Life-history traits of Pygocentrus nattereri were compared in two populations inhabiting connected tributaries of the upper Madera River: the white water Mamor e River and the clear water It enez River. As white waters provide better trophic conditions than clear waters, the size at maturity, fecundity, reproductive effort, condition and growth of P. nattereri should be greater in the more productive white water river (Mamor e) than in the less-productive clear water river (It enez). Breeding periods were highly seasonal and similar in both rivers and under strong influence of photoperiod. Oocyte size-frequency distributions, together with the frequent occurrence of recovering females indicated that an individual female spawns at least twice during the breeding season. As predicted, fish of the Mamor e were significantly larger at maturity and had higher fecundity and condition factor values than those of the It enez. Fish from both rivers matured as yearlings. The higher growth potential of females was better expressed in the Mamor e than in the It enez, where growth differences between sexes were weak. Females had a significantly better growth in the Mamor e than in the It enez. The observed life-history traits associations were consistent with the hypothesis of better trophic conditions in the Mamor e. In addition, previous genetic analyses evidenced that the colonization of the two basins is recent and that extant populations have very similar genetic backgrounds. This suggests that the observed variations in life-history traits of P. nattereri are not related to historical factors (genetic drift) between two phylogeographically distinct lineages, but rather due to the contrasting environmental conditions in the white and clear waters.

Brooders of Surub|¤ (Pseudoplatystoma fasciatum) were caught in the Ichillo River (Bolivian Amazon) and adapted to captivity conditions for 1 year in the facilities of the experimental aquaculture station of 'El Prado' (Santa Cruz de la Sierra) under natural temperature and photoperiod conditions. Induced reproduction was obtained by means of Ovaprim s (Syndel, Canada) injections and arti¢cial fertilization. Sperm and ova were obtained by gentle stripping of male and female brooders. Fertilized eggs were incubated in 60 L Zug jars. A mean hatching rate of 73.7 AE 19.0% was obtained after 24 h at 26.5 1C. For larval rearing, several protocols were tested with different settings of photoperiod, light intensity, food type and period of distribution, and stocking density. The best survival rates were obtained with Artemia nauplii feeding in total darkness. A high level of aggressiveness between larvae and precocious appearance of jumpers was observed, but these can be controlled with appropriate rearing conditions. Values (n 5 23) represent minimum, maximum, mean and standard deviation (SD).Aquaculture Research, 2008, 39, 764^776 Induced breeding of Pseudoplatystoma fasciatum J Nun ì ez et al.

-Meal frequency is a key parameter in fish larviculture, especially in highly cannibalistic species. Knowledge of the biological bases of cannibalism (growth capacity of cannibals, morphological constraints on cannibalism, prey size preference) can help predicting the risks of cannibalism for different feeding schedules under culture conditions. This study relied on the day-by-day analysis of prey size preference and bioenergetics of individual cannibals of the catfish Pseudoplatystoma punctifer (8-65 mm standard length, SL, 0.5-400 mg dry mass, DM) at 28.5• C under 12L:12D. The results were equated with the ontogenetic variations of morphological factors (head and mouth width) and feed efficiency of larvae feeding on Artemia nauplii, in order to calculate the risks of cannibalism among fish fed 2-7 daily meals. The predation capacities of P. punctifer were highest at 8 mm SL and decreased in larger fish (largest prey = 86% and 70% SL in fish of 8 and >30 mm SL, respectively). Cannibals of increasing size preferred increasingly smaller prey relative to their own size, but also to their predation capacities. These morphological and behavioural constraints were largely compensated for by bioenergetics performance. Cannibals consumed high daily food rations (as high as 171 and 29% DM in fish <1 and >300 mg DM, respectively), exhibited high gross conversion efficiencies (0.50-0.55 and about 0.70, in fish <1 and >30 mg DM, respectively), and grew rapidly (90 and 18% DM day −1 in fish <1 and >300 mg DM, respectively). The growth advantage of cannibals over siblings fed Artemia nauplii was decisive, except for high meal frequencies (6-7 daily meals). This study supports the view that the risk of cannibalism and adequate feeding strategies can be largely predicted in a particular fish species if the morphological, behavioural and bioenergetics bases of cannibalism are examined altogether in an ontogenetic perspective.