Matthew Baker and Dr. Mark Belk, Zoology

Research exploring the effects of predation on life history evolution has focused almost exclusively on geographically disjunct populations subjected to discrete differences in mortality. Sampling only the ends of a predation continuum left to guess how prey life histories in the live bearing fish Brachyrhaphis rhabdopora vary by sampling at points along the entire predation gradient.

I collected B. rhabdopora at seven sites ranging in elevation from near sea level to 850 meters in Rio Ciruelas, a 23.5 Km stream that heads on the western slope of the Tilaran mountains in western Costa Rica. Fish were successfully collected at six of the proposed seven sites during the months of January and February 1999. I found no fish in the uppermost site, 4 km below the headwaters. Rhamdia guatamalenses (catfish), Eleotris picta (la vieja) and other predator fauna are found in the lower reaches of the stream, but are absent from the upper reaches where only B.r are found. I collected a total of 903 B.r using a 3m x 1.3; 2mm mesh seine. Fish were preserved in alcohol and transported to the University of Vermont for analysis.

I measured five life history traits for the fish at each site: Male size at maturity was established as the average size of mature males, because males cease to grow once they reach maturity. Females were sorted in 2mm size classes and female size at maturity was established as the size class at which at least 50% of the individuals were carrying developed ova or offspring. Reproductive allocation was quantified as the dry mass of a single brood divided by the dry mass of the mother carrying that brood. The number of offspring were counted and each mass was measured to determine size of offspring.

Stream gradient and site distance from the head waters were calculated using a Meliograph map measurer on 1:50000 maps (Miramar and Chapernal) from the Instituto Geographico de Costa Rica. Canopy cover was determined by averaging the estimates taken by two researchers. Wet stream width and stream depth were also measured at each site.

I found that life history traits differed throughout the stream including male and female size at maturity, reproductive allocation, number of offspring, and size of offspring. Of the the four environmental factors, only predation and stream gradient explained significant variation in B.r life histories among populations.

My study shows that life history traits do, in fact, vary throughout a stream. These results have important implications for studies that consider only samples at the extremes of predation gradiants. However, my results are consistent with the conclusion that predation could be an important factor shaping B.r life histories.

In addition to predation, my study suggests that stream gradient would be important. B.r are known to occupy slow water even when the main stream is swift, so water flow likely is not responsible, rather, factors combined with gradient such as predator distribution could be responsible. My estimates of both density and canopy cover are crude, these suggest that resource availability is less important to life history than other factors.