From the electronic Killifish Mailing List, as forwarded by Cal Him
By “Tomas”
If I may venture a guess, there are really “two modes” of annualism that I observed in Venezuela and to some extent in Guyana.
First, the Llanos region receives a large amount of rain. The temporary pools fill up with water and annual fish hatch and hopefully start reproducing soon. As water rises in rivers and creeks, it eventually overflows the banks, and non-annual fish invade the annual habitats, effectively decimating the annual fish fauna. only later does the habitat dry up, thus killing the non-annual invaders.
In the Lake Maracaibo region, as well as some other areas of Venezuela, the climate is a lot drier, and the topography of the region quite different. In these places one sees ponds filling up, fish hatching and then spawning before dying, due to their habitat having desiccated. Occasionally there are flash floods. However, large numbers of the rivers in Maracaibo are more or less temporary, or have very deep channels, so there would appear to be very little chance of invasion of annual habitats by non-annuals. (However, after a flash flood in Quisiro I found a few Cyprinodon and Poecilia in pools normally inhabited by Austrofundulus limnaeus and/or Rachovia hummerlincki. These fish probably came from the mangroves, which are close by.
In both cases there is a large element of stochasticity associated with the exact timing of events, and one always finds exceptions.
— G.C.K.A. Newsletter, Janaury 1997