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Aquaculture Research. 1997, 28, 481^88 UNIV. OF MARYLAND. PERIODICALS Growth and metamorphosis of Rana catesbeiana (Shaw) tadpoles fed live and supplementary feed, using tilapia, Oreochromis niloticus {L.), as a biofertilizer M A Benitez-Mandujano & A Flores-Nava Centro de Investigacion y de Estudios Avanzados del IPN - Unldad Merida. Merida. Yucatan and Programa de Ecologia. Pesquerias y Oceanogralia del Golfo de Mexico, Universidad Autonoma de Campeche, Agustfn Melgar y Juan de la Barrera, Ciudad Universitaria, Campeche, Camp., Mexico Correspondence: Dr A Fiores-Nava. CINVESTAV-IPN. AP73-Cordemex. Merida. Yucatan. Mexico Abstract An experiment was conducted to compare growth and timing to metamorphosis of bullfrog Rana catesbeiana (Shaw) tadpoles feeding on phytoplankton and on supplementary feed. Three interconnected, round, 4 m diameter concrete tanks were used in the experiment. Tanks 1 and 2 were stocked with juvenile tilapia, Oreochromis niloticus (L.) to stimulate phytoplankton through faecal fertilization. A third tank remained without fish. Stage 25-Gosner bullfrog tadpoles were placed in 0.042 m^ cages at 1 T^. Experimental treatments consisted of: (1) tadpoles feeding solely on phytoplankton (P); (2) P + 13% body weight day'^ (bwday"^) supplementary feeding (SF); (3) P + 9.75% bw day-^ SF: (4) P + 6.5% bw day'^ SF: (5) P + 3,25% bw day-i SF; and (6) tadpoles feeding solely on supplementary feed at 13% bwday"^. Final weight was lowest in those organisms feeding exclusively on supplementary feed, followed by those feeding on phytoplankton. Treatments 2 and 3 showed the highest weight (3.65 and 3.64 g, respectively). After 70 days, 50% of the tadpoles in treatment 4 (6.5% bw day"^) reached metamorphic climax, followed by treatment 5 (33%). Only 8% of tadpoles feeding exclusively on live food reached metamorphosis. It is concluded that in the presence of abundant phytoplankton, it is possible to reduce up to 50% of the standard supplementary feeding rate and still have normal growth and metamorphosis. Tilapia represents a good alternative for biofertilization. Introduction In aquaculture, feed is one of the major components of production expenditures, in some cases representing up to 60% of the variable costs (Hepher 1988; Tacon, Stafford & Edwards 1989). There is, therefore, an important effort, by both nutritionists and aquaculturists, to develop cost-effective diets, and to implement management strategies such as water fertilization, to increase natural food and consequently reduce production costs. In commercial rearing of ranid tadpoles, feed Is a critical aspect, and consists of a wide range of protein sources which include fresh vegetables (Culley 1991), boiled egg mashes (Flores-Nava, Olvera- Novoa & Gasca-Leyva 1994) and, in most cases, commercially available pelleted feeds (Fontanello, Arruda, Mandelli & Marques 1982; Lopes-Lima & Agostinho 1992; Flores-Nava 1994a). However, ranid tadpoles are good suspension feeders and periphyton grazers (Dickman 1968; Viertel 1992), thus they make good use of natural feed, even under culture conditions (Culley 1991). Fertilization of the culture therefore appears to be a viable strategy for increasing food availability for tadpoles. In preliminary trials, the Nile tilapia, Oreochromis niloticus (L.), has been used as a bio-producer of © 1997 Blaekweli Sdenee Ltd. 481
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Growth and metamorphosis of Rana catesbeiana (Shaw) tadpoles fed live and supplementary feed, using tilapia, Oreochromis niloticus (L.), as a biofertilizer

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Page 1: Growth and metamorphosis of Rana catesbeiana (Shaw) tadpoles fed live and supplementary feed, using tilapia, Oreochromis niloticus (L.), as a biofertilizer

Aquaculture Research. 1997, 28, 481^88

UNIV. OF MARYLAND.PERIODICALS

Growth and metamorphosis of Rana catesbeiana(Shaw) tadpoles fed live and supplementary feed,using tilapia, Oreochromis niloticus {L.), as abiofertilizer

M A Benitez-Mandujano & A Flores-NavaCentro de Investigacion y de Estudios Avanzados del IPN - Unldad Merida. Merida. Yucatan and Programa de Ecologia.

Pesquerias y Oceanogralia del Golfo de Mexico, Universidad Autonoma de Campeche, Agustfn Melgar y Juan de la

Barrera, Ciudad Universitaria, Campeche, Camp., Mexico

Correspondence: Dr A Fiores-Nava. CINVESTAV-IPN. AP73-Cordemex. Merida. Yucatan. Mexico

Abstract

An experiment was conducted to compare growthand timing to metamorphosis of bullfrog Ranacatesbeiana (Shaw) tadpoles feeding onphytoplankton and on supplementary feed. Threeinterconnected, round, 4 m diameter concrete tankswere used in the experiment. Tanks 1 and 2 werestocked with juvenile tilapia, Oreochromis niloticus(L.) to stimulate phytoplankton through faecalfertilization. A third tank remained without fish.Stage 25-Gosner bullfrog tadpoles were placed in0.042 m^ cages at 1 T^. Experimental treatmentsconsisted of: (1) tadpoles feeding solely onphytoplankton (P); (2) P + 13% body weight day'^(bwday"^) supplementary feeding (SF); (3)P + 9.75% bw day-^ SF: (4) P + 6.5% bw day'^ SF:(5) P + 3,25% bw day-i SF; and (6) tadpoles feedingsolely on supplementary feed at 13% bwday"^.Final weight was lowest in those organisms feedingexclusively on supplementary feed, followed by thosefeeding on phytoplankton. Treatments 2 and 3showed the highest weight (3.65 and 3.64 g,respectively). After 70 days, 50% of the tadpoles intreatment 4 (6.5% bw day"^) reached metamorphicclimax, followed by treatment 5 (33%). Only 8% oftadpoles feeding exclusively on live food reachedmetamorphosis. It is concluded that in the presenceof abundant phytoplankton, it is possible to reduceup to 50% of the standard supplementary feedingrate and still have normal growth and

metamorphosis. Tilapia represents a good alternativefor biofertilization.

Introduction

In aquaculture, feed is one of the major componentsof production expenditures, in some casesrepresenting up to 60% of the variable costs (Hepher1988; Tacon, Stafford & Edwards 1989). There is,therefore, an important effort, by both nutritionistsand aquaculturists, to develop cost-effective diets,and to implement management strategies such aswater fertilization, to increase natural food andconsequently reduce production costs.

In commercial rearing of ranid tadpoles, feed Is acritical aspect, and consists of a wide range of proteinsources which include fresh vegetables (Culley1991), boiled egg mashes (Flores-Nava, Olvera-Novoa & Gasca-Leyva 1994) and, in most cases,commercially available pelleted feeds (Fontanello,Arruda, Mandelli & Marques 1982; Lopes-Lima &Agostinho 1992; Flores-Nava 1994a).

However, ranid tadpoles are good suspensionfeeders and periphyton grazers (Dickman 1968;Viertel 1992), thus they make good use of naturalfeed, even under culture conditions (Culley 1991).Fertilization of the culture therefore appears to bea viable strategy for increasing food availabilityfor tadpoles.

In preliminary trials, the Nile tilapia, Oreochromisniloticus (L.), has been used as a bio-producer of

© 1997 Blaekweli Sdenee Ltd. 481

Page 2: Growth and metamorphosis of Rana catesbeiana (Shaw) tadpoles fed live and supplementary feed, using tilapia, Oreochromis niloticus (L.), as a biofertilizer

Bullfrog growth and metamorphosis M A Benitez-Mandujano & A Fiores-Navn Aquaculture Research, 1997. 28. 481^88

plant nutrients. Their excreta induce phytoplanktonblooms, which in turn serve as food for bullfrog,Rana catesbeiam (Shaw), tadpoles with good results(Flores-Nava, Gasca-Leyva & Gil-Trava 1992a;Flores-Nava, Gil-Trava and Gasca-Leyva 1992b).Thus they offer an alternative for biofertilization atlow cost.

The objective of the present study was to comparegrowth and time to metamorphosis of bullfrogtadpoles fed on phytoplankton produced in responseto tilapia fertilization, and supplementary food.

Materials and methods

A total of 756, stage 25 (Gosner 1960), 0.036 gmean weight. Rana catesbeiam tadpoles from a singlecohort, were used in the present experiment. Thetadpoles were stocked at 1 T^ in galvanized steelcages (0.4 X 0.3 X 0.35 m), with a 32 mm meshsize, and an upper frame made of styrofoam forflotation. The cages were randomly distributed inthree 4 m-diameter, circular concrete tanksinterconnected through PVC piping. To keep waterquality parameters as homogeneous as possiblebetween treatments, a central drain pipe located ineach tank directed the water into an adjacentcollecting tank from which it was pumped into aheader tank and then by gravity, it returned intothe experimental tanks, thus creating a recirculatingsystem. Water depth in the tanks was kept at 0.40 m.

To stimulate natural productivity, tanks 1 and 2were stocked 1 week prior to the introduction ofthe tadpole cages, with juvenile tilapia Oreochromisniloticus at 3.3 m"^, with a total initial biomass of10 kg per tank. The fish were fed a 40% proteindiet, once every other day, at 3% bw day"\ andtheir faecal matter was allowed to leach nutrients,so that an algal bloom was established. Feeding offish stopped 24 h before the introduction of tadpolesinto the tank, and no further supplementary feedwas provided to the fish.

The experimental design consisted of fivetreatments and a control, each with three replicates(Table 1) and 42 tadpoles per cage.

Supplementary feed (SF) consisted of a high-protein moist dough; the composition and proximalanalysis are presented in Table 2, Feed was providedevery day at noon in a Petri dish placed at thebottom of each cage. Every day, the Petri disheswere replaced and the left-over feed was collected,dried and weighed to calculate the maximum foodintake.

Natural productivity (P) represented themicroalgal blooms stimulated by the excreta of boththe fish and the tadpoles in tanks 1 and 2.

Tank 3 was used as a control, where three cageswith tadpoles were fed only SF (treatment 6). Amechanical filter, placed at the inlet of this tank,consisted of a series of three cotton, 45 nm meshbags, which were used to prevent the phytoplanktonproduced in tanks 1 and 2 from contaminatingthis tank. Bag filters were replaced every 12 h bymomentarily closing the water circulation and thensubstituting them. A constant inflow of 8 ± 2 1min~^ was maintained in each tank.

Water temperature, dissolved oxygen and pH weremonitored daily by means of an ICM multimeterModel 51500. Nitrate-N, nitrite-N, total ammonia-N and phosphate were determined every 2 weeks,using the standard procedure recommended byAPHA-AWWA (1985).

Phytoplankton was counted dally in each tank,using a haemocytometer to determine relativeabundance. Identification of microalgae was carriedout to the genus level, using the keys in Prescott(1954).

Every 2 weeks, all cages were removed, all thetadpoles counted, rinsed gently but thoroughly, andweighed in an electronic balance to the nearest0.01 g.

Growth was calculated in terms of weight gained(wg) = [100 (final weight - initial weight) / initialweight]. Metamorphosis rate was determined by thecumulative percentage of organisms within eachtreatment that reached stage 42 (Gosner 1960), i.e.animals developed v»rith four limbs and a tail.

Feeding efficiency in those treatments where SFwas provided, was measured by estimating the feedconversion ratio (FCR),

A kurtosis normality test (Sokal & Rohlf 1979)was applied to the data and the results werecompared using one-way analysis of variance afterarcsin transformation of percentage figures. Themeans were compared for significant differences(P < 0.05) using Duncan's multiple range test(Duncan 1955).

Results

Water quality

T^ble 3 presents the results of water quality. Allparameters remained steady with little oscillationand within intervals reported as adequate for good

482© 1997 Blackwell Science Ltd. Aquacidtun Research, 28 , 481-488

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Aquaculture Research. 1997. 28. 481-488 Bullfrog growth and metamorphosis M 4 Benite-Mandu;ano& A F/ores-Navo

1 Description of treatmentsincluded in the experimentar designfor the present study

Treatment number Feeding strategy^ Corresponding % bodyweight day-^ ofsupplementary feed'

Tank

123456

PP + 100% SFP + 75% SFP + 50% SFP + 25% SF100% SF

-13.09.756.503.25

13.0

122223

ip. cultured productivity of the tank system: SF. supplemented feed (see Table 2).

•̂ Based on the standard rate of 13.0% bw day"' recommended by Lopes-Lima &

Agostinho (1988).

Table 2 Composition and proximal analysis of thesupplementary diet used in the present study

Ingredient

CompositionPropac^Soya bean mealFish oilSoya oilCorn starchHydrolysed fish concentrateMineral premix^Vitamin premix^Carboxymethyl cellulose

Proximal analysisMoistureCrude proteinLipidsCrude fibreAshNitrogen-free extractNet energy (J)

% Wet weight

20.9342.903.715.48

13.1278.863.001.501.50

45.7022.00

7.601.905.10

17.70

1.826

'Protein concentrate.^From: Tacon et aJ. (1984).^From: Tacon et fli. (1983).

growth of bullfrog tadpoles (Flores-Nava 1992a).Temperature was above 25.8°C; dissolved oxygenand pH remained ahove 6.6 mg 1"̂ and pH ahove7.6. Total hardness and total alkalinity were alsorelatively high, with minima of 368 and 335 mgCaC03 r ^ respectively.

Nitrite was relatively low (0.08 - 0.10 mg T^)throughout the study period, with the lowest figuresregistered in tank 3, whUe nitrate consistently

showed the highest levels (3.51 ± 0.11 mg 1 ̂ ).also in tank 3.

Reactive phosphorus showed consistently lowtrends throughout the study period, with amaximum of 0.06 mg P04-Pr^

T^hle 4 presents the results of the relativeabundance of phytoplankton in the experimentaltanks during the study. Similar trends were observedin tanks 1 and 2 (exposed to sunlight), characterizedhy a phytoplankton with few dominant species. Cellcounts ranged between 6 and 8 X 10* mt^ in tanks1 and 2, whereas a maximum of 200 cells ml"^ wasobserved in tank 3 on day 28 of the experiment,although the mean count for this tank was 46cells m r ^ Chlamydomonas sp. was the dominantspecies in tanks 1 and 2, followed hy Chlorella sp.and Scenedesmus sp.

Growth and metamorphosis

The results of growth, survival and feeding efficiencyof the experimental organisms are presented inTable 5. Average survival in all treatments was above95%. Mean final individual weight was significantlyhigher (P < 0.05) in both treatments 2 and 3, thanin the rest of the treatments, reaching 3.650 and3.648 g respectively, followed by treatment 4 (2.99g) and treatment 5 (2.80 g), these latter treatmentsnot being statistically different (P > 0.05) from eachother. The lowest final individual weight wasobserved in treatment 6 (1.49 g), while tadpolesin treatment 1 reached a mean final weight of2.15 g (Fig. 1).

Figure 2 presents the cumulative percentage ofmetamorphosed tadpoles. Rate of metamorphosis

© 1997 Blackwell Science Ltd. Aquaculture Research. 28 , 481-488 483

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Bullfrog growth and metamorphosis M A Benitez-Mandujano & A FJores-Nava Aquaculture Research. 1997. 28, 481-488

Parameters

Temperature (°C)

pHOxygen (ml!"')Nitrite (mg 1"')Nitrate (mg r')Phosphate (mg r'')Total ammonia (mgTotal hardness (mg

Total alkalinity (mg

I-')

M)

Tank1

26.58.38.50.11.30.060.05

382

35 :

± 1.80± 0.20± 3.30± 0.02± 0.75i 0.01± 0.02± 260

t 172

Tank 2

26.6 ±8.2 ±8.9 ±0.1 ±1.8 ±0.05 ±0.04 ±

374 ±

339 ±

1.70.22.50.030.520.010.20

272

178

Tank 3

25.8 ±7.6 ±6.6 ±0.08 ±3.51 ±0.05 ±0.80 ±

368 ±

336 ±

1.40.2

1.030.020.200.03

0.11268

180

•feble 3 Mean values (± SD) of waterquality for experimental tanks withRana catesbeiana tadpoles. Tanks 1and 2 were also stocked with tilapia.Oreochromis niloticus

T^ble 4 Relative abundance of phytoplankton present intanks stocked with Oreochromis niloticus and Rana

catesheiam tadpoles (1 and 2). and tadpoles only (3)

Algae Abundance

Tank1 Tank 2 Tank 3

Scenedesmus sp.Chlorella sp.Chlamydomonas sp.Golenkinia sp.Navicula sp.Selenastrum sp.Closterium sp.

'Description: **'*. very abundant:occasionally abundant; ' . scarce.

abundant;

was greatest in treatment 4. where more than 50%of the tadpoles reached stage 42 within 70 days;treatment 5 followed, with 33% reaching stage 42.Treatments 2. 3 and 6 produced 10%. 12% and8% metamorphosis, respectively; and there was nometamorphosis in treatment 1 within 70 days.

Feed conversion ratio (FCR) is considered anindicator of the apparent supplementary feedconsumption and assimilation (hut does not includeconsumption and metaholism of phytoplankton).There was a positive direct relationship hetweenamount of supplementary feed provided and FCR.with maxima of 1.10 and 1.09;1.0 (grams of feedfed:weight gain) in treatments 6 and 2 respectively.

Discussion

It is well documented that ranid tadpoles are notonly good suspension feeders, concentrating mainly

l^ble 5 Growth, survival and feed conversion ratio of Rana catesbeiana tadpoles feeding on natural and supplementary

feed. Within rows, figures with the same superscripts are not significantly different (P > 0.05)

Parameter

Mean individual initial weight (mg)Mean individual final weight (mg)Average weight gain (%)Survival (%)Feed conversion ratio'

1

37.0^2150 "̂5810.8"=

99.07 "

2

37.3="3650^9785.5 ^

100^1.09^

3

38.6"3648"9450.7 "

97.2"0.76*=

Treatment

4

33.3=*2995 ^8993.9 ""

95.4"0.63'

5

34.0"2800 ""8235.4 "^

100"' 0.30 <"

6

35.5"1498 <=4219.7 '"̂

99.1 ^1.10"

± std error

2.6510762782.9

2.260.40

takes into account only supplemental feed.

484© 1997 Blaekweli Science Ud. Aquacuiture Research 28 , 481-488

Page 5: Growth and metamorphosis of Rana catesbeiana (Shaw) tadpoles fed live and supplementary feed, using tilapia, Oreochromis niloticus (L.), as a biofertilizer

Aquaculture Research. 1997. 28. 481-^88 Bullfrog growth and metamorphosis MA Benlte2-Mflndu/ono&/I F/ores-Nava

Figure 1 Growth of Rana catesbcianatadpoles fed live phytoplankton (P)and supplementary feed (SF).Percentages are In relation to thestandard feeding rate of 13% bodyweight day~^ recommended by Lopes-Uma & Agostinho (1988).

28Time (days)

p+ioo%SF - '^p+ys ' /oSF

P+50%SF - \ ^ - P+25%SF - O - 100%SF

Figure 2 Cumulative percentage oftadpoles in each experimenttreatment that reached stage 42(Gosner 1960), that is four limbs anda tail.

60 r

50 -

40

30

20

O

10 I10

Time (weeks)

I P S P+100%SF n P+75%SF 0 P+50%SF [ 3 P+25%SF g 100%SF

on phytoplankton (Wassersug 1972; Seale & Beckvar1980; Flores-Nava etal. 1992b), but also efficientperiphyton grazers (Dickman 1968). The results ofthe present study indicate that primary productivity,particularly phytoplankton, plays an important rolein increasing tadpole growth and hastening the roleof metamorphosis under culture conditions. It isnotable that the current tendency in culturingbullfrog tadpoles is to offer solely high-protein, fish-meal-based diets (Lopes-Lima & Agostinho 1988.1992; Mazzoni & Carnevia 1990. 1992).

It is clear from otir results that a combination of

live (phytoplankton) feed and a balanced diet withhigh protein levels provides a better nutritionalprofile for bullfrog tadpoles than either of the twofeed types alone. The lowest weight gain was seenin those organisms given exclusively supplementaryfeed, thus suggesting that the supply of a high-protein pelleted feed, such as the one provided inthe present trial, was not nutritionally sufficient topromote growth and metamorphosis as rapid asIn those organisms that had access to naturalproductivity.

Comparati'vely low growth and slow rate of

© 1997 Blackwell Science Ltd. Aquaculture Research. 28, 4 8 1 ^ 8 8485

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Bullfrog growth and metamorphosis M A Benitez-Mandujgno &/I Fiores-Ngva Aquaculture Research. 1997, 28, 481-488

metamorphosis was also observed in thoseorganisms feeding only on phytoplankton; moreover,none of the tadpoles of this treatment reachedmetamorphosis within the 70-day experimentalperiod. Collins (1979) stated that the size of a firogat metamorphosis and the timing of metamorphosiscan be influenced by quality of the diet, amongother factors.

Even though the specific nutritional requirementsof bullfrog tadpoles have not been defined yet,the reported protein level optimal for diets underlaboratory conditions, ranges between 35% and43%, evaluated through biomass increment(Marschall 1978; Mazzoni, Camevia, Rosso, Salvo,Aerosa & Antoniello 1992). However, lower proteincontents (20%) in bullfrog tadpole diets have beenreported to produce good growth in outdoors rearingtanks (CuUey, Meyers & Doucette 1977). Althoughmethodological conditions may he different, andprotein quality may have varied among experiments,there is also consistent evidence, as in this work,that when tadpoles have access to microflora, theyperform much better than without it (Culley 1991;Flores-Nava et al. 1994). This clearly indicates thattadpoles complement their nutritional requirementswith the natural food present in the culture system.

It is interesting to note that treatments 2 and 3,the two treatments that produced the highest meanindividual final weights, only produced 10% and12% metamorphosis. In contrast, treatments 4 and5, with slower-growing tadpoles, produced 54% and33% metamorphosis. This means that with 50%less supplementary feed (6.5% body weight day^^)and access to natural productivity, more than fivetimes as many tadpoles metamorphosed at a finalweight that was only 18% lower than that intreatment 2.

Collins (1979) stated that at higher food resourceavailability, there will be a higher average weightof tadpoles at metamorphosis, hut, in contrast, theaverage time to metamorphosis is increased. Thiscould explain why, of the tadpoles fed the combined(P -I- SF) feed, those with comparatively lower levelsof supplementary feed (namely treatments 4 and 5),showed a faster rate of metamorphosis.

Ideally, from the aquaeulturists's point of view,four conditions should be sought in tadpole rearing:(1) high weight at metamorphosis; (2) fast rate ofmetamorphosis; (3) homogeneous size distributionwithin the tadpole population; and (4) lowproduction costs. According to our own experiences(Flores-Nava 1994b; Flores-Nava etal. 1992b,

1994) under tropical conditions. Ram catesbeiamtadpoles metamorphose within 45 to 100 days afterhatching, attaining a relatively low average weightat metamorphosis (2.2-5.0 g). This small size offroglets does not represent a problem in theproduction cycle, because they reach market size(200 g) in 6-7 months, results that comparefavourably to the growth of Rana catesbeiam inother tropical latitudes (Lopes-Iima & Agostinho1992).

In general terms, in this study, final body weightswere homogeneous in all treatments but treatment6, where a polymodal distribution was observed. Thismight have been the result of increased intraspecificcompetition for food, which is common in bullfrogtadpoles at high densities (Vera 1994).

It is possible to conclude that, under ourexperimental conditions, supplementary feed, andconsequently feed costs, could be reduced by up to50% with respect to the standard feeding regimeemployed in South America (Fontanello et al. 1982;Lopes-Lima & Agostinho 1988) whQst stillmaintaining adequate growth and metamorphosisrates.

In the present trial, tilapia were used as the mainsource of plant nutrients through lixiviation fromtheir faeces. It would be interesting to work out anutrient budget that includes all other nutrientinputs (i.e. leaching from pelleted feed and tadpolefaeces), so as to estimate the actual proportionalnitrogen and phosphorus supply from each source.In the present study, total nitrogen (TN) in feed was2121.9 mg kg"^ whUe in faeces it was 5320.4 mgkg"i dry weight. Total phosphorus (TP) was 188.4mg kg'i in the former and 247.2 mg kg"^ dryweight in the latter. The above clearly indicates thatOreochromis niloticus played an importantbiofertilizing role.

Tilapia has been successfully used as a biofertilizerin some commercial bullfrog-tadpole-rearingsystems in Southwest Mexico (Flores-Nava 1994a).As demonstrated in this study, this strategy couldhelp reduce tadpole feeding costs substantially, eitherby integrating tadpole and tilapia culture, or by usingtilapia as a biofertilizer through a maintenance-level,low-cost culture.

As far as the influence of the fish and tadpoles onwater quality in the tanks is concerned, the resultsshow that nitrogen compounds as well as reactivephosphorus remained relatively low during the studyperiod, with higher levels in tank 3. These lownutrient concentrations in tanks 1 and 2 may be

486© 1997 Blackwell Science Ltd. Aqvacullun Research, 28 , 481-488

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Aquaculture Research. 1997. 28. 481-488 Bullfrog growth and metamorphosis M/iBem£cz-Manduj(ino&^ Fiorcs-Navfl

the result of direct phytoplankton uptake, as reflected

by the high algal densities encountered. Low levels

of phosphate in tank 3 could he explained hy calcium

complexation normally occurring in the hard waters

of Yucatan (Flores-Nava 1994b).

Further research is needed to investigate the

nutritional value of different phytoplankton species

in relation to bullfrog tadpoles, so as to evaluate the

feasibility of isolating and inoculating the most

favourable algal strains in tadpole-rearing tanks.

Also, studies to determine the specific protein

requirements of tadpoles will help to define the

optimum ratio of natural to supplementary protein.

Acknowledgments

This study was supported by CONACYT-Mexico

Project No. 1746-A9209. The authors are grateful

to Dr Jack Frazier for his suggestions and revision

of the manuscript. We are also indebted to Dr

Suzanne Wendker and Mrs Fanny Merino for

identification of phytoplankton species.

References

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