In most tropical rivers subject to a single-yearly flooding, migratory fish usually spawn in the onset of the flooding and larvae drift to their nursery habitats. To define when the migratory fish species reproduce, its relationship with the water level and when nursery areas in floodplain lakes are supplied with ichthyoplankton in the Magdalena River, a twoyearly peak flooding river, we sampled the ichthyoplankton weekly throughout 2 years. The fish species Pseudoplatystoma magdaleniatum, Prochilodus magdalenae, Leporinus muyscorum, Sorubim cuspicaudus and Curimata mivartii spawn twice a year in the main channel and its larvae input into the floodplain lakes are not always associated with flooding. Spawning in the main channel and larvae inputs into the floodplain lakes suggest that the habitat used by the migratory fishes may be consistent with some hypotheses on habitats used by these fishes.

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Young red tilapias were exposed for 96 h to each one of 6 concentrations of the pesticide Diazinon in order to determine the pesticide's acute toxicity level. After the ascertaining the lethal concentration (LC50) at 96 h, a level 10 times lower was selected for the bioaccumulation study of the pesticide in male and female specimens exposed for 9 days. The elimination process was carried out for 10 days beginning right after the conclusion of the accumulation process. Analytical procedures were developed and used for the studies of acute toxicity and bioaccumulation of Diazinon in red tilapia. A lethal concentration [LC50 (96 h)] of 3.85 mg/L was found, and steady-state accumulation, at a concentration of 28.45 mg/kg, was reached at 7.72 days. In the elimination process a concentration of 0.29 mg/kg was found in tilapia tissue by the sixth day after the fish were moved to clean water, and it continued to decrease quickly toward nondetectable levels.

In order to know the flora of planktonic cyanobacteria, 24 water samples (May 2010-October 2013) from the Riogrande II Reservoir, Colombia (75832 0 30 0 W-75826 0 10 0 W and 6833 0 50 0 N-6828 0 07 0 N) were studied. The reservoir provides water to 1.4 million inhabitants in Medellin, Colombia (40 % of the total population). Among the cyanobacteria, we identified 11 morphospecies belonging to three families of the order Chroococcales (Merismopediaceae (2 species), Microcystaceae (3) and Synechococcaceae (1)), one of Oscillatoriales (Pseudanabaenaceae, 1 species) and one of Nostocales (Nostocaceae, 4 species). The genera with the highest number of taxa were Microcystis Kützing ex Lemmermann and Dolichospermum (Ralfs ex Bornet et Flahault) Wacklin, Hoffmann & Komárek, both known to form dense blooms, many of them toxic. That is why the aim of this research is to understand the cyanobacterial species richness in the reservoir.

The objective of this study was to determine whether fish collected from the La Miel or Nechí Rivers (Colombia) differed in muscle methyl mercury (meHg) concentration. Two fish from six different species were collected from markets adjacent to each river. Overall, fish collected from the market adjacent to the Nechí River contained higher levels of meHg. This result however is being driven by very high meHg concentrations in four individual fish, three of which are Pimelodid, long-whiskered catfish. These catfish may represent ideal sentinel organism for the detection of meHg contamination in Colombian rivers.