Sclerochronology is a method used to estimate the season of death (season of capture) of archaeological individuals based on a modern growth model. This method has been increasingly accepted in South America and has mainly been applied to coastal archaeological sites (on the Atlantic and Pacific Ocean). This is the first time that this method has been applied to a freshwater species, the marbled swamp eel (Synbranchus marmoratus), in archaeology. Excavations undertaken at Loma Salvatierra, a human-built platform located in the Bolivian Amazon and occupied from 500 until 1400 AD, have yielded 111 zooarchaeological vertebrae of the marbled swamp eel, which is one of the most widely distributed species recovered in South American continental archaeological sites. In order to estimate the fishing season for these archaeological individuals, we developed a modern osteological reference collection, made up of 61 specimens with known capture dates sampled monthly over a one-year period, about 60 km from Loma Salvatierra. The vertebrae present periodic growth patterns with a succession of dark and light bands alternately. Consequently, the vertebrae are a reliable basis for the estimation of the marbled swamp eel fishing season. The analysis of the marginal increments of vertebrae in present-day fish allowed us to elaborate a modern growth model showing that the seasonal growth of the marbled swamp eel is related to the hydrological cycle, whereby the fast growth period coincides with the onset of rainfall in the region. On the basis of this modern-based model, the analysis of zooarchaeological vertebrae demonstrates that fish were captured over several seasons. Demonstrating that human groups occupied villages year-round does not mean that they were not mobile but shows year-round fishing in the savanna. This year-round fishing practice raises questions concerning the generalized idea of fishing as an exclusively dry-season activity. As wild resources are generally seasonal, the evidence of the year-round fishing of swamp eels might suggest year-round fishing at Loma Salvatierra and contributes to the understanding of late-Holocene mobility patterns in pre-Columbian times.