While many tropical freshwater fishes spawn during periods of flood associated with rainy seasons, there are some fishes that are exceptions to the rule and spawn only during dry seasons.
One such species is Creagrutus guanes, a small tetra inhabiting mountain streams of the Magdalena River drainage in Colombia, whose reproductive biology has been studied by Mauricio Torres-Mejia and Martha Ramrez-Pinilla.
Publishing their results in the most recent issue of the journal Copeia, the authors collected the tetras once a month over a period of 13 months and then made macro- and microscopic observations of gonads, as well as measuring the weights of gonads, mesenteric fat, and liver.
Based on the analyses of their data, the authors concluded that Creagrutus guanes is a species whose reproductive activity is concentrated in periods of low rainfall (December"March and June"August).
Traditional hypotheses of why some freshwater fishes prefer to spawn during the dry season invoke the fact that during dry seasons tropical rivers have a higher concentration of larval food (zooplankton and algae) and a more stable habitat (no flash floods), both factors favouring the survival of offspring.
The authors found circumstantial evidence that weakens support for the hypothesis that reproduction is synchronized with food abundance for adults (because the most significant weight gains in females were found soon after the onset of the wet seasons).
It is also doubtful that the hypothesis that reproduction during dry season prevents juveniles from being washed out by heavy rains holds true for C. guanes, as according to the authors, this species prefers backwaters and embayments, where flow speed is null even during the strongest floods, and it is unlikely that juveniles can be washed out in those conditions.
For more information, see the paper: Torres-Mejia, M & MP Ramrez-Pinilla (2008) Dry-season breeding of a characin in a Neotropical mountain river. Copeia 2008, pp. 99"104.