Rearing fry of South American catfish (Rhamdia sapo) on natural zooplankton populations

Aquaculture, 70 (1988) 323-331 Elsevier Science Publishers B.V., Amsterdam -

323 Printed in The Netherlands

Rearing Fry of South American Catfish (Rhamdia supo)
ABSTRACT Zagarese, H.E., 1988. Rearing fry of South American catfish (Rhamdiasapo) on natural zooplankton populations. Aquaculture, 70: 323-331. Two experiments were performed on rearing South American catfish (Rhamdia sapo) fry in 8m2 concrete tanks. Live zooplankton was found to be an adequate food for Rhamdia fry. However, fry showed a natural preference for benthic organisms when they were available. No differences in selectivity due to fry age were observed. Differences in selectivity between small and large fry were only d.etectedfor Daphniaparuula, the largest zooplanktonic item. This suggested that food partitioning may occur only when large prey items are available.

INTRODUCTION

The food of larval freshwater fishes is very similar among different species, being mainly planktonic organisms (Le Louarn, 1980). The success or failure in culture of certain species such as the striped bass (Morone saxatilis) depends primarily on the establishment of zooplankton in rearing ponds (Geiger, 1983 ) . The first weeks after stocking fry in ponds is the life history stage in which natural food is most important. Bonneau et al. (1972) found that channel catfish (IctaZurus punctatw) started accepting artificial feed only 5 weeks after stocking.. L. Luchini (personal communication, 1985) found that for the South America:n catfish (Rhamdiu supo) this period is about 2 weeks. The acceptance of supplementary feed is not related to an extensive learning process in the case of channel catfish (Bonneau et al., 1972) and also does not seem to depend on fish size (Bryan and Allen, 1969). The determining factor appears to be availability of natural food (Bonneau et al., 1972 ). The same can

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be said for the South American catfish. Luchini and Avendafio Salas (1985) have reared Rhamdia indoors by providing them exclusively with artificial feed beginning on the first day. However, if these fry are then stocked in ponds which have an adequate density of natural items, the fish stop feeding on supplementary feed and turn to natural food for about 2 weeks (L. Luchini, personal communication, 1985 ) . Analysing these observations one may ask if this preference for natural food would confer any advantage in culture systems, such as higher growth rates or survival. Malhotra and Munshi ( 1985) observed higher survival of Aspidoparia morur (a small Asian carp) and Luchini and Avendafio Salas (1985) obtained better growth rates of Rhamdia sczpowhen they used live zooplankton as food, in preference to artificial feeds. The object of this paper was to study the feeding of Rhamdia sapo fry reared without artificial feed, with special emphasis on the contribution of zooplankton. The requirements in size and type of prey, the existence of selectivity and possible variation in relation to age and size of the fry were determined. MATERIALS AND METHODS

Two experiments were undertaken as part of a project on the culture of South American catfish at the Aquaculture Experimental Station of the National Institute of Fishery Research and Development (INIDEP) at Salto Grande (31 o S, 58” W, Argentina). The fish were reared in concrete tanks 8 m2 in area and 0.5 m deep. The first experiment lasted 8 days; 15-day-old Rhamdia fry (fed until that day with artificial food) were stocked at a density of 12/m2. The second experiment lasted 25 days; 4-day-old fry were stocked at 375/m2. The tanks were treated with organic (cow manure) and inorganic (potassium nitrate and superphosphate) fertilizers and inoculated with zooplankton from Salto Grande reservoir. Tanks were filled a few days before stocking to avoid predaceous insects. Temperature was measured twice a day (08.00 h and 18.00 h) . Dissolved oxygen and pH were periodically monitored. Water was added to compensate for evaporation and seepage. About 10 fry were sampled daily from each tank for stomach analysis. In the first experiment a number of fry equal to that removed was replaced from a batch of sibling fry maintained in aquaria with natural food. No replacement was required for the second experiment because of the high densities used. Daily zooplankton samples consisting of two 23-l Schindler trap samples were taken from opposite ends of the tanks. Samples were filtered through a 58-pm net and preserved in 4% formalin. All items in each stomach were counted microscopically, while a number of subsamples, adjusted according to Cassie (1971) to achieve a maximum error of lo%,were counted for each of the zooplankton samples taken from the tanks. The linear selectivity index, Li= ri -pi (Strauss, 1979), where ri and pi are

325

the percentages of species i in the gut content and in the water, respectively, was calculated for each tank and for each sampling date. Only those zooplankton items that were effectively preyed on by the fry were considered for these calculations. A linear regression of the natural log of fry size against age was calculated separately for each tank. This permitted an arbitrary classification of the fish in each tank and date into two groups based upon whether they were larger or smaller than predicted by the regression equations. The groups were then compared to determine if differences existed in percent composition of each of the prey species. Only those dates in which the considered prey contributed more than 5% to the diet of at least one of the two groups of fry were taken into consideration. The diets of both groups of fry were further compared by computing the percent overlap (a) according to Schoener (1970) as suggested by Keast and Eadie (1985) where cu=loo(l-1/2(p,~-p,~)) wherep,,

is the proportion

of the ith prey in group x andp+

the same in group

Y. RESULTS

During the first experiment the temperature varied from 17 to 3O”C, and survival of Rhamdia fry was less than 6% after 8 days. This was probably due to handling errors and the experiment was terminated. However, it was possible to conclude the following: (1) the diet composition of the fry showed no variation during the sampling period. Starting with the first day the fry ingested relatively large prey; instars III and IV and occasionally pupae of chironomid.s, together with ostracods represented the most abundant items in the gut contents, and (2) the daily amount of zooplankton removed by the fry was less than 1.6 g which represented less than 3.5% of the least quantity of zooplankton present in the environment during the sampling period. This amount was calculated assuming a mean fry weight of 250 mg, a daily intake rate of 26% of the fry weight (Amutio et al., 1985; H.E. Zagarese, unpublished data, 1986) and the proportion of zooplankton in the guts (21.3%, Table 1). During the second experiment the temperature varied between 16 and 29’ C, and survival of fry after 25 days was greater than 45% in both tanks. Growth equations were length = exp ( 1.715 + 0.053 day) (r-= 0.93; r~= 241) for tank 1, and length=exp (1.748+0.050 day) (r=0.93; n=239) for tank 2. During the first 2 days the diet was almost exclusively restricted to the colonial algae Eudorina sp., but the analysis of the gut contents showed that the cells were intact, with most of them maintaining the colonial structure and the mucilage. Other organisms of similar size to the Eudorirm colonies, such as rotifers and copepod nauplii, were seldom found in the gut contents although

326 TABLE 1 Diet composition (% ) of Rhamdia sapo fry Taxa

1st Exp.

2nd Exp.

Tank 5

Tank1

Bosmina (huaronensis

6.6

1.7

3.8

2.9

16.0 5.8 6.9 60.8 3.7 1.9 3.2

2.5 5.7 6.0 38.6 11.2 28.4 3.8

+ longirostris)

Daphnia laevis Daphniaparvula

1.5 10.3 31.6 45.4 1.7

Moina micrura Acanthocyclops

Tank 2

robustus

Chironomids Ostracods Others

- 200

1

200

3 160

- 160

E

- 120 -60

-40

D. -

laws

F 3 2

- 40

II-

Days

d z

Days

,100

DWS

Fig. 1.Feeding selectivity (dark circles) and abundance (organisms/l, open circles) of Daphnia laevis and D. parvula during the second experiment. Left-hand panels: tank 1, right-hand panels: tank 2.

they were abundant in the water (H-E. Zagarese, unpublished data, 1986). From the third day onwards, animal prey species completely replaced the algae in the diet. No posterior switches in diet composition were observed, ostracods and chi-

321 250

I I 200

.200

a

150

I150

100

- 100

50

- 50

0

-0

0

5

10

13

,” I : z

20

aI

0300 120 240 60 180

6o

0

5

10

15

A. robustus

% g:! :

20

Days

Fig. 2. Feeding selectivity (dark circles) and abundance (organisms/l, open circles) of Moina micrura and Acanthocyclops robustus during the second experiment. Left-hand panels: tank 1, right-hand panels: tank 2.

ronomids (mainly instars I and II ) were ingested from the third day onwards. The selectivity index values (Figs. 1 and 2) showed no definite trends during the period considered. The only species for which selectivity was positive during most of the sampling period was Acanthocyclops robustus. Table 1 shows the average composition of gut contents over the whole sampling period. The large and small fry groups showed no significant differences in diet composition. The only exception was with respect to Duphnia paruula which represented 16% of the diet of the large fry and only 5% for the small group during days 9-20 in tank 1, and days 11-20 in tank 2. Mean percent overlap (cu) between the two groups was 72.9 in tank 1 and 70.5 in tank 2, which is considered biobogically significant (Wallace, 1981). However, the overlap values decreased dramatically during days when D. parvula was abundant. DISCUSSION

No estimates of the absolute abundance of chironomids and ostracods were available, but if we consider that the number of these organisms present in the plankton samples can be used as an indicator of their abundance (the Schindler trap generally captures large quantities of these organisms although it is

328

not the most adequate sampling device for them), we can assume that the quantity of benthic organisms at the beginning of each experiment was roughly the same. During the first experiment fry were stocked at a low density (12/ m’), and the diet was composed mainly of benthos even though the zooplankton density was high. In the second experiment in which the stocking density was much higher ( 375/m2), fry fed mainly on zooplankton. From these observations it can be concluded that there was a preference for benthic prey, but when the availability “per capita” of benthic organisms decreases, the fry accept zooplanktonic prey. The same was observed by Bonneau et al. (1972) in the case of channel catfish that feed mainly on ostracods when these are abundant, and subsequently turn to zooplankton (Alona, Bosmina, and Cyclops) when the number of ostracods decreases. During the second experiment, the observed growth of Rhamdia fry can be considered good, being equal to or greater than that obtained by other researchers (Varela et al., 1982; Amutio et al., 1985; Luchini and Avendaiio Salas, 1985) using different feeds and rearing systems. In the only reported case of faster growth rates (Luchini and Avendafio Salas, 1985), zooplankton (especially Moina sp. ) was also provided as food. Survival was similar to that obtained by Amutio et al. (1985) and can be considered acceptable, particularly since fry were reared in an extensive system where invertebrate predation can be an important source of mortality (Hartig et al., 1982; Hartig and Jude, 1984). It can be concluded that, in spite of the preference for benthic prey, zooplankton is an adequate food for Rhamdia fry. It is generally accepted that selectivity varies with fish age and size (Wong and Ward, 1972; Eggers, 1977; Werner, 1979; Hansen and Wahl, 1981; Mills and Forney, 1983). In this study no differences in diet composition were found between large and small fry of equal age. It may be that the size of both groups did not differ enough to exhibit differences in selectivity, or that the prey size range maintained itselfbelow the maximum size that could be capturedby both small and large fry. This second explanation is supported by the fact that the only species for which differences in intake were found was D. paruula, which was the largest zooplankton prey. Furthermore, the diet overlap between the two groups of fry decreased when D. parvula was abundant, suggesting food partitioning related to this prey. No changes in selectivity that might be attributed to fry development were observed (Figs. 1 and 2 ); the variation in selectivity values showed no defined trend along the time axis. The only switch in diet composition happened around the third day. During the first 2 days only Eudorina was found in the gut contents and from the third day onwards it never appeared again. The role of this alga in the feeding of fry is not clear since most probably none of the cells was digested. In addition, organisms of similar size to Eudorina colonies, such as rotifers and copepod nauplii, were not found in the gut contents in spite of their abundance in the environment. We do not know whether Eudorina col-

onies were actively caught or whether they drifted in with the respiratory current of the fish. Eggers (1977) has pointed out the inefficiency of small fishes in capturing moving prey. This argument was invoked also by Malhotra and Munshi (1985) to explain the higher survival rates in fry of Aspidoparia morar when fed with live zooplankton instead of a balanced ration. According to these authors, the inability of fry to capture moving prey would prevent them from feeding before they had completely developed the ability to digest the food. This inability to capture moving prey would explain why organisms of similar size to that of Eudorirza colonies, such as rotifers and copepod nauplii, were not found in the gut contents in spite of their abundance in the water. It also demonstrates that the retention ability of gill rakers cannot be used as the only criterion for explaining selectivity. From the preceding observations it can be concluded that once the fry are capable of capturing moving prey (which occurred on the seventh day from birth; the third after stocking) it is unnecessary to provide the fry with a gradient of increasing prey size with time. Teska and Behmer (1981) arrived at the same conclusion in relation to the culture of white fish (Coregonus clupeaformis). An interesting aspect of the feeding of Rhamdia fry was their preference for Acanthoc.yclops robustus over the rest of the zooplanktonic prey since, according to many predation models (Werner and Hall, 1974; Confer and Blades, 1975; O’Brien et al., 1976; Drenner et al., 1978), this prey should contribute in a much smaller proportion to the diet of a planktivorous fish, whether due to its smaller size compared with other species, its greater escaping ability (Hall et al., 1979) or on account of its locomotion in a “hop and sink” manner (Zaret, 1975; Kerfoot et al., 1980). According to these models, the fry would require greater time to capture a cyclopoid and consequently, these should be negatively selected (Janssen, 1978). The cause of preference observed was probably related to the great abundance of A. robustus in the water during most of the sampling period (Ivlev, 1961; Bonneau et al., 1972). However, cyclopoids were positively selected even on those days when other species were more abundant in the environment. Hansen and Wahl (1981) have suggested that fish experience ca.n significantly influence prey selection, resulting in increased efficiency in capturing those prey with which they are most familiar. Thus, certain small pre:y continue to be preyed upon even after larger prey become abundant. An alternative point of view is presented by Mills et al. (1987) who found no relationship between prey selection and conditioning in young yellow perch (Perca flauescens) and suggest that some sort of innate species preference exists. ACKNOWLEDGMENTS

I thank my major advisor, Roland0 Quiros, for generous discussions and criticisms throughout all phases of this study. I also wish to express my thanks

330

to Silvina Menu Marque and Maria Cristina Marinone for expert technical assistance and critical-reading of the manuscript. I am grateful to Laura Luchini for providing Rhamdia fry and for allowing me to use the Aquaculture Experimental Station facilities. Finally, I wish to extend my sincere thanks to my friend Adrian Inchaurza for preparation of the figures.

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