Fish move in both directions of a river, swimming both upstream and downstream. The choice of direction is not random but dictated by specific biological needs, often forming a significant part of the fish’s life cycle. Whether a fish travels against the flow or with the current depends on the species, its developmental stage, and the ultimate destination required for survival.
Migration Against the Flow (Upstream)
The most well-known example of movement against the current is the spawning migration of anadromous fish, such as Pacific salmon. These fish spend their adult lives in saltwater but must return to freshwater to reproduce. This journey requires immense physical effort, often navigating hundreds or thousands of kilometers against the river’s flow. The motivation for this travel is the need to reach specific, shallow, gravel-bottomed stream beds where their eggs can be safely laid.
The precision of this upstream homing migration is guided by a powerful sense of smell (olfaction), which allows the fish to identify the unique chemical signature of their natal stream. Young fish “imprint” on these distinct scents before migrating to the ocean. They use this olfactory memory to pinpoint the exact tributary upon their return years later, ensuring the next generation hatches in the same environment. For many species, the upstream movement is a one-way trip, as the reproductive effort depletes energy reserves, leading to death shortly after spawning.
Movement With the Current (Downstream)
Fish also deliberately travel in the downstream direction, moving with the current for dispersal and reproduction. This movement is less energetically demanding than swimming upstream and serves different life-cycle purposes. One major cause of downstream travel is the dispersal of juveniles, such as young salmon. They drift or actively swim from their freshwater hatching grounds toward larger rivers or the ocean where they will mature.
Movement toward the sea is also the reproductive strategy for catadromous species, like the freshwater eel. Eels spend their adult life in rivers and then migrate downstream to a specific oceanic location to spawn. Post-spawn adults of some anadromous species also undertake a downstream journey, known as “kelting,” to return to the ocean after reproduction, provided they survive the demanding upstream travel and spawning. The downstream current provides an efficient mechanism for rapid relocation to areas offering better feeding or growth opportunities.
Energy and Navigation: How Fish Handle River Flow
A fish’s ability to navigate and endure a river’s flow relies on specialized biology and sophisticated hydrodynamic strategies. Sustained, long-distance swimming, such as upstream migration, is powered primarily by red muscle fibers. These fibers are rich in oxygen-carrying myoglobin and suited for aerobic, continuous activity. This slow-twitch muscle tissue is located just beneath the skin along the fish’s flanks.
When a fish needs a sudden burst of speed to overcome a rapid or a waterfall, it recruits the larger mass of white muscle fibers. These fibers are fast-twitch and operate anaerobically for powerful, short-duration movements. To conserve energy during long migrations, fish actively seek out low-velocity zones, such as the quiet water of an eddy behind a rock or the slower current along a riverbank seam. This use of hydraulic refugia allows them to rest and recover before re-entering the main, faster flow.
Beyond physical endurance, fish use several environmental cues to guide their movement through complex river systems. They detect subtle changes in water temperature and flow rate to orient themselves and monitor their progress. Many species use a combination of these cues, along with olfactory memory, to maintain a precise course and ensure they reach their intended destination.