Patterns in longitudinal distribution of American Eel (Anguilla rostrata) population characteristics in rivers of Puerto Rico

American Eel (Anguilla rostrata) population characteristics and distribution remain drastically understudied in the Caribbean region. We conducted the first island-wide study to evaluate the distribution of density, length, and sex in relation to distance from the mouth of 23 stream reaches in five rivers in Puerto Rico. We found 287 American Eel at a mean density of 156.7 fish/ha. Sex ratios favored females (52:3) but most of the eel sampled were undifferentiated or undetermined. Generalized linear modeling indicated that upstream habitats with low eel density were optimal environments for production of large and female eel. Our findings indicate that Puerto Rico could significantly contribute to the spawning population and strengthen knowledge of American Eel in the Caribbean. Our findings could be used to guide local and range-wide conservation efforts to ensure stream connectivity and access to optimal upstream habitat for survival of the population.