INFLUENCE OF AMBIENT OXYGEN ON FEEDING AND GROWTH OF THE TILAPIA, OREOCHROMIS NILOTICUS (LINNAEUS)

G. G. Tsadik and M. N. Kutty

AFRICAN REGIONAL AQUACULTURE CENTRE, PORT HARCOURT, NIGERIA
CENTRE REGIONAL AFRICAIN D'AQUACULTURE, PORT HARCOURT, NIGERIA

UNITED NATIONS DEVELOPMENT PROGRAMME
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
NIGERIAN INSTITUTE FOR OCEANOGRAPHY AND MARINE RESEARCH
PROJECT RAF/82/009

JULY 1987

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INFLUENCE OF AMBIENT OXYGEN ON FEEDING AND GROWTH OF THE TILAPIA, OREOCHROMIS NILOTICUS (LINNAEUS)*

G. G. TSADIK** AND M. N. KUTTY

ABSTRACT

Oreochromis niloticus (8.1 ± 0.5 g) grown in fresh water at 28 ± 2C for 35 days at various ambient oxygen concentrations, below air saturation in static water aquaria, were seriously affected in their feeding, assimilation and growth by ambient oxygen. While the ambient oxygen maintained was reduced from about 90% air saturation (about 7 mg/l) to about 20% air saturation (about 1.5 mg/l) food consumed and assimilated decreased 40 and 60% respectively. Correspondingly, assimilation efficiency decreased from 80 to 50%, and conversion efficiency from 15 to 5%, while uncorrected FCR increased from 1.45 to 6.75.

Under a simulated diel flux of oxygen (from about 20 to 200% air saturation) brought about by an induced bloom of plankton, growth rate of O. niloticus was considerably reduced when compared with those maintained at DO near saturation constantly. The present results suggest that DO levels below about 50% air saturation and diel flux of DO, often prevalent in tropical fish ponds treated with high doses of organic/inorganic fertilizers, would cause considerable reduction in pond production of tilapias.

* This study formed a part of a thesis submitted for M. Tech. (Aquaculture) degree of the African Regional Aquaculture Centre/Rivers State University of Science and Technology, Port Harcourt, Nigeria.

^** Present address: Senior Aquaculturist, Department of Fisheries Resources Development, P. O. Box 1055, Addis Ababa, Ethiopia.

INFLUENCE DE L'OXYGENE DISSOUS AMBIANTE SUR L'ALIMENTATION ET LA CROISSANCE DU TILAPIA; OREOCHROMIS NILOTICUS (LINNAEUS)

G. G. TSADIK ET M. N. KUTTY

RESUME

Des individus d'Oreochromis niloticus (8.1 ± 0.5 g) élevés en eau douce à 28 ± 2°C pendant 35 jours à des concentrations variées d'oxygène dissous, inférieurs à saturation en air, dans des aquariums d'eau stagnante, ont été sérieusement affectés dans leur alimentation, assimilation et croissance par la concentration ambiante en oxygène. Lorsque la teneur en exygène dissous était réduite de 90 % de la saturation en air (environ 7 mg/l) à environ 20 % de la saturation en air (environ 1.5 mg/l) la nourriture consommée et assimilée diminuait de 40 à 60 % respectivement. De manière correspondante, l'efficacité de l'assimilation diminuait de 80 à 50 % et l'efficacité de la conversion de 15 à 5 %, tandis que le taux de conversion alimentaire non corrigé augmentait de 1.45 à 6.75.

Dans des conditions simulées d'un flux diurne d'oxygène (d'envirion 20 à 200 % de saturation en air), provoquées par une efflorescence induite de plancton, le taux de croissance de O. niloticus était considérablement réduit, en comparaison avec ceux obtenus dans des conditions constantes proches à la saturation en air. Les résultats présents suggèrent que des taux d'oxygène dissous inférieur à 50 % de la saturation en air et des flux diurne d'oxygène dissous existants dans les étangs de pisciculture tropic trait´s ave Des doses ´levyées d'engrais organiques mineraux, causeraient une réduction importante dans la production des tilapias en étangs.

INTRODUCTION

Metabolism and growth of fishes are dependent on the availability of ambient oxygen (Doudoroff and Shumway, 1970; Fry, 1971; Davis, 1975; Brett, 1979; Kutty, 1981; Wickins, 1981). Thus all factors affecting changes in dissolved oxygen, including the lowering of oxygen and resultant hypoxia and the diel flux of oxygen can affect production of fish in ponds. There is considerable information on how ambient oxygen limits growth in fishes. For example, Herrmann (1975) found that growth of the salmonid, Onchorhyncus kisutch is proportional to dissolved oxygen between 4 and 8 mg/l at 20°C. An aspect which needs close scrutiny is the effect of fluctuating levels of ambient oxygen, as it occurs in diurnal flux of oxygen in any water body, specially evident in fertilized ponds or in waters where phytoplankton development is prominent. There is evidence to show that diel flux of oxygen reduces growth in Micropterus salmoides (Stewart et al, 1967), salmonids (Dorfinan and Whitworth, 1969; Warren et al, 1973), esocids (Adelman and Smith, 1970) and in carps (Iatazawa, 1971). While this has been recognized in these teleosts in the temperate environment, little attention is paid on the growth of tilapias affected by hypoxia or the diurnal flux of oxygen so prominent in fertilized tropical ponds where tilapias are grown. This relative neglect might be owing to the well known hardiness of tilapias under extremes of environmental conditions and the assumption that tilapia performance is not severely affected by such oxygen changes.

Several tilapias are reported to tolerate oxygen levels of 0.1 – 0.5 mg/l (O. mossambicus, 0.1 mg/l, Mayurama, 1958; O. niloticus, Magid and Babiker, 1975; O. mossambicus 0.4 mg/l at 30C and 0.6 mg/l at 35C in closed respirometers - Mohammed and Kutty, 1982; Tilapia guineensis and Sarotherodon melanotheron and O. niloticus - nil oxygen, water depleted of oxygen by a addition of tobacco waste in low concentrations, but fish allowed access to air - Kutty, unpublished). In all these cases of very low oxygen tolerance, the fish has apparently had access to air. Stickney et al (1977) recorded heavy mortalities in tilapia ponds fertilized with pig dung apparently because the fish could not surface as the ponds were covered with duckweed. Our unpublished observations also show that both T. guineensis and S. melanotheron die in 1 – 2 hours in closed respirometers, but if given access to air they survive for 8 – 16 days. O. niloticus fingerlings allowed access to air, survive for over a week (LT₅₀ 8.6 days) in water depleted of oxygen.

Thus it is evident that while survival of tilapias are affected by low oxygen, they can survive long periods with very little oxygen in water if they are allowed access to air, but how exactly these low oxygen conditions affect growth of tilapias is not known.

In the present investigation, tests have been made to study the feeding, assimilation and growth of O. niloticus under oxygen concentrations below air saturation and also in simulated diel oxygen flux in the laboratory.

MATERIALS AND METHODS

Fish

O. niloticus (8.1 ± 0.45 g (SD) ) obtained from the state fish farm, Okigwe, Nigeria, were acclimated to fresh water at 28 ± 2 C in aerated aquarium tanks and were fed with a pelleted feed. The pelleted feed given had the following composition: crude protein - 38.5%; nitrogen free extract - 26.2%; lipids - 5.4%; moisture - 13.4%; crude fibre - 9.0%; ash - 7.5% (the pellets and their composition were given by the Nigerian Institute for Oceanography and Marine Research).

Experimental set-up and procedure

Eight fish selected for each test were kept in 50 litres of water in a static water aquarium tank (90 × 30 × 30 cm) for 5 weeks under different oxygen regimes. Water changes were made daily or as needed. The fish were fed with the pellets referred above, with demand feeder or manually. The faecal matter was removed daily, dried and weighed. All fish were weighed every 7th day until the termination of the experiment. Tests were done in four different oxygen conditions:

1. Under high DO: oxygen level maintained near air saturation by continuous aeration. Two separate tests were done (A) DO at 7.3 ± 2.6 (SD) mg/l - feeding fish with a demand feeder and (B) DO at 6.9 ± 1.8 (SD) mg/l - feeding fish ad lib manually.

2. Under medium DO: oxygen level maintained by controlling the duration of aeration and water used. DO maintained at 3.4 ± 1.0 mg/l. Fish were fed with a demand feeder.

3. Under low DO: oxygen level maintained by avoiding aeration and using low DO water to replenish or change tank water. Two tests were done: (A) DO at 1.3 ± 1.2 mg/l and (B) DO at 1.1 ± 0.6 mg/l.

4. Under fluctuating DO: a diurnal oxygen flux was induced by inducing a phytoplankton bloom by application of a single dose of fertilizer (N:P:K:15:15:15). Fish introduced 15 days after application of fertilizer, when tank kept exposed to daylight outside laboratory, indicated a clear DO flux. Two tests were done: (A) D.O. flux: 3.3 ± 1.5 mg/l (minimum) and 15.9 ± 2.8 mg/l (maximum). Fish were fed with demand feeder. (B) D.O. flux: 1.3 ± 0.3 mg/l (min.) and 16.1 ± 2.1 mg.1 (max.). Fish were not fed (supplementarily).

Fig. 1. Daily fluctuation in concentration of dissolved oxygen in aquarium prior to application of NPK fertilizer, during induced bloom 15 days later, and after fish introduction.

D.O. fluctuations over the day in an aquarium which was kept in daylight with some presence of algae, before NPK fertilizer application, same 15 days after application of NPK fertilizer and after introduction of tests fish (8 tilapias) are shown in fig. 1. Estimations of feed consumed (as applicable), faecal output and weight change were made for each series of experiments.

RESULTS

Results of experiments under the various oxygen regimes are given in Table I, which shows values of average weight of fish (W); weight increase (production) (P); food consumption (C); faecal output of individual fish (F) and the derived values, relative growth rate (P/W/Day), assimilation (A = C - F), metabolism (R = A - P), assimilation efficiency (%) (A/C), gross growth efficiency (%) (P' - dry weight/C), net growth efficiency (%) (P'/A) and food conversion ratio (C/P).

The increase in mean weight of individual fish during the course of the growth tests for the high, medium and low levels of DO, is shown in fig. 2. The fastest rate of growth was at high DO and slowest growth in the low DO are obvious.

In fig. 3 are given the changes in mean weight of fish under the two tests at high DO and the two tests at fluctuating DO. It is obvious that the difference in the conditions of the two tests at high D.O. (demand feeder and manual feeding) or at low DO (fish fed with demand feeder and no supplementary feed) has not made any serious difference in the growth within the sets, but the difference in growth of high DO tests and fluctuating DO tests are marked.

DISCUSSION

A comparison of the results of experiments under different oxygen levels clearly shows that growth is drastically affected by DO. The relative growth rate of both tests at high oxygen, are the highest, 0.032 and 0.034 g/day, 0.014 g/day for medium DO and 0.006 and 0.004 g/day for the two tests at low DO, showing that growth reduction is more the lower the DO (Table I). As referred to already, it is evident that in both tests at fluctuating levels of DO, the growth rates were considerably suppressed (0.017 and 0.020 g/day), indicating that the fluctuating DO does affect growth of tilapia tested.

TABLE I

SUMMARY OF DATA ON GROWTH, METABOLISM, ASSIMILATION AND FOOD CONVERSION OF O. NILOTICUS UNDER VARIOUS REGIMES OF DISSOLVED OXYGEN. ALL WEIGHTS GIVEN ARE IN GRAMS

NOTE: Weight increase of fish (Production) has been indicated as either wet (P) or dry (P'). In the latter case values were estimated assuming a water content of 78.94% in wet weight for the whole fish. Under treatment, High DO(A) = Fish were fed with demand feeder and High DO(B) = Fish were fed manually; Fluctuating DO(A) = Fish were fed with supplementary NIOMR pelleted feed and Fluctuating DO(B) = with no supplementary feed.

Fig. 2. Increase in weight of O. niloticus grown under high (near air saturation), medium and low ambient oxygen. Mean Values (± SD; n = 8) are indicated.

Fig. 3. Weight increase of O. niloticus grown under high ambient oxygen (near air saturation) and under simulated conditions of diel flux in oxygen. Data from 2 separate sets of experiments (A and B) are indicated. Curves drawn through mean values (± SD; n = 8) indicated (see test).

The comparison of the various values can be considered to be valid as the weight of fishes used are identical. However, a more correct picture can be obtained if the values are corrected for a standard weight. This has been done and the values estimated for a 10 g fish are presented graphically in Fig. 4. The results shown in Fig. 4 do not change the trends indicated in Table I. What comes out clearly is that total food consumed and assimilated by the fish is considerably reduced (40 and 60% respectively) with ambient oxygen level reducing from about 90% to about 20% air saturation. Growth and metabolism (indirect measure of energy spent in respiration i.e. standard metabolism plus energy for activity/behaviour/stress (see Brett, 1979; Kutty, 1986) also decrease considerably (about 85 and 60% respectively) with a corresponding reduction in DO, but it appears that faecal output per se increases ( about 45%) with reduction in the level of ambient oxygen.

A better appreciation of this aspect can be obtained by studying the assimilation efficiency which reduces from about 80% to about 50% with reduction in DO level from 90 to 20% air saturation, suggesting that the fish is not able to assimilate the food consumed when DO is low. Also it comes out that there is a corresponding decrease in conversion efficiency from about 15 to 5%. Other aspects dealt with in Table I need not be repeated, but the food conversion ratio (FCR), which is a crude version of conversion efficiency can be pointed out. The FCR is inversely proportional to the ambient DO level, ranging from 1.45 to 6.75 at the highest and lowest DO tested.

The values of correlation of the various parameters such as food consumption, faecal output, assimilation, growth, FCR etc with changes in ambient oxygen is shown in Table II. It is seen from the table that all values except metabolism (correlation not significant) all the other parameters are very highly correlated with ambient oxygen. Except for FCR, where ‘r’ is negative, all the rest have high positive ‘r’ values.

It is a common situation in tropical ponds that high doses of fertilizer or manure application often causes blooms of plankton which result in reduction of DO in the night, see Boyd, (1979) for obtaining extrapolated low values of DO. Often there is a concomittant diel pulse of DO, causing a prominent flux as shown in our simulated set-up (Boyd, 1979; Ali, 1986 for Aluu ponds). This oxygen condition can be expected to cause reduction in growth of fish and thus fish production in pond (Kutty, 1986). From this context, it is important to study how exactly this kind of DO change in the field causes the reduction in pond fish production. Also in the present study there is the possibility of high ammonia build-up in certain tests where NPK was added. We do not know how much this effect was but there is some indication that O. niloticus of the same size and the same conditions as tested presently, can tolerate NH₃-N levels upto 20 mg/l (near neutral pH) (EIFAC, 1986) and their growth then is affected by NH₃ levels. In tests with low ambient oxygen and high ambient ammonia (combined and separate) it was found (Lutakumwa, 1986) that growth reduction caused by low DO (80% less than that at high DO) was more than that caused by high NH₃ (57% less than that at high DO), thereby indicating that the low DO effect on O. niloticus in a DO pulse system is real. This aspect needs further elaboration.

Fig. 4. Fate of food consumed (g dry weight/fish/day) (bottom panel), assimilation efficiency and conversion efficiency (top and middle panels) in O. niloticus grown at ambient oxygen concentrations below air saturation. Values plotted are corrected for a 10g fish.

TABLE II

Correlation coefficients (‘r’) for the various parameters (Y) listed with ambient oxygen (X)

The ‘r’ value is not significant at 1% level; all other values are significant at 1% level (Snedecor and Cochran, 1967).

The present study brings out clearly the effect of decrease in DO on feeding and growth of O. niloticus. The effect of fluctuating DO is also noteworthy.

ACKNOWLEDGEMENTS

The first author (G.G.T.) is grateful to the Ministry of Agriculture, Government of Ethiopia for sponsoring him for the ARAC post-graduate course in Aquaculture (1983/84) and to the African Development Bank for the award of a fellowship through ARAC.

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