Chinese Journal of Oceanology and Limnology

Vol. 20, No. 4, P. 344 - 347, 2002

REDUCED GROWItt IN HYBRID TII_APIA ( ORECL-7-1ROMIS ~ I C U S • O. NR.(TIICUS ) AT

INIERMEDIATE ST(X;KING DENSITY *

W A ~ Yan(T-~) t ' ' " , O5I Y i - b o ( ~ ; ~ ) tt, YANG Yun-xia(Jf~;~) t+, CAI Fa-sheng(i~.)3~i~)*** ( "PKey Ixzboratary of Ecology and Physiology in Aquaculture, Ministry of Agriculture of Otina,

Shanghai Fisheries Unizerdty , Shanghai 200090, C~ina ) ( ~'State Key Laboratory of Freshzmter Ecology and Biotechnology,

Im-titute of Hycb'dn'ology , the Chinese Academy of Science, Wuhan 430072, China ) ( J'r162 oflVlzrine Biology, University ofShantou~ Shantou 515063, China )

Received Mar. 12, 2001; revision accepted Jan. 8, 2002

Abstract Hybrid tilapia were reared at densities of 1, 5 or 10 fish per tank for four weeks.

1Vbrtality was 0 at 1 and 10 fish per tank, but was 25 % at 5 fish per tank. Specific growth rate was high-

est at 1 fish per tank, and lowest at 5 fish per tank. The lower growth rate at the intermediate stocking

density was associated with reduced feed efficiency, but there was no reduction in feed intake or digest-

ibility. The results suggested increased metabolic cost caused by aggressive behaviour at intermediate

stocking density, which can be suppressed by a further increase in density.

Key words: growth, feed efficiency, stocking density, tilapia

IN1RODUCTION

In most studies, growth rate of fish was found to decrease with increasing density, especially when the density was above a critical point (Refstie and Kittlesin, 1976; Refstie, 1977; Trzebia- tow-ski et al., 1981; Vijayan and Leatherland, 1988). However, in a cold-water carnivorous fish, the Arctic charr, Salvelinus alpinus, growth rate was low at low stocking densities (Brown et al . , 1992; Jorgensen et al . , 1993; Wallance et al. , 1988). The reduced growth at low densities was

caused by strong aggressive behaviour, which was suppressed at high densities. Tilapia are warm-water omnivorous fish, but also display strong aggressive behaviour, resulting

in reduced growth and increased mortality (Breine et al., 1996). The purpose of the present study was to investigate the effect of stocking density on the growth and feed utilisation in hybrid tilapia ( Oreochromis mossambicus • O. niloticus ). The hypothesis posed was that growth performance would be poorest at an intermediate stocking density as a result of aggressive behaviour.

MATERIALS AND MEII-IODS

The experiment was carried out from 31 August to 29 September 1998 at the Nanao Marine Bi- ology Station, Shantou, China. Hybrid tilapia obtained from Puning Fish Hatchery, Shantou, were reared in an 18 m 2 outdoor concrete tank and fed twice a day a commercial tilapia diet (Meiyan Feed

Corporation, Shantou, China) containing 30% crude protein and 3.0 % crude lipid for four months prior to the experiment.

* This project was financially supported by the State Key Laboratory of Freshwater Ecology and Biotechnology of China and by

Shanghai Fisheries University (Project SFU. 200003).

* * Correspondending author, Tel./fax: + 86 21 65710764, E - mail: wangyan@shfu, edu. cn

No. 4 WAI'~ et al. : EFFECT OF SI(K;KING DENSITY ON GROWIIt OF TILAPIA 345

Two weeks before the experiment, 120 fish were transferred to twelve 240 1 rectangular fibreglass

tanks (80 • 50 cm; water depth 60 cm; water volume: 240 1), and fed the floating experiraental feed (dry matter content 94% ) to satiation twice a day. The feed consisted of 24 .7% fish meal, 28 .5%

soybean meal, 3 5 . 6 % wheat flour, 1 . 8 % soybean oil, 6% vitamin premix, 2 . 67% mineral

premix, and 0.86 % Cr203 as marker for digestibility measurement. Analysed contents of crude pro-

tein (micro Kjeldahl method), lipid (ether extraction in a Soxtec system), ash (combustion at

550~C for 12 h), and energy (bomb calorimetry) were 31 .6%, 3 . 2 % , 16.2% and 15.4 k J ' g 1

( dry matter), respectively. Content of Cr203 was detemained as described by Fumkawa and Tsduka-

hara (1966).

At the start of the experiment, 64 fish weighing 41.24 _+ 0.90 g (mean _+ S. E) were starved for 24 h and randomly stocked into each of the 12 tanks at three densities: 1, 5 and 10 fish per

tank, with four replicate tanks per density. Five groups of five fish each were sampled for initial

body composition analysis. Each day the fish were hand-fed at 8:00 and 16:00 until they no longer ac-

cepted feed. Faeces that ra'mined intact were collected by pipetting once a day, dried at 70~ to con-

stant weight and preserved at 5~C. Faeces that were broken were siphoned out. Water in each tank was ccrnpletely replenished with aerated well water once a day. Aeration was provided intermittently for 90

rain every 120 rain. Water temperature ranged from 24.8~2 to 28.0 ~ (mean 26. I T ). Photoperiod was

16L: 8D. Dissolved c~ygen was n'easured weekly, and was 6.53 + 0.11 (rrean _+ S. E. ), 5.85 + 0.23

and 4.53+ 0.19 mg/1 at 1, 5 and 10 fish per tank, respectively.

The experiment lasted for 4 weeks. At the end of the experiment, fish from each tank were starved for 24 h and batch weighed. One to five fish were taken from each tank for body composition

analysis. The sampled fish were autoclaved, ha'nogenised and dried to constant weight at 105~C (Cui et al . , 1996). Protein, lipid, ash and energy contents were determined for the fish samples

using methods described for feeds, and protein, energy and Cr203 contents were determined for fae-

cal samples from each tank. Calculated growth performance parameters were : specific growth rate = 100 (lnW~ - ZnWo )~ l,

feed efficiency = 1 0 0 ( W , - W o ) I ~, digestibility of dry matter = 1 0 0 - 1 0 0 Y1 "Y2 -~, and di-

gestibility of protein and energy = 100 - 100Y~ �9 Y2 1. C1 1 �9 C 2 ' where W, is final and Wo initial

wet weight of the fish, t is the duration of growth period in days, I is total dry matter intake from

feed, Y1 is Cr203 content in feed, Y2 is Cr203 content in faeces, C1 is protein or energy content in

feed, and C2 is protein or energy content in faeces (Wang et al . , 2000).

One-way analysis of variance was used to test the effects of different treatments on the various

variables, and Tukey' s procedure was used for multiple comparisons. Proportions were arcsine

transformed prior to analysis. Differences were regarded significant when P < 0.05.

REN_JLTS

One fish reared alone in a tank was found abnormal in activity and feeding response. Its feed intake (2 .17 % B ' W d -1 ), specific growth rate ( 0 . 9 6 % �9 d -1 ) and feed efficiency ( 4 7 . 1 8 % )

were much lower than those in the other three replicate tanks (see Table 1). Data on this tank was

excluded from analysis. Mortality was zero in tilapia reared at 1 and 10 fish per tank, but was 25.0 -+ 9 .6% (mean _+

S.E. ) at 5 fish per tank. Specific growth rate was highest in tilapia at 1 fish per tank, and lowest

at 5 fish per tank. Feed intake at 10 fish per tank was lower than at lower densities, but was not

significantly different between 1 and 5 fish per tank. Feed efficiency was lowest at 5 fish per tank,

and was not significantly different between 1 and 10 fish per tank. Digestibilities of dry matter, pro-

346 CHIN. J. OCEANgL. LllVlN3L., 20(4), 2002 Vol.20

te in and energy were lower at 10 fish per

Table 1 Growth performance densities ( mean _+ S. E. )

t ank t han at lower densi t ies (Tab le 1 ) .

and digestibility in hybrid tilapia reared at different

Density 1 fish/tank 5 fish/tank 10 fish/tank

Initial weight (g)

Final weight (g)

Specific growth rate ( %" d - 1 )

Feed intake ( % B" W" d - 1 )

Feed efficiency ( % )

Dry matter digestibility ( % )

Protein digestibility ( % )

Energy digestibility ( % )

41.74_+ 0.81 41.17_+ 0.39 41.25_+ 0.45

82.40+ 1.98" 66.64_+ 0.90 c 73.96_+ 1.79 b

2.41+ 0.13 ~ 1.72- + 0.04 ~ 2.08_+ 0.07 b

2.86-+ 0.14 " 2.80_+ 0.08 " 2.48_+ 0.04 b

87.03_+ 0.69 " 65.08_+ 3.21b 87.97-+ 1.84a

82.21 _+ O. 48" 82.64 -+ O. 29" 78.66 -+ O. 55 b

92.32 _+ O. 31 a 91.97 -+ O. 70 ~ 87.77 _+ O. 62 b

88.69 -+ O. 14" 87.90 _+ O. 66 ~ 83.15 _+ O. 48 b

s .E. - standard error. Letters after each value indicate results of multiple comparisons (Tukey HSD test) . Values with the same let- ter are not significantly different frcrn each other at the 0.05 level.

At the end of the experiment, no significant differences were found in crude lipid ( P = 0 . 0 6 5 ) ,

a s h ( P = 0 .478) or energy content ( P = 0. 749) among fish reared at different densities, t -~vever, rmis-

ture content was significantly higher, while crude protein content in the fish reared at 10 fish per tank

was lower than tha t in the f ish reared at 1 f ish per t ank (Tab le 2 ) .

Table 2 Body composition and energY content of hybrid tilapia reared at different densi-

ties (mean_+ S . E . )

Density Moisture Crude protein Crude lipid Ash Energy

1 fish/tank 72.78+ 0.93" 15.78_+ 0.12 " 5.18_+ 0.61 4.93_+ 0.04 4.94_+ 0.17

5 fish/tank 75.28 _+ 0.58 ,b 15.12 _+ 0.50 ,b 3.39 _+ 0.30 5.05 --+ 0.19 4.35 _+ 0.26

10 fish/tank 76.85 -+ 1.08 b 14.17 --+ 0.32 b 2.61 _+ 0.79 5.21 + 0.14 3.74 --+ 0.37

1. s.E. = standard error; 2. Moisture, protein, lipid and ash contents were expressed as % wet weight, and energy content was ex- pressed as kJ g 1 wet weight; 3. Initial values were: Dry matter, 23.94_+0.75; protein, 14.76+0.38; lipid, 2.80+0.52; ash, 5.43 + 0.18; energy, 3.92 + 0.27 (mean + S. E. ) ; 4. Letters after each value indicate results of multiple comparisons (Tukey HSD test). Values with the same letter are not significantly different.

DISCUSSION

Inconsis tent resul ts have been reported on the effect of stocking densi ty on the growth of t i la-

pia . Most s tudies reported a decrease in growth rate with increasing dens i ty (Hargreaves et a l . ,

1991; Breine et a l . , 1996 ) , or a lack of densi ty effect wi th in the range of densi ty tested

(McGeach in et a l . , 1987 ; Wa tanabe et a l . , 1 9 9 0 ) . However, growth rate of Nile t i lapia (Oreo-

chramis niloticus ) increased when stocking dens i ty increased from 60 to 140 f ish m 3 (Sadek et a l . ,

1 9 9 2 ) . Growth rate of juveni le Sarotherodon galilaeus was reported to be h igher at 70 fish per t ank

t han at 35 fish per t ank ( H o e n e r et a l . , 1 9 8 7 ) . The different results may reflect differences in spe-

cies and strain of t i lapia , body size and developmenta l stage, culture condi t ions and range of dens i ty

tested. The higher growth rates and lower mortal i ty at higher densi t ies reported in the present and

scrne other s tudies suggested possible effects of social interactions, wi th aggressive behaviour be ing

suppressed at h igh densi t ies . The results supported the hypothesis tha t growth rate was lowest at an

intemaediate densi ty, and increased wi th a decrease or increase in densi ty .

In Arctic charr , the decrease in growth rate at low densi t ies was acccrnpanied by a decrease in

feeding rate ( Jo rgensen et a l . , 1 9 9 3 ) . In the present study, t i lapia reared at the intermediate den-

sity, which showed the lowest grow~th rate, had feeding rates s imilar to those reared at 1 f ish per

tank , while fish reared at the highest densi ty had reduced feeding rates . Ti lapia reared at the inter-

No.4 WAI'~ et al. : EFFECT OF b-'g3CTdr~ DENSITY ON GROW~ OF TILAPIA 347

mediate density also had digestibilities similar to those reared at 1 fish per tank, but the feed effi-

ciency was much lower than fish reared at lower and higher densities. The results suggested that,

different from Arctic charr, the reduced growth rate in tilapia reared at the intermediate density was

mainly caused by a higher metabolic expenditure, which was probably a result of a high level of ag-

gressive behaviour. The decreased feeding rate and digestibility in tilapia reared at the highest den-

sity, 10 fish per tank, suggested constraints caused by crowding and reduced water quality.

In conclusion, results of the present study suggested that, at relatively low stocking densities,

grow~ and survival of hybrid tilapia are adversely affected by aggressive behaviour. Density manipu-

lation can be employed to suppress aggressive behaviour by maintaining the fish at a relatively high

density in practical culture.

ACKNE)WLEIX;EMENT

We thank Zhuang Donghong, Lin Weixiong, Xie Shouqi and Zhu Xiaoming for their help at

various stages of this project.

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