Natural Fish Migration Barriers

Natural fish migration barriers such as beaver dams and waterfalls interfere with the upstream movement of fish. An unimpeded passage is especially important for anadromous fish, which are those species that live most of their lives in tidal waters but must move into rivers and streams to spawn. Anadromous fish species such as salmon, American shad, alewife, blueback herring, white perch and yellow perch all migrate into rivers and streams during the early spring in order to spawn. Resident fish species migrate to feed, reach rearing areas to spawn and to seek refuge from predators or detrimental environmental conditions such as the freezing over of a lake or stream.

  1. Natural Barriers

    • Waterfalls, cascades, debris and beaver dams represent the most significant natural fish barriers. Eroded streamside banks will also create natural barriers to fish movement. In the case of waterfalls, the vertical water drop may be too high for fish to jump or throw themselves over. Anadromous fish are normally capable of jumping up water drops of up to 1 foot high, but require both sufficient water depth and flow in order to do so. Waterfalls may also be too powerful for fish to jump up, particularly if water is moving at a velocity that is higher than the fish's swimming ability. Tall waterfalls will be impassable at any level of flow.

    Additional Barrier Considerations

    • Specific sections of a stream can become a barrier during low or high flows and natural features such as cascades can become impassable during high flows due to increased water velocity. Cascades can also become impassable during low flows, as the water may not be deep enough. As each species has its own swimming and leaping abilities, a barrier that prevents one species from crossing may be passable by other species.

    Negative Effects of Migration Barriers

    • Natural barriers either prevent migratory fish species from reaching their spawning grounds, or prevent their young from gaining access to the coast, which is as important in their life cycle. If migrating fish are unable to make it upstream, they will inevitably be too exhausted to spawn. If these fish spawn in densely populated downstream areas, their offspring will be forced to compete for available food and shelter. Once migration is delayed or halted by barriers, the life cycle of these anadromous species will be disrupted, causing a reduction in their population.

    Impacts on Resident, Non-Migratory Populations

    • Natural barriers impact on resident, non-migratory fish populations by preventing the juveniles and adults from moving either upstream or downstream to adjust to changing conditions, such as temperature fluctuations, available cover and competition for available food. Resident fish need to move freely within the stream networks to prevent population fragmentation. If a disturbance occurs in an isolated stretch of stream, particularly a man-made disturbance such as a sewage spill, the small remaining population of resident fish may be eliminated from that section permanently. If the natural barrier remains intact and fish are not able to reach a specific section of river or stream, the natural biological balance in that section will quickly be thrown out of balance.