n y l c h l o ri d e ( P V C ) t u b i n g w a sa t t a c h e d t o t h e e x t e n d e r s b y t w o , 9 c m U b o l t s . F o u r c y l i n d ri c a l e le c -t r o d e s w e r e a t t a c h e d t o t h e e n d o f th e P V C f r a m e . E a c h e l e c t r o d e w a sc o n s t r u c t e d f r o m 1-2 m s t ai n le s s s t e e l r o d s 1 2 m m i n d i a m e t e r . A 6 0b y 15 b y 15 c m s t y r o f o a m p o n t o o n m o u n t e d a b o v e e a c h e n d e le c t r o d e ,p l u s a n a d d i t i o n a l w i n c h s y s t e m , k e p t t h e a r r a y t il t e d s li g h tl y u p w a r d i nr e l a ti o n t o t h e w a t e r s u r f a c e d u r in g t r aw l in g . E a c h p o n t o o n w a ss t a b il iz e d o n t h e P V C f r a m e b y a 1 2 m m d i a m e t e r r o d .


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Co mmercial electro-trawl/or crawfish 141Winch systemThe trawl skid shoes, sliding along the bottom with associated vegeta-tion and mud problems, caused most of the mechanical resistance.However, since electrical currents would force crawfish off the bo tt om,most mechanical resistance could be alleviated by holding the trawl skidshoes several centimeters above the bottom out of the way of mostvegetation and mud. Only water resistance during trawling remained.

A winch system was built to adjust the trawling system to varyingwater depths. Trawl depth was adjusted by a boat trailer winch, and theanode array was adjusted by a secondary boat anchor winch. Eachwinch containe d 6 mm diamet er stranded aircraft cable which wasattached to the trawl by a cable connector. The anchor winch was usedto support an array level during trawling as well as for removing theanode array from th e water during crawfish removal.Field proceduresTrawl handling in vegetationTo test densities of vegetation in which the push-trawl could effectivelybe used, 8 vegetation zones were established in the 22 ha pond on 27March 19 81 :2 with none or little, 2 light, 2 med ium , and 2 heavy.Areas of zones ranged fro m 900 to 1020 m 2. Ten vegetat ion sampleswere take n from each zone at 10 m intervals. Plants root ed within a0.25 m 2 quadr at were pulled up and stalks longer than 35 cm werecounted.

On bo th 29 March and 12 April 1981 the trawl was push ed over thearea of each z one at a speed of about 1.5 m s 1. Unmarke tab le and soft-shell crawfish were removed and returned to the water. Remainingcrawfish were counted and weighed. The relationship between vegeta-tion dens ity and trawl effi ciency based on catch per area was ascertained.Speed testsTo determine optimal trawling speed, each pond at Ben Hur Farm waspartitioned into a shallow and deep zone, with average depths of 0-7and 1.1 m, respectively. Each of these zones c onta ined a 388 m 2 plotthat measured 40 by 9.7 m. This allowed for four 40-m sweeps throughthe plot since the trawl was 2-4 m wide. Nonemergent vegetation waslocat ed in all zones b ut did not a ffect operating efficiency. Each plot


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142 C. D. Cain, Jr., J. W. A vault, Jr.w a s t r a w l e d o n c e p e r d a y f r o m 2 M a y to 5 M a y 1 9 81 a t e a c h o f f o u rs p e e d s : 0 . 5 , 1 . 0, 1 .5 a n d 2 . 0 m s 1 . D a i l y in i t ia l t e s t s p e e d f o r e a c hp l o t w a s a l t e r n a t e d f o r f o u r d a y s so t h a t e a c h p l o t w a s t r a w l e d i n it ia l lya t a l l f o u r s p e e d s .

F o r e a c h t r a w l in g p e r i o d , t r a w l d e p t h a n d e l e c t r o d e le v el w e r ea d j u s t e d . O n e o p e r a t o r g u i d e d t h e b o a t o v e r t h e a r e a o f e a c h p l o t a t ap r e d e t e r m i n e d t h r o t t l e s e t ti n g , w h i l e a n o t h e r o p e r a t o r p r e s s e d t h e f o o ts a f e t y s w i t c h t o c l o s e t h e c i rc u i t, a n d t i m e d e a c h s w e e p to c h e c k s p e e d .

N e w l y - m o l t e d c r a w f i s h a n d u n m a r k e t a b l e c r aw f i sh (le ss t h a n 7 6 m mt o t a l l e n g t h ) w e r e c u l l ed , c o u n t e d a n d w e i g h e d . A ll r e m a i n i n g c r a w f is hw e r e p u t i n t o o n i o n s a c k s a n d h e l d in a c o o l e r a t 4 C . P e r c e n t m o r t a l i t ya f t e r 2 4 h w a s c a l c u l a t e d f o r e a c h p l o t a t e a c h s p e e d .

O p t i m u m s p e e d w a s d e t e r m i n e d , w h e r e c a t c h w a s m a x i m i z e d b u tw i t h a m i n i m u m m o r t a l i ty a n d l ea s t n u m b e r o f u n m a r k e t a b l e c r a w f is h .T h i s s p e e d w a s u s e d fo r c o m p a r i s o n s b e t w e e n t r a p p i n g a n d t ra w l in g .T r a p p i n g v e r s u s t r a w l i n gP o n d s B -1 a n d B -3 a t B e n H u r F a r m w e r e u s e d f o r t r a p p in g a n d tr a w l-in g c o m p a r i s o n s . C r a w f i s h w e r e t r a p p e d i n e a c h p o n d w i t h d o u b l ef u n n e l t r a p s c o n s t r u c t e d f r o m 1 .9 c m h e x a g o n a l m e s h p o u l t r y n e t t in ga n d b a i t e d w i t h a b o u t 2 3 0 g o f c u t g i zz a r d s h a d D o r o s o m a c e p e d i a n u r n .T r a p s w e r e n u m b e r e d in e a c h d e p t h z o n e a n d s e t a t a r a te o f 38 p e r h a .T h e y w e r e c h e c k e d a f te r 1 6 - 2 4 h . C r a w f is h i n e a c h t ra p w e r e c o u n t e d ,w e i g h e d , a n d r e t u r n e d t o t h e w a t e r a t t h e c a p t u r e l o c a t i o n . L e n g t h o fo n e c r a w f i s h f ro m e a c h tr a p w a s r e c o r d e d . T r a p s w e r e t h e n e i t h e rr e m o v e d f r o m t h e w a t e r o r n o t r e b a i t e d .

A f t e r c r a w f is h h a d b e e n a c c l im a t e d t o p o n d c o n d i t io n s f o r 6 - 8 hp o s t - t r a p p i n g , t h e e l e c t r i f i e d t r aw l w a s p u s h e d o v e r t h e a r ea o f e a c hh a l f -z o n e a t t h e p r e d e t e r m i n e d e l e c tr ic a l p a r a m e t e r s a n d s p e e d o f1 .5 m s 1. T h e c a t c h w a s c u l l e d , p u t i n t o s a c k s a n d p e r c e n t m o r t a l i t yw a s c a l c u l a te d in t h e s a m e m a n n e r a s t h a t u s e d in v e g e t a t i o n a n do p t i m u m s p e e d t es ts . L e n g t h s o f 2 0 c r a w f is h f r o m e a c h z o n e w e r er e c o r d e d . N o c r a w f i s h c a u g h t b y t ra w l i n g w e r e r e t u r n e d t o p o n d s .

S a c k s w e r e a ls o fi ll ed w i t h 9 - 2 0 k g o f c ra w f i s h c a u g h t f r o m t h e s a m ep o n d s ( h a r v e s t e d o n o t h e r d a y s ) a n d s t o r e d i n a c o o l e r a t 4 C . P e r c e n tm o r t a l i t y w a s c a l c u la t e d a f t e r 2 4 h.

W a t er t e m p e r a t u r e a n d c o n d u c t i v i t y m e a s u r e m e n t s w e r e t a k e n w i tht h e c o n d u c t i v i t y m e t e r b e f o r e t r a w li n g . S ix r e p e t i t i o n s o f t h e t r a p p in ga n d t ra w l in g s c h e m e w e r e p e r f o r m e d f r o m 8 M a y to 2 6 M a y 1 9 81 .


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Com me rc ia l e l ec t ro - t rawl ]o r c raw f i sh 143S t a t i s t i c a l an a lys i sS t a t i s ti c a l a n a l y s e s o f c a t c h w e r e b y c o m p l e t e l y r a n d o m d e s ig n a n a ly s iso f va r ia n c e w i t h a s p l it - p lo t a r r a n g e m e n t o f t r e a t m e n t s f o r b o t h s p e e dt e s ts a n d t r a p - t r a w l c o m p a r i s o n s . C a t c h b a s e d o n v e g e t a t io n d e n s i t yw a s a n a l y s e d w i t h r e g r e s s i o n a n a l y s i s .

R E S U L T S A N D D I S C U S S I O NT r a w l h a n d l i n g i n v e g e t a t i o nC r a w f i s h c a t c h d e c l i n e d a s v e g e t a t i o n d e n s i t y i n c re a s e d f r o m 4 t o 1 5 6s t a lk s p e r m 2. A h i g h l y s i g n if i c a n t ( P < 0 . 0 1 ) q u a d r a t i c r e l a t i o n s h i pe x i s ts b e t w e e n s ta l k d e n s i t y o f a ll ig a t o r w e e d a n d c a t c h p e r a re a o fc r a w f i s h f r o m p o n d A - 1 in H e n d e r s o n , L o u i s i a n a ( F ig . 3 ). B e s t r e s u l tsw e r e o b t a i n e d in z o n e s w i t h l it t l e o r n o v e g e t a t i o n ( 4 s t a lk s p e r m 2)w h e r e a s m u c h a s 0 .4 k g p e r 1 0 0 m 2 o f c r a w f is h w e r e c a u g h t p e r t ra w l .I n d e n s e s t v e g e t a t i o n c a t c h w a s p o o r e s t - a s l it tl e a s 0 .0 8 k g p e r1 0 0 m 2.

C r a w f i s h o c c u r in t h e g re a t e s t n u m b e r s in s h a l lo w , t u r b i d , f r e s h w a t e rm a r s h e s s u p p o r t e d w i t h a g o o d s t a n d o f v e g e t a t i o n ( P e n n , 1 9 4 3 ) .A l t h o u g h c r a w f is h m a y b e f o u n d a n y w h e r e w i t h i n th i s h a b i t a t , t h e y a reu s u a l ly c o n c e n t r a t e d in d e n s e v e g e t a t i o n w h e r e t h e m o s t f o o d o c c u r s ,a n d t h e y a r e s af e r fr o m p r e d a t o r s . T h e r e f o r e , c a t c h s h o u ld h a ve b e e ng r e a t e s t in d e n s e r v e g e t a t i o n z o n e s a n d le a s t in l it tl e o r n o v e g e t a t i o nz o n e s .

T h e r e v e r s al o f t h is r e l a t io n s h i p in th i s s t u d y i m p l i e s t h a t c a t c he f f ic i e n c y m a y b e m o r e d e p e n d e n t u p o n t h e m e c h a n i c a l a b il i t y o f t h et r a w l i n a c e rt a in z o n e , r a t h e r t h a n u p o n r e l a ti v e a b u n d a n c e o f c r a w -f is h . T r a w l h a n d l i n g a n d m a n e u v e r a b i l i t y w e r e d i f f ic u l t i n z o n e s w i t hd e n s i t i e s o f 1 0 0 - 1 5 6 s t a lk s p e r m 2. A l l ig a t o r w e e d w a s o f t e n m a t t e do n t h e w a t e r s u r f a c e a n d s l o w e d t h e t r a w l s p e e d c o n s i d e r a b l y , s o m e t i m e st o a c o m p l e t e s t o p .

A s v e g e t a t i o n d e n s i t y d e c r e a s e d , t ra w l m a n e u v e r a b i l i t y i n c re a s e d . I nz o n e s o f 3 6 - 1 0 0 s t al k s p e r m 2 t h e s k id s h o e s c o u l d b e l o w e r e d s e v er a lm o r e c e n t i m e t e r s b e n e a t h t h e w a t e r s u r fa c e a n d c o n s e q u e n t l y m o r ec r a w f i s h w o u l d b e c a u g h t . I n li g h t z o n e s o f 8 - 3 5 s t a lk s p e r m 2 v e g e t a -t i o n p r o b l e m s w e r e v i r tu a l ly e l i m i n a t e d .


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144 C . D . C a i n , J r . , J . W . A v a u l t , J r .

0.45

,~ 0.35

0 . 2 5e-

0.15

0.05

I I !0 50 I00 150

D e n s i t y ( s t a l k s / m 2 )Fig. 3. Relationship between stalk density of alligator weed A l t e r n a n t h e r ap h i l o x e r o i d e s and crawfish catch per area from pond A-1 in Henderson, Louisianaon 29 March and 12 April 1981. The regression curve indicates the predicted

average catch at each vegetation density. The regression equation is as followsY = 3 5 .5 5 - - 0 . 3 3 5 X + 0.00106X 2

R 2 = 0.93

Ideally, one would like to maximize crawfish catch while minimizingmecha nical probl ems with the trawl. In view of the above discussion,and since lengths of cr awfish were similar in all zones (P > 0.05) , areasrelatively free of vegetation, preferably adjacent to vegetated areas,should be used for electro-trawling.Speed testsC a t c hHighly significant (P < 0.01) differenc es in catch occ urre d in speed anddepth effects, but not in their interaction. Highest catches were obtainedat 1-5 m s-1 fol low ed by 2-0 m s 1 (Fig. 4). When averaged over days,poor est catc hes wer e at 0.5 m s 1. Catches fro m plots 1 and 3 (deep


11/18

Comm erc ia l elect ro- t raw l or crawf ish 1452 . 0

1 . 5

1 .0

0 . 5

>2 . 0

Fig. 4.

0 . 5 m / s

i 2 3 4P l o t

2 . 0

1 . 5

1 . 0

0 . 5

1 . 0 m / s

i 2 3 4P l o t

1.5

1.0

0 . 5

1 . 5 m / s

,-..,%,

:-:-:-:.

f=-: ~

I",'o'.%

i 2 3 4P l o t

2 .

i .

i .

O .

2 . 0 m / s

1 2 3 4P l o t

Average catch of crawfish at four trawling speeds in four plots frompon ds B-1 and B-3 at Ben Hur farm from 2 May to 5 May 198 1.

z o n e s ) w e r e c o n s i s t e n t l y l es s t h a n c a t c h e s f r o m p l o t s 2 a n d 4 ( s h a l l o wz o n e s ) a t d if f e r e n t s p e e d s .S t e w a r t ( 1 9 7 5 ) t r aw l e d fo r N o r w a y l o b st e r s N ep h r o p s n o r v eg i c u s

f r o m 0 . 7 5 t o 1 .0 m s a , a n d K o a n d K i m ( 1 9 7 0 ) t r a w l e d f o r s h r im pP e n e a u s / a p o n i c u s f r o m l e ss t h a n 0 . 5 t o 2 . 0 m s 2 . T h e l a t t e r w o r k e r s h a dp o o r c a t c h e s a t s p e e d s a r o u n d 1 .5 m s 1. C a t c h e f f i c i e n c y is b a s e d o nm a n y f a c t o r s i n c l u d i n g tr a w l d e s i g n an d d i m e n s i o n s , s p e e d a n d en v i r o n -m e n t a l c o n d i t i o n s ( M a r t y s h e r sk i i a n d K o r o t k o v , 1 9 7 1 ) . T r a w l d e s i g na n d d i m e n s i o n s a re s p e c i f ie d b y t h e t y p e o f f is h p u r s u e d . I n g e n e r al ,t h e l a r g e r t h e t r a w l ( w i d t h a n d h e i g h t o f a p e r t u r e ) r e l a t i v e t o s i z e a n d


12/18

146 C. D. Cain, Jr., J. W. A vault, Jr.speed o f the organism pursued, the more ef ficient it will be at relativelyslow speeds. Because crawfish ponds generally allow for smaller trawlsonly, speed must be increased since trawl apertu re is reduced.Physical conditions cause much variability in catches, even in thesame area (Aron and Collard, 1969). Crawfish catch was significantlyhigher (P < 0-01) in shallow zones (0.6 m) as compared to deep zones(1.2 m) at all speeds (Fig. 5). Regression analysis reveals that a highlysignificant (P < 0.01) quadrat ic relationship exists between water depthand crawfish catch.

Optimum speed was empirically calculated to be 1.8 m s 1 for thedeep zone. In deeper water negative effects of electrofishing efficiencyoccur. Most importantly, the effective electrical field does not reach asfar radially from each point on the electrode (Vibert, 1967; Novotnyand Priegal, 1974). Therefore, crawfish experience a lower currentdensity and may escape before galvanotaxis occurs. Catch may havebeen improved by increasing variables of speed, electrode size or elec-trode voltage. The latter two options pose problems due to limitationsof electrical components and safety hazards (Novotny and Preigal,1974). However, combinations of the above options may produce themost satisfactory results.

In shallow zones, op timum speed was calculated to be 1.5 m s 1.However, a highly significant (P< 0.01) cubic relationship exists inshallow zones which suggests that catches may increase again at higherspeeds. Generally, in shallow waters organisms have less escape area.However, there exists a maximum speed where crawfish have enoughtime to exhibit anodic taxis before the trawl passes over them.U n m a r k e t a b l e c r aw f i s h a n d m o r t a l i t yAlthough an optimum speed can be determined where catch is maxi-mized, numbers of unmarketable crawfish and mortalities should beminimized for optimum efficiency. Aron and Collard (1969) relatecatch effic iency as a function of organism size and escape capabilities.They suggest that larger fish are better able to escape conventionaltrawls towed at lower speeds. Also, undersized fish that are caught bythe trawl usually escape through the properly chosen mesh size due toincreased water force. However, higher flow forces cause highermortality in adult and juvenile cod and herring (Mal'kyavichus e t a l . ,1968).

In the presen t study, less than 1% unmarketable crawfish were caughtand numbers were not significantly different (P > 0.05) at various test


13/18

Comm erc ia l elec t r o - t r aw l o r craw f i sh 1472.0

1.5

I.O

0 . 5

= o

< 2.0

1.5

1 . 0

0 . 5

D E E P( P l o t s I a n d 3 )

I I I I

SRALLOW( P l o t s 2 an d 4)

I I J 10 . 5 1 . 0 1 . 5 2 . 0

Speed ( m / s e e )

F i g . 5 . R e l a t i o n s h i p b e t w e e n t r a w l i n g s p e e d a n d a ve ra g e c r a w f is h c a t c h i n d e e pz o n e s ( p l o t s 1 a n d 3 ) a n d i n s h a l lo w z o n e s ( p l o t s 2 an d 4 ) f r o m p o n d s B - 1 a n d B -3a t B e n H u r f a r m f r o m 2 M a y t o 5 M a y 1 9 8 1 . o , A v e r a g e c a t c h f r o m r e s p e c t i v e p l o t s .R e g r e s s io n e q u a t i o n s :

F o r p l o t s 1 a n d 3 ,

F o r p l o t s 2 a n d 4,

Y =- - 0 . 5 1 8 + t . 638X- O . 46X zR z = 0.93

Y= 1 .162 + 3-508 X-- 1 .194X zR 2 = 0. 87


14/18

148 C. D. Cain, Jr., J. [4]. A va u lt, Jr.s p e e d s . S e l e c t e d m e s h s iz e a l l o w e d m o s t u n h a r v e s t a b l e c r a w f i s h t o p a sst h r o u g h t h e n e t , a n d f e w w e r e r e ta i n e d .

M o r t a l i t y w a s lo w ( 2 % o r l es s) a n d n o t s i g n if i ca n t ly d i f f e r e n t ( P > 0 . 0 5 )a t t e s t s p e e d s . H o w e v e r , t ra w l t i m e s w e r e le s s t h a n 3 r a in , a n d a n i m a l sw e r e n o t s t r e s se d f o r t h is s h o r t ti m e . M o r t a l it i e s a r e p r o b a b l y r e l a t e dm o r e t o e l e c t r i c a l e x p o s u r e t i m e r a t h e r t h a n t o w a t e r f o r c e s .S o f t - s h e l l c r a w f i s hE l e c t r ic a l c u r r e n t d o e s n o t d i s c r im i n a t e b e t w e e n c ra w f i s h in d i f f e r e n ts t a g e s o f e c d y s i s . A l l s t a g e s w e r e o b s e r v e d f r o m t h e c a t c h . C a t c h e s o fs o f t - s h e l l e d c r a w f i s h r a n g e d f r o m 0 to 1 2 i n d i v id u a l s p e r tr a w l a n d w e r en o t s i g ni fi ca n t l y d i f f e r e n t ( P > 0 . 0 5 ) b e t w e e n d i f fe r e n t s p e e d s o rd e p t h s . W h e n c r a w f i s h b e c o m e s o f t , t h e y g e n e r a l ly h i d e to d e t e rc a n n i b a li s m . T h e r e f o r e , o n e w o u l d s u s p e c t h i g he r n u m b e r s i n r e la t iv e l yh i gh v e g e t a t i o n a r e as . N o n s i g n i fi c a n c e b e t w e e n s p e e d s o r d e p t h s in t h ep r e s e n t s t u d y is p r o b a b l y d u e t o h o m o g e n e i t y o f s p ar se v e g e t a t io nb i o m a s s w h i c h r e m a i n s, a n d n o t t o c a t c h e f f i c i e n c y v a ri a bl e s o f s p e e da n d w a t e r d e p t h . R e s u l t s m a y b e d i f f e r e n t w h e n h e a v y v e g e t a t io n isp r e s e n t e a r li e r in t h e s e a s o n , s i n c e s o f t -s h e l l e d c r a w f i s h m a y p r e f e rt h e s e a r e a s .

T r a p v e r s u s t r a w lC a t c hC a t c h o f m a r k e t a b l e c r a w f is h f r o m e a c h d e p t h z o n e f r o m p o n d s B-1a n d B - 3 o n v a r i o u s d a y s i s s h o w n i n T a b l e 1. H i g h l y s i g n i f ic a n t d i f fe r -e n c e s ( P < 0 -0 1 ) o c c u r r e d in ge a r, d a y a n d d e p t h e f f e c t s , b u t n o t int h e i r i n t e r a c t i o n s . T h i s in d i c a t e s t h a t c a t c h w a s c o n s i s t e n t o v e r a ll l ev e lso f f a c t o rs . T r a w l c a t c h e s w e r e g r e a t e r t h a n t r a p c a t c h e s i n all b u t f o u rc o m p a r i s o n s , a n d t r a w l c a t c h e s w e r e 5 4 % h i g h e r o v e ra l l ( 3 9 5 a g a in s t2 7 5 . 5 k g , r e s p e c t i v e l y ) .

C r a w f i s h d i sp e r sa l r e l a ti o n s h i p s w i t h i n a p o n d a r e n o t w e l l - k n o w n ,b u t v a r i a b i li ty s e e m s d u e t o c h a n g e s i n w e a t h e r c o n d i t i o n s , s p a w n i n ga c t i v i ty a n d f o ra g i n g a n d d ie l m o v e m e n t s (P e n n , 1 9 4 3 ; H u n e r a n d B a rr ,1 9 8 1 ) . T h e r e f o r e , d a i ly t r a p c a t c h e s v a ry a c c o r d in g l y . H o w e v e r , t ra w lc a t c h e f f i c i e n c y is n o t a s d e p e n d e n t u p o n b e h a v i o r a l p a t t e r n s a s a r ep a s s iv e h a r v e s ti n g m e t h o d s ( M a s o n e t a l . , 1 9 7 3 ) . F a c t o r s s u c h a s t r a w l


15/18

Co mmercial electro-trawl for crawfish 149T A B L E 1

Mar ke t ab l e Cr awf i s h Caugh t ( kg ) Com par i ng Tr app i ng and Tr awl i ng f r om P ondsB-1 and B-3 a t Ben Hur Fa r m f r om 8 May t o 26 M ay 1981 a

Day Pond Depth (zone) Treatment (gear)Trap Trawl Total

8 May B-1 D eep (1) 6 .4 19 .0 25 .4Shal low (2) 17 .5 16 .0 33 .5

B-3 De ep (3) 9 .2 10 .0 19 .2Sha l low ( 4 ) 17 . 2 20 . 5 37 . 7

9 May B-1 Deep (1) 10-2 17 .0 27 .2Shal low (2) 14 .4 16-4 30 .8

B-3 De ep (3) 9 .7 13 .5 23 .2Shal low (4) 15 .5 23 .0 38 .5

10 M ay B-1 Deep (1) 8 .9 14 .0 22 .9Sha l l ow ( 2 ) 9 . 4 30 . 0 39 - 4

B-3 De ep (3) 8 .6 6 .0 14 .6Sha l low ( 4 ) 21 . 9 15 . 0 36 . 9

17 May B-1 Dee p (1) 9 .7 25 .0 34 .7Sha l low ( 2 ) 14 .9 36 . 5 51 . 4B-3 Dee p (3) 9 .5 14 .0 23 .3

Shal low (4) 13-3 20 .0 33 .325 May B-1 De ep (1) 8-8 13 .0 21 .8

Shal low (2) 6 .1 14 .0 20 .1B-3 De ep (3) 8 .4 7 .5 15 .9

Shal low (4) 9 .3 16 .0 25 .326 M ay B-1 De ep (1) 7 .5 8 .5 16 .0

Shal low (2) 8-0 16-0 24-0B-3 De ep (3) 4 .8 12-0 16 .8

Shal low (4) 8 .3 12 .0 20-3To t a l 257 . 5 395- 0 652 . 5

.~ = 11 .0 17 .0a W a t e r t e m p e r a t u r e = 2 4 C ; w a t e r c o n d u c t i v i ty = 3 2 0 m h o s c m-2. Electr icalspec i f i ca t ions : 0 .2-0-3 ma cm-2; 4 pul ses per s ; 41 .5 ms width .


16/18

150 C D. Cain, Jr . , J . W. Av au lt , Jr .d e s ig n a n d d i m e n s i o n s , t o w s p e e d a n d p h y s i c a l c o n d i t io n s p l a y a m a j o rr o le (M a r t y s h e r s k ii a n d K o r o t k o v , 1 9 7 1 ).

T r a w l c a t c h v ar ia b i l it y , t h e r e f o r e , w a s m o s t d e p e n d e n t u p o n c ra w -f is h re l a ti v e a b u n d a n c e in d i f f e r e n t a r ea s o v e r d a y s . C a t c h e s w i t h b o t hg e a r t y p e s d e c l i n e d s i g n i f ic a n t l y ( P ~> 0 . 0 1 ) o v e r t h e t h r e e - w e e k p e r i o d ,w i t h n o i n t e r a c t io n s w i t h o t h e r f a c t o r s . T r a p c a t c h e s s h o w e d a s t e a d yd e c r e a s e , w h i le t r a w l c a t c h d e c r e a s e s w e r e n o t c o n s i s t e n t (T a b l e 1).T h e m a x i m u m t ra w l c a t c h o f 4 3 . 5 k g o c c u r r e d o n 1 7 M a y 1 98 1. T hism a y h a v e o c c u r r e d d u e t o l o w e r te m p e r a t u r e s . O n 1 6 M a y 1 9 8 1 as t o rm f r o n t d e c re a s e d m a x i m u m a t m o s p h e r i c t e m p e r a t u r e b y 9 C , a n dp r o d u c e d 2 .5 c m o f r a in f al l. T h i s m a y h a v e i n c r e a s e d c r a w f i s h a c t i v i tyt o w h e r e t h e y w e r e m o r e v u l n e r a b le . T r a p c a t c h o n t h e s a m e d a ys h o u l d h a v e b e e n e v e n m o r e p r o n o u n c e d in re l a t i o n t o o t h e r d a y s , s in c et r a p c a t c h e s a r e m o r e d e p e n d e n t u p o n c ra w f i s h b e h a v i o r p a t t e rn s .H o w e v e r , t h is d i d n o t o c c u r . I n c r e a s e d t r a w l c a t c h o n 1 7 M a y 1 9 8 1 isp r o b a b l y d u e t o c o m p l e x b e h a v i o r p a t t e r n s a s s o c i a te d w i t h e l e ct r i c alc u r r e n t s a n d t h e t r a w l .

B o t h g e a r t y p e s c a u g h t s i g n if ic a n t ly m o r e c r a w f i sh i n s h a l lo w z o n e st h a n in d e e p z o n e s ( P < 0 - 01 ). W i tz ig ( 1 9 8 0 ) f o u n d t h a t w a t e r d e p t hc e a s e d to b e a n i m p o r t a n t f a c t o r in c o n t r o ll i n g c r a w f i s h d i s t r ib u t i o na f t e r a b o u t F e b r u a r y - t h e p o i n t a t w h i c h m o s t v e g e t a t i o n h a d d is a p -p e a r e d . H o w e v e r , d u r i n g t h e l a t e s t p a r t o f t h e c r a w f i s h s e a s o n i n t h iss t u d y , m o r e v e g e t a t i o n s e e m e d t o b e in th e s h a l lo w a r ea s , t h a n in t h ed e e p e r a r e a s .M o r t a l i t yD u r i n g s p e e d t e s t s, m o r t a l i t y w a s l o w ( 3 % o r le s s) a n d w a s n o t si gn if i-c a n t l y d i f fe r e n t b e t w e e n g e a rs o r d e p t h s . H o w e v e r , m o r t a l i t y f r o mt ra w l i n g w a s h ig h e r t h a n t ra p m o r t a l i t y o v e r d a y s ( P < 0 . 01 ) . W e d o n o tu n d e r s t a n d t h i s d i f fe r e n c e b u t c r a w f is h r e m o v e d f r o m t h e n e t m a y h a v eb e e n s tr e s se d b y h ig h e r a t m o s p h e r i c t e m p e r a t u r e s l a te r in th e d a y.T r a w l i n g p e r i o d s w e r e s h o r t , l e s s t h a n 1 0 m i n , a n d a n im a l s w e r e p r e -s u m a b l y n o t s tr e s se d f r o m w a t e r f o r c e e f f e c t s f o r t h is s h o r t p e r i o d .

R E C O M M E N D A T I O N SF u t u r e r e se a r ch s h o u ld c e n t e r o n e x t e n d e d l a b o r a t o r y s t ud i e s a n d o nt h e e l e c t ri c a l a n d m e c h a n i c a l e f f i c i e n c y o f th e t r a w l in g s y s t e m .


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Co mmercial electro-trawl for crawfish 151S p e c if ic c r a w f i s h r e a c t i o n s i n v a r y in g w a t e r c o n d u c t i v i t i e s a n d

t e m p e r a t u r e s r e la t iv e t o c o m b i n a t i o n s o f e le c t r i ca l p a r a m e t e r s m u s t b ea s c e r t a i n e d . E l e c t r i c a l c o n d i t i o n s in th e f i e l d c o u l d t h e r e f o r e b e b e t t e rc o n t r o l l e d .

E l e c t r o d e n u m b e r , s iz e a n d a r r a y a r r a n g e m e n t s h o u l d b e i n v e s t ig a t e dt o d e t e r m i n e a n o p t i m a l e l e c t r o d e s e t- u p . T h i s a r r a n g e m e n t s h o u l do p t i m i z e c a t c h e f f i c i e n c y .

R E F E R E N C E SAllen , D. M. & Inglis, A . (1958) . A p ush net for qu ant i ta t ive sampling o f shr imp in

sha l low estuar ies . Limnology and Oceanography, 3 , 2 3 9 - 4 1 .Aron , W. & Co l l ard , S . (1969) . A s tudy o f the in f luence o f ne t speed on ca tch .

Limnology and Oceanography, 1 4 , 2 4 2 - 9 .Ch apo ton , R . B . (1964 ) . Surface t rawl for ca tching juveni le Am erican shad. Pro-

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