Tropical fish are renowned for their species diversity, interesting behaviors, and beautiful coloration. Most people associate tropical fish primarily with coral reefs, but species inhabiting tropical freshwaters account for 20-25% of the world’s total fish species diversity! Despite their impressive diversity, scientists are only beginning to understand the ways in which these fishes affect the functioning of tropical freshwater ecosystems.
There is a growing list of threats to tropical freshwater fish diversity, including overfishing, habitat degradation, introduced species, and river impoundments. At the same time, the number of humans dependent upon the rich fisheries of tropical rivers and lakes grows every year. These shifts raise critical question about whether every species plays a unique role in its ecosystem, or instead most species are equivalent in their functional roles.
Pete McIntyre’s research addresses the functional contributions of tropical fishes to their ecosystems through nutrient recycling. Biologists have long recognized that animals are not very efficient at retaining the nutrients in their food, and some of these nutrients are returned to the environment in forms that are readily available to fuel new productivity of plants. In the case of fishes, most species excrete a substantial proportion of their dietary nitrogen as ammonia (NH3) that is released continuously across the gills. In ecosystems where nitrogen availability limits the productivity of algae, the recycling of dietary nitrogen by fishes could be a critical part of the nutrient cycle.
We collect fish from riffles by stirring the rocks under which they hide in Rio Las Marias, Venezuela. By exhaustively collecting from many such quadrats, we can estimate the typical density of benthic fishes in the river.
An unnamed species of suckermouth catfish (Lasiancistrus sp., Loricariidae) in Rio Las Marias, Venezuela. This species eats algae and detritus, and is fairly abundant in the river.
In collaboration with Alex Flecker (Cornell University) and Mike Vanni (Miami University, Ohio), he studied the excretion of NH3 and dissolved phosphorus by fishes in a piedmont river in Venezuela. This site is home to around 80 species of fishes, including a variety of catfishes and tetras. They are investigating the determinants of nitrogen and phosphorus recycling rates, including species identity, body size, body composition, and diet. Their work has revealed great variation among species, much of which is explained by body size and composition (see Vanni et al. 2002. Ecology Letters 5: 285-293).
Their field site is not linked into a power grid, so they have always had to rely on portable field equipment that requires little power. For the last two years, they have been using a Turner Designs AquaFluor handheld fluorometer to obtain high-resolution NH3-N data in the field using only battery power. Using the OPA detection method developed by Holmes et al (1999), they have been very pleased with the instrument’s precision and linearity up to ~90 µg N/L when checked against a Turner 10-AU or autoanalyzer. Others in their research team have been equally pleased with the accuracy of low-level measurements (1-8 µg NH3-N/L) taken with the AquaFluor. The AquaFluor’s portability for air-travel and battery power for fieldwork made it a critical part of his research in South America and Africa.
They have now expanded their Venezuelan project to measure NH4 recycling by almost 50 species of fishes, and the results indicate that fishes play a critical role in quickly regenerating nutrients in this N-limited ecosystem. This information is now being combined with surveys of the fish community to determine the importance of individual species in the ecosystem.
Schools of fish in a deep pool, where they can avoid fishermen.
Rhamphichthys marmoratus, the largest of the knife fishes in Rio Las Marias, Venezuela. These fishes use weak electrical fields to navigate and hunt.
Dozens of species of fish specie live in small riffles like this one. Some species feed on algae growing on the rocks, others eat the riparian vegetation, and many hunt for insects and crustaceans.
We catch fish from the river and incubate them in bags of water to measure their nutrient excretion.
Author: Dr. Pete McIntyre
Institution: Cornell University, Rio Las Marias, Venezuela