Aquaculture 194 Ž2001. 349–362 www.elsevier.nlrlocateraqua-online
Food particle size, feeding frequency, and the use of prepared food to culture larval walking catfish žClarias macrocephalus/ Rakpong Petkam a,) , G. Eric E. Moodie b a b
Department of Zoology, UniÕersity of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Department of Biology, UniÕersity of Winnipeg, Winnipeg, Manitoba, Canada R3B 2E9
Received 22 May 2000; received in revised form 19 September 2000; accepted 19 September 2000
Abstract The effects of food particle size, food type Žlive or prepared., and feeding frequency, on the growth and survival of Clarias macrocephalus larvae were observed over 13- to 21-day experimental periods. Larvae were reared in glass aquaria and plastic tubs under aerated conditions at water temperatures from 258C to 298C. Live food Ž Artemia of different size ranges. was given to excess on various feeding schedules. Larvae were also fed combinations of live Ž Artemia. and prepared food; larvae were switched to prepared food on day 6, 11, and 16 after receiving Artemia from the onset of exogenous feeding. Larvae fed Artemia had high growth and survival. Prepared food was acceptable at the start of exogenous feeding but resulted in relatively poor growth and survival. Artemia in the size range from 160 to 315 m are more suitable for C. macrocephalus larvae than sizes - 160 m. A schedule of one feeding per day is satisfactory if feeding is in excess. Live food was necessary at the start of exogenous feeding to improve survival. Larvae fed Artemia from the start of exogenous feeding until day 11, followed by prepared food until the end of the experiment, grew significantly faster and had better survival than those fed other food combinations, Artemia, or prepared food alone. The results indicate that simple changes in culture procedures can improve the production of C. macrocephalus larvae. Larvae should be fed once daily on an excess of Artemia in the size range from 160 to 315 m from first feeding to day 10, followed by prepared food in powder form from day 11 to 21. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Walking catfish larvae; C. macrocephalus; Particle size; Prepared food; Artemia
) Corresponding author. Present address: Department of Fisheries, Khon Kaen University, Khon Kaen 40002, Thailand. Tel.: q66-43-362109; fax: q66-43-244474. E-mail addresses:
[email protected] ŽR. Petkam.,
[email protected] ŽG.E.E. Moodie..
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 0 . 0 0 5 2 4 - X
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1. Introduction Over the past 30 years, population increase, over-fishing, and water pollution have resulted in the decline of many wild fish populations in Southeast Asia and elsewhere. Fish farming is helping to match the declining catches to increasing demand. The expansion of aquaculture throughout Thailand, has resulted in a demand for live larval food and substitutes for live food. Major environmental changes affecting the production of natural Žwild. live food and the presence of contagious pathogens associated with wild plankton have led to a shift from natural live food to cultured live food for larvae, particularly where large numbers of larvae are required. Good quality foods such as Artemia have been adopted for larval culture of many species but such foods are costly and inconvenient for many farmers. Prepared food has been successfully used to rear larvae of some species. However, a combination of live and prepared food is often necessary for others. Walking catfishes Ž Clarias sp.. constitute one of the dominant fish genera grown in Thailand. Walking catfishes can be cultured at high density in saline and poorly oxygenated water and produce a high harvest per unit area over a short culture cycle. Farming of this genus has expanded widely throughout the country ŽLeenanone, 1981; Areerat, 1987.. Five species of Clarias occur in Thailand but only two native species are farmed, Clarias macrocephalus and C. batrachus. The biology of C. macrocephalus has been described by Sidthimunka Ž1971., Diana et al. Ž1985., and Pillay Ž1990.. The introduced African species, C. gariepinus, is widely farmed because of its superior growth. However, C. macrocephalus has a higher market value because of its tender flesh and better flavour ŽDiana et al., 1985; Areerat, 1987.. Consequently, hybrids of C. macrocephalus and C. gariepinus are also cultured. Although Thai consumers prefer C. macrocephalus, its production is limited relative to that of C. batrachus and C. gariepinus and hybrids. This results from a high larval mortality rate, slow larval and adult growth, and a limited supply of C. macrocephalus larvae. The expansion of intensive and extensive farming of C. macrocephalus has overtaken the production capacity of wild brood stocks as well as fry. As a result of the high demand, cultured larvae production is becoming the main source of fry. However, shortages of larvae remain despite production by the private and public sectors. The objective of this study was to determine whether live or prepared food could be presented to C. macrocephalus so as to improve larval survival and growth. The study examined appropriate food particle size, feeding frequency, and the feasibility of using a readily available, locally prepared food to rear C. macrocephalus larvae.
2. Materials and methods 2.1. LarÕal source Eggs were obtained from walking catfish brood stock Ž C. macrocephalus, x s 200 g. collected from a river the previous year. A luteinizing releasing hormone analogue was
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used in an overall procedure similar to that commonly used for C. gariepinus and other species ŽFermin, 1991.. Fertilized eggs were subsequently spread over submerged netting in hatching tanks. Hatching occurred within 24 h of fertilization. Two days after hatching, larvae from two parents were pooled and removed with a tablespoon in groups of five which were allocated to the treatment containers. Each consecutive allocation went to a different container until each held exactly 600 larvae at a density of 20 larvaerl. In Experiment 1, a shortage of larvae resulted in only 320 larvae being available for allocation to each experimental unit. One day was allowed for recovery and complete yolk absorption. Dead larvae were replaced to maintain a density of 20 larvaerl prior to the start of the experiments. Larvae generally began exogenous feeding 4 days after hatching. All subsequent references to larval or juvenile ages refer to days after hatching. All experiments began at the onset of exogenous feeding. 2.2. Food sources Prepared food consisted of a powdered Thai Starter Feed Žcrude protein G 40%, lipid G 10%, ash F 8%, and moisture F 12%, from C.P., 36 Soi Yenchitra, Chan Road, Toong Wat Don, Sathon, Bangkok 101120, Thailand.. Artemia franciscana ŽPlatinum Grades I and III, Argentemiaw . cysts were obtained from Argent Laboratories, Redmond, WA, USA as well as Artemia sp. from a Chinese source. 2.3. Rearing facilities Initial experiments were conducted in glass aquaria Ž30 = 45 = 30 cm deep. filled to a capacity of 30 l. Treatments and replications were randomly assigned to the aquaria. Subsequent experiments were conducted in circular blue plastic tubs Ž50 cm in diameter and 20 cm high. also filled to a capacity of 30 l. In Experiment 1, each tub contained only 16 l of water in order to maintain the desired larval density of 20 larvaerl. Water quality in aquaria was maintained by replacing 20% of the volume every 2nd day. Later, 80% of the water in the tubs was replaced every 2nd day and 30% on intervening days. The water supply was city Žtap. water stored in an open tank for at least 24 h prior to use. The study was conducted at the Hatchery Section, Department of Inland Fisheries, Faculty of Agriculture, Khon Kaen University, Thailand. The experiments took place indoors, however, the fish were exposed to outdoor temperatures and natural illumination. 2.4. Sampling and water quality measurements Samples for each experiment were obtained by randomly removing 30 larvae from each container on each sampling date. Ten larvae from each aquarium were preserved in a 10% formalin solution. To reduce shrinkage, a 5% formalin concentration was used in the tub-based experiments ŽGlenn and Mathias, 1987.. Total lengths were measured with the aid of a stereo microscope 24 h after preservation.
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The remaining 20 larvae from each sample were used to estimate dry weight. Larvae reared in aquaria were dried at 1008C for 24 h. Larvae reared in tubs were dried at 808C to avoid excessive loss of volatile fatty acids. After 24 h, samples were removed from the oven and allowed to cool for about 5 min in a desiccator. They were then weighed Žas a group of 20. in an air-conditioned laboratory. Survival during the experiment was estimated from daily mortality. Final survival Žreported here. was determined by the number alive at the end of each experiment. Aquaria and tubs were cleaned daily and dead larvae removed before the first feeding. Water temperature and pH were measured with a digital meter. Water temperatures ranged from 258C to 298C and pH from 6.5 to 8.5 throughout the study. Dissolved oxygen ŽDO. and ammonia concentration Žmgrl. were measured using a Hach kit. Three water samples were collected randomly and analyzed for ammonia concentration by standard methods ŽAmerican Public Health Association, 1965. to verify the accuracy of the Hach kit readings. Dissolved oxygen levels were maintained at G 3.9 mgrl ŽDO - 1 mgrl reduces growth of C. macrocephalus larvae. Mortality occurs when DO falls below 0.2 mgrl.. 2.5. The experiments Three experiments were designed to determine the food particle size which would produce optimum growth and survival. Two experiments were based on live Artemia and one on prepared food. A fourth experiment compared growth and survival among larvae fed combinations of live and prepared foods. The fifth experiment examined the effects of time of feeding and number of daily feedings. 2.5.1. Experiment 1 Artemia nauplii were graded by means of nylon mesh sieves over the following size ranges: F 1%, 1–2%, 2–3%, and 3–4% of initial total larval length. Five treatments were applied: Ži. unsieved Artemia, Žii. Artemia F 59 m, Žiii. Artemia 59–183 m, Živ. Artemia 183–205 m, Žv. Artemia 205–295 m. There were three replicates for each treatment. Food was given to excess every 4 h for 15 days. Larvae were randomly sampled from each experimental aquarium on days 1, 3, 5, 7, 9, 11 and 13. 2.5.2. Experiment 2 Artemia nauplii were size graded by sieving as in Experiment 1, however, in this experiment, the larvae in each treatment all received the same number of nauplii. Nauplii numbers were estimated after sieving by counting individuals in several sub-samples. Larvae in each treatment were then provided with a volume of water containing equal densities of nauplii. Four treatments were applied consisting of Artemia size graded as follows: Ži. unsieved, Žii. F 160 m, Žiii. 160–214 m, Živ. 214–315 m. There were three replicates per treatment. Artemia was provided to excess every 6 h for 21 days. Larvae were randomly sampled from each tub on days 1, 6, 13 and 21.
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2.5.3. Experiment 3 Prepared food was size graded by pressing it through sieves to produce four treatments: unsieved prepared food, prepared food F 183 m, prepared food 183–205 m, and prepared food 205–295 m. There were four replicates of each treatment. Food was given to excess every 4 h for 13 days. Larvae were randomly sampled from each aquarium on days 1, 3, 5, 7, 9, 11 and 13. 2.5.4. Experiment 4 The growth and survival of larvae fed various combinations of nauplii and prepared food was compared in order to evaluate the possibility of reducing farmers’ dependency on costly live foods. The duration of the experiment was 21 days. Six treatments were applied as follows: Ži. Artemia nauplii throughout, Žii. Artemia nauplii for the first 15 days followed by prepared food in powdered form for 6 days, Žiii. Artemia nauplii for the first 10 days followed by prepared food in powdered form for 11 days, Živ. Artemia nauplii for the first 5 days followed by prepared food in powdered form for 16 days, Žv. prepared food in powdered form throughout, Žvi. prepared food in paste form throughout. There were three replicates per treatment. Larvae were fed to excess at 6 h intervals. Random samples were removed from the tubs on days 1, 5, 6, 10, 11, 15, 16 and 21. The extent of feeding among the sampled larvae was assessed by means of a subjective examination of stomach fullness under a dissecting microscope. Stomachs were classified as AemptyB, Ahalf-fullB or AfullB. 2.5.5. Experiment 5 An experiment designed to determine the optimum time of day for feeding and the importance of a single or several daily feedings was conducted by supplying larvae with equal densities of Artemia according to the following treatment schedule: Ži. one morning feeding at 0700, Žii. one evening feeding at 1700, Žiii. two feedings at 0700 and 1700, Živ. three feedings at 0700, 1200, and 1700, Žv. four feedings at 0700, 1000, 1400 and 1700. There were three replicates per treatment. Larvae were randomly sampled on day 1, 5, 9, 13, 17 and 21. Tubs were cleaned at dusk, except for those used in treatment Žii. which were cleaned at dawn to control food availability during day and night. 2.6. Statistical analysis The experiments utilized a completely randomized design. The effects of different treatments on total length and mean dry weight, specific growth rate, relative growth rate and increment of both total length and dry weight were tested by means of one-way ANOVA ŽSAS Institute, 1996.. When statistically significant differences existed at a probability level - 0.05, Duncan’s multiple range test was used to compare treatment means. Hartley’s test was used to test the homogeneity of variance ŽNeter et al., 1990.. Percent of survival at the end of an experiment was normalized by arcsin transformation and analyzed by means of one-way ANOVA. The correlation between mean length and
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Table 1 A comparison of final total length, dry weight, and survival Žmean"SE. of larvae fed unsieved and size graded Artemia nauplii for 15 days after hatching ŽExperiment 1.. Means in the same columns with the same superscript are not significantly different Ž P F 0.05. Treatments Ži. Unsieved Žii. F 59 m Žiii. 59–183 m Živ. 183–205 m Žv. 205–295 m
Total length Žcm. bc
1.372"0.128 0.969"0.015d 1.240"0.084 c 1.666"0.077 a 1.546"0.052 ab
Dry weight Žmg. a
4.473"0.615 1.030"0.087 b 2.093"0.397 b 4.690"0.318 a 4.720"0.719 a
Survival Ž%. 67.41"6.91 14.63"5.26 28.80"13.57 44.54"7.83 35.37"13.28
Table 2 A comparison of final total length, dry weight, and survival Žmean"SE. of larvae fed unsieved and size graded Artemia nauplii for 21 days after hatching ŽExperiment 2.. Means in the same columns with the same superscript are not significantly different Ž P F 0.05. Treatments
Total length Žcm.
Dry weight Žmg.
Survival Ž%.
Ži. Unsieved Žii. F160 m Žiii. 160–214 m Živ. 214–315 m
1.319"0.007 1.297"0.015 1.377"0.030 1.379"0.035
3.253"0.373 2.983"0.133 3.627"0.081 3.360"0.269
84.33"1.17 77.33"5.13 81.17"0.44 78.17"2.35
survival among the replicates within an experiment was analyzed to determine whether differences in growth could be attributed to differences in survival as well as treatment effects.
3. Results The effects of treatments on growth are reported here in terms of total length and dry weight. Specific growth rate, relative growth rate, and increments of length and weight responded similarly ŽPetkam, 1993.. The size of Artemia nauplii significantly influenced total larval length and dry weight in Experiment 1 Ž FŽ4,10. s 11.41, P s 0.001; FŽ4,10. s 12.80, P s 0.001, respectively; Table 1. but had no significant effect in Table 3 A comparison of final total length, dry weight, and survival Žmean"SE. of larvae fed unsieved and size graded prepared food for 13 days after hatching ŽExperiment 3.. Means in the same columns with the same superscript are not significantly different Ž P F 0.05. Treatments Ži. Unsieved Žii. F183 m Žiii. 183–205 m Živ. 205–295 m
Total length Žcm. 1.052"0.036 1.031"0.023 0.991"0.040 1.034"0.034
Dry weight Žmg. a
1.080"0.053 0.865"0.012 bc 0.768"0.026 c 0.970"0.056 ab
Survival Ž%. 18.85"6.34 3.27"1.85 15.58"5.21 16.67"3.46
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Table 4 A comparison of final total length, dry weight, and survival Žmean"SE. of larvae fed different diets for 21 days after hatching ŽExperiment 4.. Means in the same columns with the same superscript are not significantly different Ž P F 0.05. Treatments Ži. Žii. Žiii. Živ. Žv. Žvi.
Total length Žcm. cd
1.489"0.023 1.553"0.021bc 1.771"0.045a 1.633"0.029 b 1.259"0.023 e 1.417"0.037 d
Dry weight Žmg. c
3.843"0.226 4.487"0.121c 6.617"0.156 a 5.830"0.394 b 2.170"0.321d 4.157"0.054 c
Survival Ž%. 84.26"1.54 a 87.87"1.33 a 85.92"0.81a 57.78"8.93 b 12.78"3.43 c 17.22"3.43 c
Ži. Artemia to day 21; Žii. Artemia to day 15 then powdered prepared food to day 21; Žiii. Artemia to day 10 then powdered prepared food to day 21; Živ. Artemia to day 5 then powdered prepared food to day 21; Žv. powdered prepared food to day 21; Žvi. prepared food as a paste to day 21.
Experiment 2 ŽTable 2.. In both experiments, larvae which received larger nauplii tended to have greater lengths and dry weights. There was no significant effect on survival in either experiment Ž FŽ4,10. s 2.62, P s 0.099.. Larvae in all treatments accepted nauplii well and generally had full stomachs within about 30 min of being offered food. The best survival resulted when larvae received unsieved Artemia wTreatment Ži., Experiments 1 and 2x. There was no significant correlation between growth and survival in the 15 groups. The particle size of prepared food had no significant affect on total length in Experiment 3 Ž FŽ3,12. s 0.56, P s 0.651.. Maximum total length Ž1.05 cm. occurred in
Fig. 1. The effect of food combination on larval dry weight Žmg..
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treatment Ži. Žunsieved food, Table 3.. Dry weight was significantly greater among larvae which received unsieved food Ž FŽ3,12. s 10.69, P s 0.001.. Treatment Žiii. resulted in poor growth throughout the 13-day experimental period. Survival was poor Ž- 19%. in all treatments. There was no significant correlation between growth and survival in the 16 aquaria. A mixed diet of Artemia nauplii followed by prepared food resulted in better growth than a diet composed only of Artemia or prepared food. A sequence in which larvae received Artemia for the first 10 days followed by powdered prepared food for the
Fig. 2. Stomach fullness of larvae. Ža. One day after the start of exogenous feeding. T1, T2, T3, T4 received Artemia, T5 received powdered prepared food, T6 received prepared food as a paste. Žb. One day after the first presentation of powdered prepared food to larvae age 16 days ŽT2., age 11 days ŽT3., age 6 days ŽT4., age 1 day ŽT5..
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Table 5 A comparison of final total length, dry weight, and survival Žmean"SE. of larvae receiving food once in the day, once ŽD. at night ŽN., twice, three and four times in the day, for 21 days after hatching ŽExperiment 5.. Means in the same columns with the same superscript are not significantly different Ž P F 0.05. Treatments Ži. Once ŽD. Žii. Once ŽN. Žiii. Twice Živ. Three Žv. Four
Total length Žcm. 2.180"0.023 2.163"0.046 2.140"0.029 2.191"0.052 2.212"0.061
Dry weight Žmg. a
7.783"0.291 6.050"0.265c 6.600"0.340 bc 6.650"0.401bc 7.000"0.465ab
Survival Ž%. 76.03"2.55 68.41"8.93 87.38"3.70 91.74"1.79 79.44"4.58
remaining 11 days wtreatment Žiii., Table 4x grew significantly larger in both total length Ž1.77 cm, FŽ5,12. s 87.11, P - 0.0001. and dry weight Ž6.62 mg, FŽ5,12. s 41.90, P 0.0001. than larvae which received the two foods over different intervals The poorest growth in terms of total length and dry weight occurred when larvae received powdered food throughout the experiment wtreatment Žv.x. Of the two groups of larvae which received only prepared food in either a powder wtreatment Žv.x or a paste form wtreatment Žvi.x, those receiving it in the paste form showed significantly better growth in both total length and dry weight ŽTable 4, Fig. 1.. Dissection of the stomachs of larvae indicated prepared food in both powder and paste form was accepted at an early stage of exogenous feeding. However, larvae receiving Artemia wtreatments Ži., Žii., Žiii. and Živ.x had significantly more food in their stomachs than those receiving prepared food ŽFig. 2.. Older larvae tended to feed more successfully on prepared food ŽFig. 2b.. Larvae which received nauplii for at least 10 days had significantly better survival Ž84.2–87.9%. than those which received nauplii for 6 or fewer days Ž12.8–57.8%.. There was a significant positive correlation between growth Žtotal length. and survival Ž r 2 s 0.52, b s 0.004.. Differences in growth were therefore effects of treatments rather than of differential survival. Different feeding schedules during the 21 days after hatching resulted in significant differences in dry weight Žbased on two-factor ANOVA FŽ4,10. s 5.48, P s 0.013. but had no significant effect on total length Ž FŽ4,10. s 0.93, P s 0.485; Table 5.. There was no significant interaction between treatments and time. One feeding at 0700 h wtreatment Ži.x gave the best dry weight; one night-time feeding wtreatment Žii.x gave the poorest dry weight. There were no significant differences in survival levels among the various treatment groups Ž FŽ4,10. s 2.86, P s 0.081.. Feeding only once in 24 h and at night resulted in the lowest fry weight, smaller total length and poorest survival. There was no significant correlation between growth and survival among the 15 tubs.
4. Discussion The availability of live natural and cultured food is seasonally dependent and good quality live food such as Artemia is relatively expensive. Live Artemia and zooplankton
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can be used to rear C. gariepinus larvae ŽHogendoorn, 1980.. C. gariepinus may grow significantly better, however, using optimally formulated dry feed ŽUys and Hecht, 1985; Appelbaum and Van Damme, 1988; Pector et al., 1993; Hoffman et al., 1994.. There is therefore reason to hope prepared feed can also be used to rear other Clarias species. In Southeast Asia, mashed boiled egg yolk is traditionally used to rear Clarias ŽViveen et al., 1990.. A mixed diet of Artemia plus a dry artificial feed for C. macrocephalus produced a higher specific growth over the first 14 days than a diet of either item alone ŽFermin and Bolivar, 1991.. Clarias larvae have a very short, small gut, the frequency of feeding must therefore be high enough to maintain their metabolic requirements at all times. Feeding prepared food twice a day was found to be economically optimal for 2-week-old Ž0.3 g. C. macrocephalus, whereas more frequent feeding with proportionately restricted amounts of prepared food Ž9% BW. did not affect growth but the feed conversion ratio increased ŽTabthipwon, 1990.. Three daily feedings with live food Ž Moina. have been suggested for rearing C. macrocephalus larvae ŽMollah and Tan, 1982.. More frequent feeding may be preferable in the case of C. gariepinus ŽUys and Hecht, 1985.. Continuous feeding for 24 h per day produced the fastest growth for 0.5 g C. gariepinus larvae according to Hogendoorn Ž1981.. Growth improved when large Artemia were available. Repetition of the experiment to avoid the confounding effect of unequal food density among treatments which existed in the first experiment confirmed that C. macrocephalus larvae offered larger sized Artemia and fed to excess, achieved better growth. A particle size equivalent to 2.2% of total larval length is optimal for C. gariepinus ŽUys and Hecht, 1985.. The size of decapsulated and dried Artemia cysts Ž200–250 m . may be a more appropriate feed for C. gariepinus larvae than larger Artemia nauplii Ž470–550 m . ŽVerreth et al., 1987.. It is difficult to maintain these size relationships, however, as larval size increases while the size of Artemia nauplii, which are not usually allowed to feed, remains constant. The analysis of growth parameters indicate an Artemia size range from 160 to 315 m is more suitable than Artemia F 160 m for C. macrocephalus larval reared under laboratory conditions. Excessively small food particles are not consumed efficiently ŽHalver, 1989. and a prepared food size of 125 m appears to be too small for C. macrocephalus larvae ŽUnprasert and Sinthusen, 1989.. Further investigations should be undertaken using wider size ranges of larval food items and older larvae. The differences in growth among larvae receiving different food sizes may result from a higher energy requirement when larvae feed on smaller sizes of food compared to those feeding on bigger food items ŽPitcher and Hart, 1982.. Higher survival in the tub experiments probably resulted from improved water quality due to a higher volume of water replacement and because the tubs were easier to clean. C. macrocephalus larvae, like those of many species ŽRottmann et al., 1991. are more successfully cultured on live food. Larvae fed only prepared food in powder form showed poor growth and survival. Those fed prepared food in paste form had satisfactory growth but poor survival. The importance of the physical form of artificial food is well known ŽSantiago et al., 1987.. The problems larvae experience when fed prepared food may result from physiological and morphological immaturity of the digestive system ŽLauff and Hofer,
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1984; Hofer, 1985; Lam, 1991., the absence of proteolytic enzymes present in live food or both ŽDabrowski and Glogowski, 1977; Hepher, 1988.. The superior growth of larvae fed Artemia may result more from the larva’s overall food intake, food digestibility, and assimilation rather than Artemia’s nutritional value. Artemia’s enzymes may contribute to an activation of zymogens or digestive hormones such as bombesin ŽKolkovski et al., 1997; Garcia-Ortega et al., 1998.. There is no consensus on this subject as proteases derived from live food have been shown to make only a small contribution to enzymatic activity in sea bass and sardine larvae ŽCahu et al., 1995; Kurokawa et al., 1998.. Opinions also differ on the importance of live food for C. gariepinus. Live Artemia or zooplankton is required according to Msiska Ž1981.. However, Uys and Hecht Ž1985. suggested prepared food could be used to successfully rear C. gariepinus larvae. C. batrachus larvae fed on live food and Artemia exhibited significantly superior growth relative to those fed artificial diets according to Alam and Mollah Ž1988. and Bairage et al. Ž1988.. In our study, a mixed diet of Artemia followed by prepared food produced the best gains in length, weight, and survival. The optimal age at which to shift from live to artificial food in C. macrocephalus larval rearing is still uncertain. The length of the feeding period on live food may be crucial if larvae are to maximize growth and achieve high survival. For the foods used in this study, supplying Artemia for the first 10 days followed by powdered prepared food for the remaining 11 days of the 21-day experimental period resulted in the best growth and survival. A preliminary study by Fermin and Bolivar Ž1991. of C. macrocephalus larvae found a mixed diet of Artemia and dry food produced the best specific growth and survival relative to larvae fed either zooplankton or a dry artificial diet alone. However, Fermin and Bolivar Ž1996. subsequently showed that although C. macrocephalus larvae could be weaned from Artemia to dry food after 4 days with no affect on survival, larvae fed Artemia for 10 days had superior growth. Co-feeding larvae with live and inert diets has also been shown to improve growth ŽRosenlund et al., 1997; Canavate and Fernandez-Diaz, 1999.. The present study suggests prepared food may not be consumed efficiently at the start of exogenous feeding but that prepared food is superior to Artemia later in larval life. Similarly, Okoye et al. Ž1991. found a diet of zooplankton alone could not support the growth and survival of C. gariepinus larvae in outdoor tanks and that artificial diets containing at least 30% crude protein were necessary to supplement live food in order to produce good growth and survival. Conversely, Van Damme et al. Ž1990. found that the best growth rate of C. gariepinus larvae could be obtained when larvae fed on Artemia alone. Feeding Artemia and a dry diet resulted in intermediate growth and poor growth was obtained when larvae fed only on a dry diet. An early shift from Artemia to a prepared diet between the age of 1.8 and 4.1 days may halt the growth of C. gariepinus ŽVerreth and Tongeren, 1989.. The rationale for the examination of the effect of feeding frequency on C. macrocephalus larval rearing success was based on the fact the larvae have a small stomach but a high metabolic rate. One would predict feed should therefore be given at frequent intervals. In the present study, however, varying the feeding frequency did not influence growth or survival. Conversely, three feedings per day appeared to be optimal for C. macrocephalus larvae reared over a similar temperature range Ž26–318C. by Mollah and
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Tan Ž1982.. The different results may be a consequence of different feeding level applied. Satiation feeding was utilized by Mollah and Tan Ž1982., whereas food was supplied to excess in the present study. This resulted in uneaten Artemia surviving well enough to remain palatable until the next feeding time. Given that C. macrocephalus larvae are to some extent scavengers, the effect of an increase in feeding frequency may thus not have been directly tested. Increasing the feeding frequency from two to four times per day did not significantly increase growth and survival, therefore, one daily feeding is acceptable at least under laboratory conditions and when excess live food is supplied. Similar results were found in shrimp culture in static system where multiple feeding may not be advantageous ŽVelasco et al., 1999.. Growth Žin total length. and survival were not significantly affected by day vs. night-time feeding. However, C. macrocephalus fed on Artemia during the day-time had significantly better weight gain than those fed at night. In this respect, C. macrocephalus resembles C. batrachus which is also diurnally active ŽLeenanone, 1981.. Larvae of C. gariepinus in contrast are primarily nocturnal tactile feeders ŽBritz and Pienaar, 1992.. In summary, this study shows Artemia in the size range from 160 to 315 m produced better growth than Artemia - 160 m. Artemia alone resulted in better survival than prepared food in either powdered or paste form. A combination diet beginning with Artemia and then switching to prepared food on day 11 resulted in better growth than combinations of other time periods, prepared food or Artemia alone. A feeding schedule of one daily feeding is acceptable when larvae are fed to excess on live Artemia. This study shows simple changes in culture procedures can improve the production of C. macrocephalus larvae. Further study of optimal food particle size over a wider size range of fish, ideal feeding frequency for prepared food, the best time to switch to prepared food during the period from 5 to 10 days of age may make the culture of this species more attractive to fish farmers in Thailand and elsewhere.
Acknowledgements We thank Professors F.J. Ward, K.W. Stewart, and Dr. M. Giles for their valuable help. We are also grateful to the Thai project coordinators, Dr. Phanna Waikakul and Dr. Manochai Keeratikasisorn. The project was supported by the Canadian International Development Agency ŽCIDA..
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