FISH COMMUNITIES AND ENVIRONMENTAL CHARACTERISTICS OF A TROPICAL RAIN-FOREST RIVER IN COSTA-RICA

Fish communities associated with different habitats in the Rio Puerto Viejo, Costa Rica, were sampled in 1962. Forty three fish species, representing 32 genera in 17 families is a fairly rich ichthyofauna for a river of moderate size in Middle America. The macroinvertebrate fauna is also diverse, although aquatic vegetation consisted mainly of filamentous algae. A comparison of physico-chemical characteristics of the habitats and their fish communities revealed numerous examples of habitat separation of similar species. Extreme habitats (springs, swamps and an oxbow backwater) provide refuges for a few species which tolerate high CO2 and low dissolved O2 concentrations. A detailed analysis of food habits revealed how each species in a given habitat was able to reduce niche overlap within the fish community. Fruit and leaf-eating habits are present, but not nearly as diversified as in the Amazon. Some fish species reproduce in all months; others intermittently with peak periods; and catadromous species migrate to the sea once a year. Pronounced seasonal within-river migrations, similar to those of the Amazon are not present, perhaps because of the lack of well-defined high and low water periods of long duration in the Puerto Viejo. It is hypothesized that short-term low water temperatures during flooding may cause adhesive egg laying species to postpone spawning and thus avoid destruction of eggs that might have been laid above low water level. Egg scatterers may be stimulated to spawn by the same low temperatures, thus increasing survival of eggs and young by making them less visible and providing more potential food particles suspended in the turbid flood waters.