Larger fish upstream in a small stream: what are the causes of this longitudinal pattern?

Stream fishes often display distinct spatial patterns in size distribution, with one possible pattern being larger individuals dominating the headwaters. This “larger fish upstream pattern” (LFUP) has been widely documented, yet the ecological drivers remain unclear. We investigated factors contributing to LFUP in the fish community of a South American stream. Using historical data (1994–1998) from electrofishing surveys across six sites, we analyzed size distribution across eight species, with predictors including upstream distance, food availability, predation pressure, and intraspecific competition. Linear mixed models indicated that four species exhibit consistent LFUP, primarily driven by upstream distance or food availability. Species with pelagic or lithopelagic spawning displayed LFUP, supporting the hypothesis that LFUP is linked to reproductive strategies requiring upstream movement to maintain population stability. Our findings support that habitat connectivity and complexity are crucial for conserving small pelagic and lithopelagic fish species, as suggested in previous works. This study contributes to the field by highlighting mobile behavior in small tropical stream fishes and underscores the need for conservation strategies that account for movement patterns and habitat requirements in stream systems worldwide.