The Mystery of Dos Bocas Reservoir, Puerto Rico: Explaining Extreme Spatial Heterogeneity in Largemouth Bass Distribution

Dos Bocas Reservoir, Puerto Rico, has pronounced in-lake variability in fish distributions, especially in piscivore abundance, between the two primary reservoir basins. We quantified fish community distributions by electrofishing in each basin and the confluence, and assessed water quality, contaminant concentrations, habitat availability, and habitat selection. Fish community composition varied markedly among the three areas. We found almost no largemouth bass (Micropterus salmoides) in Basin 1 (0.5 fish/h), moderate catch rates in the Confluence (11 fish/h), and relatively high catches in Basin 2 (35 fish/h; ANOVA, P = 0.005). We detected significant differences in pH, conductivity, and Secchi depth (Paired t-Tests, P < 0.05), but none were outside of the range suitable to most fish species. Analysis of common chemical contaminants including polychlorinated biphenyl (PCB) congeners, common pesticides, and mercury revealed little to explain the unusual distributions. When radio-tagged naive largemouth bass (n = 6) from another reservoir were released in Basin 1, they quickly abandoned Basin 1 and were located 11 days later in the Confluence and Basin 2 where they remained for the duration of the study. Radio-tagged largemouth bass (n = 6) released in Basin 2 remained near their stocking site. We hypothesize that differences in watershed development may result in short-term water quality events, including high sedimentation and possibly acidification of Basin 1. These short-term events may contribute to variation in fish assemblages. We recommended focusing on largemouth bass management within Basin 2 and promoting fisheries for Tilapia, sunfish, and crappie in Basin 1.