Aquaculture, 38 (1984) 293-306 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

293

DIGESTIBILITY IN SAROTHERODON NILOTICUS FRY: EFFECT OF DIETARY PROTEIN LEVEL AND SALINITY WITH FURTHER OBSERVATIONS ON VARIABILITY IN DAILY DIGESTIBILITY

SENA S. DE SILVA and MALA K. PERERA

Department of Zoology, University of Ruhuna, Meddawatte, Matara (Sri Lanka)

(Accepted 26 October 1983)

ABSTRACT

De Silva, S.S. and Perera, M.K., 1984. Digestibility in Sarotherodon niloticus fry: effect of dietary protein level and salinity with further observations on variability in daily digestibility. Aquaculture, 38: 293-306.

Dry matter and apparent protein digestibility coefficients were determined in Saro- therodon niloticus fry of mean weight 21 mg maintained on four artificial diets with pro- tein content ranging from 9.6% (Diet 1) to 30.4% (Diet 4) at salinities of O”/,,, 5%, and 10°/oo. The total digestibility estimates made using faecal material accumulated through the night did not significantly differ (P > 0.05) from those estimates based on faecal mate- rial collected during the day time, between feedings. The mean apparent total, protein and energy digestibility of the different groups of fry ranged from 56.7% to 69.1%, 71.7% to 87.1% and 73.6% to 83,9%, respectively. The dry matter digesti~ity, in all experimen- tal groups of fry, showed a significant decrease when the dietary protein content changed from 9.6% to 30.4%. Salinity had no significant effect on the digestibility of any of the four experimental diets.

In S. niloticus maintained on Diets 3 and 4 at salinities of 5O/,, and 1Oo/oo the total and protein digestibility showed daily variation. Generally a day or two of high digestibility was followed by a day of low digestibility. The possible utilization of this rhythmicity, whether apparent or real, in aquacultural practices to reduce the feed costs is suggested.

INTRODUCTION

In intensive aquaculture, nutritionally adequate or wholesome diets have to be presented and it is equally essential that such diets are effectively di- gested. The physiology of digestion in finfish is well understood (Barrington, 1957; Kapoor et al., 1975; Fange and Grove, 1979). Similarly, the digesti- bility of a number of standard diets, used in various stages of the life-cycle, in salmonid and channel catfish culture is also well documented (Smith and Lovell, 1972; Windell et al., 1978; Cho and Slinger, 1979; Smith et al., 1980; Buddington, 1980).

Tilapias constitute a major group of species that are cultured in the trop- ics, Digestibility studies on tilapia species have mostly been confined to in- vestigations of fish fed on plants or on materials of similar origin (Mann,

0044-8486/84/$03.00 o 1984 Elsevier Science Publishers B.V.

294

1966; Kirilenko et al., 1975; Buddington, 1979,198O; Bowen, 1981). Surotherodon niloticus is considered to have considerable potential for

intensive aquaculture and is cultured widely, particularly in cages, in most parts of the tropics (Cache, 1982; Guerrero, 1982). It is a euryhaline species (Farmer and Beamish, 1969; Chervinski, 1982), and as such is also a favour- able candidate for culture in saline inland waters of the tropics. However, very little is known of its response to artificial feeds and its nutrient require- ments (Jauncey and Ross, 1982) and even less of these aspects in the fry and fingerling stages.

In this paper results are presented of investigations on digestibility in S. ni- loticus fry maintained at three salinities, viz., freshwater, 5’/,, and 10°/oo, and on four artificial diets of varying protein content, carried out over a 16- week period. In addition, investigations were made on the possible existence of a ‘digestibility rhythm’, as found in the Asian cichlid, Etroplus suratensis (De Silva and Perera, 1983).

MATERIALS AND METHODS

One- to two-week-old S. niloticus fry were brought to the laboratory from a Ministry of Fisheries hatchery and kept undisturbed for a period of 48 h after which they were divided at random into three equal lots. Two lots of fry were acclimatized over 48-h and 72-h periods to salinities of 5°/oo and 10°/oo, respectively. The salinities were selected on the basis of the findings of Farmer and Beamish (1969) and on technical feasibilities. The required salinities were obtained by mixing well water with filtered sea water using a Beckman electrodeless induction salinometer. During the acclimatization period the mortality was almost nil.

Each lot of fry was divided into four equal groups, each consisting of 35- 40 fry which were carefully introduced into circular fibreglass tanks of 25 cm depth and 30 cm diameter. Every group of four tanks maintained at a given salinity was incorporated into a recirculating system which consisted of the four experimental tanks and a tank of 25-1 capacity to receive the out- flow (settling tank), all of which were connected to a reservoir tank of 130-l capacity from which water was pumped by four Eheim pumps to the experi- mental tanks. The mean rate of flow was around 1 1 min-’ . In addition, each experimental tank was aerated for 8 h day-’ . Temperature and salinity were monitored twice daily and pH once in 3 days. Every sixth week each ‘system’ was cleaned up and water replenished completely. A 12-h light: dark cycle was maintained.

Each group of fry at a particular salinity was maintained on one of the four artificial diets, assigned randomly (Table I). The diets were fed, finely ground, twice a day at 08.30 h and 15.30 h, totalling 6% of the body weight per day; the 6% ration was chosen on the basis of available data on fry growth and is thought to be well above the maintenance ration required for S. niloticus fry. After the first month fry were weighed alive, individually,

295

under anaesthesia (MS 222, 1 : 10 000) and the quantity of feed was ad- justed accordingly every fortnight. An additional 10% of the ration was given during the second week, between two weighings. Details of growth performance will be presented in another paper (Perera and De Silva, in

prep. ).

TABLE I

Composition of the experimental diets

Ingredients (%) Diet

1 2 3 4

Fish meala 20.0 40.0 60.0 70.0 Dextrin 61.0 41.0 21.0 11.0 Cod-liver oil 10.0 10.0 10.0 10.0 Kaolin 3.0 3.0 3.0 3.0 Vitamin mixb 1.0 1.0 1.0 1.0 Mineral mixC 2.0 2.0 2.0 2.0 Cr*O, 3.0 3.0 3.0 3.0

Composition (by dry weight)

Protein (%) Lipid (%) Ash (%) Energy (kJ g-‘)

(determined)

9.6 22.0 28.0 30.4 11.6 11.7 13.9 14.1 13.2 18.3 26.1 29.7 20.9 20.6 19.2 19.5

aCommercial preparation - approximately 44% protein and 29.2% ash (by dry weight). bCommercial preparation available locally for poultry. ‘(in grams) NaCl, 43.5; MgSO,*7H,O, 137.0; NaH,PO,*H,O, 87.2; K,HPO, 239.8; CaCH,PO, l 2H,O, 140.0; Ca(C,H,O,)*5H,O, 327; AlCl,, 0.20; ZnSO,.7H,O, 4.0; MnSO,0.7; KI, 0.20; CaCl,, 1.00; ferric citrate, 30.0.

On completion of feeding, and when all the food presented was apparent- ly completely consumed (the time taken differed between different experi- mental groups as well as with growth), the tanks were cleaned. After the 28th day of feeding, the faecal material accumulated in the tanks was siphoned out every morning before commencement of feeding, and washed in 20 ml of water to remove salts adhering to the surface. The faecal material was then dried at 80°C overnight and finely ground. As the amount of faecal matter collected in this way was very small, collections over a 5-day period were pooled together for each experimental group for subsequent analyses. In addition, during the last 4 weeks of the experiment, faecal material col- lected during the day, between the two feedings, was pooled for each 5-day period and treated as above. However, the amount of faecal material thus

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TABLE IV

Percent dry matter, nutrient and energy digestibility of the four experimental diets deter- mined on faecal material collected as indicated in Table II for 5’. n~~ot~cus reared at differ- ent salinities. The S.D. is given in parentheses. For energy estimates the range is given. The same superscript indicates that the total or nutrient digestibility estimates based on faecal material collected through the night are not significantly different (P < 0.01) from those based on faecal material collected during the day, for a particular experimental group

Faecal Diet 1 Diet 2 material

FW 5%0 lo%, FW 5%0 loo/o,

Night faeces Total 67.7a 67.9’ 68.13 66.8a 69.1a 66.3’

(i’ 4.0) (* 2.6) (* 2.9) (t 3.9) (* 4.2) (i’ 5.7)

Protein 74.9 71.7 72.9 87.1 86.6 86.9 (2 6.9) (2 6.2) (c 4.9) (i 3.4) (* 3.4) (t 3.8)

Energy 75.0 73.6 76.0 74.8 75.3 76.3 (74.1-76.0)(73.0-74.0)(75.6-76.4) (74.4-75.2) (75.0-76.0) (76.1-76.6)

Day faeces Total 65.1a 68.1a 70.1a 63.4a 66.5a 58.8b

(+ 7.5) (* 1.9) (* 4.3) (+ 5.2) (t 2.7) (5 6.2)

collected was only sufficient to carry out C&O3 analyses even for those fry maintained on Diets 3 and 4 at salinities of 5°/oo and 10°/oO which gave the best growth performance (Perera and De Silva, in prep.). During the last 22 days of experimentation faecal material collected during the night was not pooled as earlier, but analyses were carried out on each night’s collection in- dividu~ly, The experimen~l details are summ~ised in Table II.

Analyses carried out on aliquots of dried faecal material were Cr,O,, pro- tein, and caloric contents. Cr203 was estimated according to Furukawa and Tsukahara (1966); protein by the biuret method, spectrophotometric~ly, as modified by Raymont et al. (1964) using bovine serum albumen as the standard; and caloric contents by ignition in a ballistic bomb calorimeter (Gallenkamp, U.K., Ltd.). Wherever sufficient material was available all of the above analyses were carried out in duplicate or triplicate. Faecal material remaining after these analyses was pooled in two lots, that collected (a) be- tween 04-03-83 and 02-05-83 and (b) between 03-05-83 and 30-05-83, for de~minations of caloric contents, which were carried out in quadruplicate for each lot (Table II).

Dry matter digestibility and the digestibility of the nutrient components were determined according to the equations of Conover (1966) and Maynard and Loosli (1972). For convenience and clarity a code is used for each group of fry where the Diets l-4 are designated as Dl, D2, etc. and the salinity is designated as SO, S5, etc. Thus the group maintained on Diet 4 in fresh-

299

Diet 3 Diet 4

FW 5%0 lo%, FW 5%0 lO%o

61.3a 67.6a 65.4= 56.7a 60.ga 62.5a (* 4.7) (+ 4.0) (t 6.9) (* 6.1) (+ 4.5) (+ 7.8)

84.5 86.6 85.6 83.3 84.2 85.6 (+ 2.8) (A 1.9) (* 2.7) (+ 1.9) (* 2.2) (* 2.4)

77.4 80.8 83.0 75.1 77.6 83.9

(76.8-78.2) (80.0-81.8) (82.6-83.5) (74.8-75.5) (77.3-78.0) (83.0-84.2)

48.6b 59.4a 60.7a 50.3b 57.0= 67.ga (* 3.9) (i 7.7) (i. 4.3) (+ 5.3) (f 7.9) (+ 3.9)

water will be denoted by the code D4SO while that on Diet 4 at lo’&, salinity will be denoted by D4SlO.

RESULTS

In this study, apart from the replicates, totals of 382, 288 and 24 determi- nations of CrZ03, protein, and caloric content were made, respectively, on different samples of faecal material (Table III). The mean digestibility coeffi- cients of total organic matter (dry matter), protein, and energy obtained for different samples of faecal material based on that accumulated through the night as against that collected during the day, between feedings, are given in Table IV. Of the 12 experimental groups of S. niloticus fry, the dry matter digestibility estimates based on night faeces differed significantly (P< 0.01) from estimates based on daytime faeces only in three instances, viz. in the groups D2S10, D3SO and D4SO. The dry matter digestibility estimates be- tween day and night differed from each other at a significance level of 5% in only one of these groups (D3SO). The overall dry matter digestibility coeffi- cients and the digestibilities of the component nutrients are given in Table V. As mentioned earlier (page 295), sufficient faecal material could not be col- lected during the daytime to determine the apparent digestibility coefficient of protein.

The variation of the percent dry matter digestibility and the protein di-

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301

gestibility in relation to the dietary protein content at the three salinities is shown in Figs.1 and 2, respectively. An apparent trend for the dry matter digestibility to decrease with increasing dietary protein content is evident. The protein digestibility at all three salinities increased very significantly as the dietary protein content increased from 9.6% to 22.0% and thereafter decreased. The apparent protein digestibility of a particular diet was not significantly different (analysis of variance; P>O.Ol) between the three experimental salinities. Very similar trends were observed in the apparent digestibility of energy.

Daily variability

The daily variability in dry matter and apparent protein digestibility was investigated for fry maintained on Diets 3 and 4 at salinities of 5°/oo and

10”/00 - the experimental groups in which sufficient faecal material could be collected for Crz03 and protein analyses. The variations observed are shown in Fig. 3. The total and protein digestibility varied from day to day and generally a day or two of high digestibility alternated with a day or two of low digestibility (Table VI). This rhythmicity was not always clear cut. The maximal deviation was evident in the group on Diet 3 at 5°/oo where the total digestibility remained at a low level for about 4 days.

0

o FW . 5 “1.0

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% Dietary protem

Fig. 1. Relationship of the overall apparent total digestibility of the different diets to the dietary protein content at the three experimental salinities.

o Fw

. 5%.

A 10 “i..

Fig. 2. Relationship of the overail apparent protein digestibility of the experimental diets to the dietary protein content of S. niloficus fry reared in freshwater, 5’&, and lo’/,, salinity. _ -

Total

1 6 10 15 2b

Day

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r

303

TABLE VI

Data pertaining to the extent of daily variability in percent dry matter and apparent pro- tein digestibility of S. niloticus fry at salinities of 5%, and loo/,,, maintained on Diets 3 and 4. 2 - the highest percentage difference observed in digestibility between adjacent days

Digestibility Salinity @)

5%0 10%

Diet Diet

3 4 3 4

Total Mean Range S.D. z

Protein Mean Range S.D. z

68.4 59.3 69.4 68.9

56.5-76.2 50.0-71.4 57.1-78.4 48.2-75.3 5.2 4.9 6.0 7.7

18.3 17.5 27.3 21.7

76.0 70.2 76.2 77.3 63.1-85.0 59.9-79.7 66.7-84.6 56.1-85.4

5.2 5.2 5.1 7.5 16.3 20.9 17.0 23.0

DISCUSSION

In the present study digestibility estimates, either of dry matter or of pro- tein, were made using faecal material collected over the period between presentation of the last feed of one day and the first feed of the following day, and pooled together over such 5-day periods, and using material col- lected between the two feedings during the day, approximately 7 h apart, and again pooled over a &day period. The 5-day period was chosen arbitrar- ily as the minimum number of days required to collect sufficient faecal ma- terial for analyses. The methods of collection of faeces and the effects of leaching of nutrients from faecal material have been adequately treated (Cho et al., 1975; Austreng, 1978; Windell et al., 1978), but remain contro- versial. Cho et al. (1982) have shown that the errors due to the effects of leaching on the digestibility estimates are smaller than those which could arise due to contamination of food particles, etc.

The lack of significant differences (P> 0.05) in the dry matter digestibility estimates made from faecal material of night and day-time ‘origins’ in all but one of the experimental groups of S. niloticus fry supports Cho et al’s (1982) evaluation. Similarly, in the present study lack of consistantly lower or higher digestibility estimates obtained from one group of faecal material

Fig. 3. The daily variability of the total and protein digestibility of S. niloticus fry main- tained on Diets 3 and 4 at 5O/,, and lo’/,,, salinities.

304

provides further evidence that leaching has an insignificant, if any, effect on the estimates. The authors also wish to point out that collection of faecal material at short intervals of time without the use of a faecal trap, may ex- cite the fish which may bring about premature evacuation of some material and hence result in lower estimates of digestibility.

The digestibility of diets and their nutrient components have been shown to be dependent on body size (Windell et al., 1978). Such studies have al- most always been carried out on adult or juvenile fish. Making allowance for the size of the experimental S. niloticus fry, the dry matter and apparent protein digestibility estimates recorded in this study compare favourably with those reported earlier for cichlid species (Jauncey, 1982).

The present study has shown that digestibility of the dry matter of the experimental diets as well as of their nutrient components is related to the dietary protein contents at all three salinities tested; the apparent protein digestibility increased initially, and thereafter decreased with increasing dietary protein content. This could imply either that dietary protein level affects total digestibility or that the dextrin level in the diets brings about an equivalent effect. Jauncey (1982) reported that in S. mossambicus juve- niles the protein digestibility was not affected by the dietary protein con- tent. In trout total digestibility was found to decrease with increasing carbo- hydrate and lower protein levels (Page and Andrews, 1973).

Almost all digestibility studies have been based on pooled faecal material collected over a number of days. De Silva and Perera (1983) reported for the first time the possible existence of rhythmicity in digestion. Daily rhythms in a number of metabolic activities have been demonstrated in animals (Van Wormhoudt et al., 1972; Gohar et al., 1983). The observed rhythmicity in digestibility, on the other hand, could very well be an apparent phenomenon manifesting the individual variability of the experimental group. Even so, the present observations may be of significance as, in aquaculture, response to feeds is evaluated for a group and not for individuals. The earlier observa- tions on the rhythmicity in the Asian cichlid (De Silva and Perera, 1983) as well as those presented here made with widely different feeds and markers make us believe that the daily rhythmicity is a true phenomenon and not an artefact related to the methodology.

It is suggested that experimentation on alternative feed strategies where diets of high protein content are alternated with diets of low or medium pro- tein content may yield a hitherto unexpected method of reducing the feed costs in aquacultural practice.

ACKNOWLEDGEMENTS

We wish to express our most sincere thanks to Messrs B. Amerasinghe, A. Rodrigo and P.B. Vincy for their cooperation, particularly in assisting in the maintenance and care of the fry. This work was made possible by a grant to the first author by the International Development Research Council, Canada,

305

initiated and administered by the Asian Regional Office in Singapore. We are grateful to Dr. B. Davy, Associate Director of the IDRC, and the personnel in the Singapore office for their cooperation and interest at all stages of the project.

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Barrington, E.J.W., 1957. The alimentary canal and digestion. In: M.E. Brown (Editor), The Physiology of Fishes, Vol. I. Academic Press Inc., London, pp. 109-161.

Bowen, S.H., 1981. Digestion and assimilation of periphytic detrital aggregate by Tilapia mos~~bica. Trans. Am. Fish. Sot., 110: 239-245.

Buddington, R.K., 1979. Digestion of an aquatic macrophyte by Tilapia zillii (Gervais). J. Fish Biol., 15 : 449-455.

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Cho, C.Y., Slinger, S.J. and Bayley, H.S., 1982. Bioenergetics of salmonid fishes: energy intake, expenditure and productivity. Comp. Biochem. Physiol., 73B: 25-41.

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