Introduction Materials and methods

1. Introduction

The matrinxa, Brycon cephalus, has been reared by researchers and producers in ˜ South America due to its good adaptation to captivity, fast growth, and omnivorous Ž feeding Werder and Saint-Paul, 1978; Reimer, 1982; Eckmann, 1984; Saint-Paul, . 1986 . The amount of fingerlings produced has been the main problem for commercial development of this species, since the transformation from larvae to fingerling is the most difficult stage of production. Probably the main reason for the low yield of larviculture is the aggressive behavior of this and other species of the genus Brycon Ž . Woynarovich and Sato, 1989 . Consequently, the use of an appropriate stocking density Ž . StD in ponds could be a solution to this problem. StD affects growth rate and probably is one of the most important reasons for failure Ž . to produce fingerlings Hepher and Prugnin, 1981 . According to Araujo-Lima and ´ Ž . Ž . Goulding 1997 , survival of tambaqui larvae Colossoma macropomum in ponds is Ž . highly variable 0–66 , and the reason for this mortality remains unexplained. The same authors stated that StD may be the major cause of this variable survival. Therefore, it is essential to evaluate the influence of StD on individual fish growth and the effects of this parameter on biomass gain by the group as a whole, since individual growth rate can be higher at low StD, but biomass production of the group can be higher at higher Ž . StD Jobling, 1994 . The aim of this study was to determine the effect of StD on water quality, survival and growth of larvae of matrinxa, B. cephalus, and to contribute to the production ˜ technology for this species.

2. Materials and methods

2 Ž . The experiments were conducted in nine 64 m ponds 8 m = 8 m = 1.2 m located Ž . at the Centro Nacional de Pesquisa em Peixes Tropicais CEPTArIBAMA , Brazil. All ponds had concrete walls, a clay floor, a concrete kettle drain system, and water coming from a dam through individual open masonry channels. Water was filtered through a nylon screen of ca. 300 mm mesh to prevent the entry of predators. The ponds were left Ž 2 . empty for 5 days before the experiments. Dolomitic lime 30 grm was then added and the ponds were filled up to 1.2 m depth. Water was renewed only to compensate for losses by evaporation and infiltration, and when oxygen levels were lower than 4 mg y1 Ž y2 . Ž y2 . l . Cattle feces were added to the ponds 1 500 g m and 9 175 g m days after Ž y2 . the addition of lime. The ponds also received triple superphosphate 5 g m 2 days after the first organic manuring. The larvae were placed in the ponds 7 days after filling. Ž . Spawning to obtain the eggs was induced, as described by Bernardino et al. 1993 . All the eggs were from the same cohort and were incubated in Hungarian-model Ž . hatcheries 60 l with running water. Following the procedures of Woynarovich and Sato Ž . Ž . 1989 , curimbata Prochilodus scrofa larvae were added to the hatcheries 34 h after ´ hatching to feed the larvae of B. cephalus and to prevent cannibalism. Sixty hours after hatching the larvae showed partial absorption of the yolk sac, swelling of the gas bladder, and horizontal swimming, and therefore were considered to be ready for the experimental ponds. The larvae were randomly divided into treatments of 30, 60 and 120 larvae m y2 Ž . Ž three replicates per treatment . They were fed until satiety after day 5 it was possible to . Ž observe the larvae feeding on the water surface three times a day 0800, 1300 and 1700 . Ž . Ž . h with fish meal 35.1 crude protein prepared at CEPTArIBAMA Table 1 . Granule size of the fish meal was 1 mm up to day 14, and 3 mm thereafter. Samples of larvae were collected on days 0, 7, 14 and 21, and length and weight of the larvae were measured. The size of samples collected was proportional to the StD: low StD s 30, medium StD s 60, and high StD s 120 larvae, to avoid changes in StD during the Ž . experiments. The coefficient of variation of length CVl was calculated to analyze the homogeneity of the larvae in each treatment. CVl was obtained according to the following equation: CV s SDrX 100 Ž . where SD is the standard deviation and X is the mean length. Ž . Specific growth rate G is commonly used to evaluate the effect of different Ž treatments on fish growth Troschel and Rosch, 1991; Elliot and Hurley, 1995; Johnson . and Dropkin, 1995 . G was calculated for each collection according to Jørgensen and Ž . Jobling 1993 . On day 21 the ponds were completely drained and all surviving Ž . fingerlings on this day fishes can be considered as fingerlings were collected to Ž . Ž y2 . determine survival and production fingerlings m . Food conversion was calcu- lated according to the following equation: Food conversion s food g rweight gain g . Ž . Ž . Temperature was determined with a manual thermometer and dissolved oxygen with an oxygen meter, YSI model 57, once a day. There is a diurnal change in the dissolved oxygen concentration in fish culture ponds, and lower concentrations are always Ž . observed about 0600 h Boyd, 1982 . Water was always measured at 0800 h to determine the near-minimum concentrations of dissolved oxygen. The following physicochemical parameters of the water were analyzed at three day intervals: pH, using a pHmeter Fisher model 110; and total alkalinity, total hardness, total ammonia, and Table 1 Composition of the feed used for B. cephalus larvae Compounds used Fish flour 49.0 Corn 37.0 Textured soybean protein 8.0 Wheat flour 4.6 Soybean oil 1.0 Mineral and vitamin mixture 0.4 y1 Ž . Digestive energy kcal kg 31.88 Ž . Crude protein 35.11 Table 2 Physicochemical parameters of pond water during the larviculture of B. cephalus Ž . Mean valuesS.E.M. Means identified by different letters on the rows were significantly different P - 0.05 as determined by ANOVA and Tukey’s comparison of mean values. y2 Ž . Parameters StD larvae m 30 60 120 a a a Ž . Temperature 8C 28.070.21 27.820.22 27.910.22 y1 a a a Ž . Dissolved oxygen mg l 5.260.39 5.570.33 5.080.31 a a a Ž . pH units 6.280.03 6.330.06 6.280.04 y1 a ab b Ž . Total hardness mg l CaCO 23.251.09 20.371.06 19.380.82 3 y1 a b b Ž . Total alkalinity mg l CaCO 24.041.05 21.460.44 21.570.44 3 y1 a a a Ž . Total ammonia mg l 0.180.02 0.150.02 0.180.02 a a a Ž . Transparency cm 95.882.49 97.253.35 98.003.64 Ž . transparency according to Boyd 1982 . According to the same author, if these physico- chemical parameters of the water are at adequate levels, they do not influence food Ž . ingestion and growth in spite of diurnal changes , and it is not necessary to determine them every day. All data are expressed as mean S.E. Treatment groups were compared by one way ANOVA, and means were compared by Tukey’s multiple range test, with the level of Table 3 Ž y2 . Ž y2 . Growth of B. cephalus larvae at different StDs, i.e., low 30 larvae m , medium 60 larvae m and high Ž y2 . 120 larvae m StD Days U n 7 14 21 Ž . Length mm a a a Ž . 30 3 30 7.160.05 18.580.27 33.910.45 50.10.58 b b b Ž . 60 3 60 7.160.05 17.560.13 27.190.38 40.240.64 ab c c Ž . 120 3 120 7.160.05 17.890.13 25.200.18 35.460.42 Ž . Weight mg a a a Ž . 30 3 30 2.30.1 8010 56030 172080 a b b Ž . 60 3 60 2.30.1 6010 31020 1000100 a b b Ž . 120 3 120 2.30.1 7010 22010 790100 Ž . Coefficient of variation of length a a a Ž . 30 3 30 9.320 11.861.99 11.41.41 8.631.64 a a b Ž . 60 3 60 9.320 9.520.69 16.41.41 19.110.15 a a b Ž . 120 3 120 9.320 13.290.79 14.852.01 20.953.93 Ž . Specific growth rate a a a Ž . 30 3 30 – 51.672.72 26.611.25 15.843.74 a ab a Ž . 60 3 60 – 47.141.17 22.152.19 16.251.25 a b a Ž . 120 3 120 – 48.721.02 16.282.62 18.063.49 U Number of replicates per treatment. Number in parenthesis indicates size of the sample in each pond. Means Ž a b ab c . Ž . identified by a different superscript in the columns , , or were significantly different P - 0.05 as determined by ANOVA and Tukey’s comparison of mean values, for each parameter analyzed. significance set at P - 0.05. The statistical software program Sigma Stat, version 2.0, was used for data analysis.

3. Results