Organism Movement

The timing of some animal movements may be predictable – reflecting daily or seasonal movements – but vary according to species. Seasonal spawning migrations can be affected by environmental conditions such as water temperature or velocity. Other movements are in direct response to changing conditions (food availability, temperature, oxygen levels, water levels and flow velocities) either to take advantage of opportunities (access to floodplains) or to avoid adverse conditions.

For all animal specie,s within river and stream ecosystem movement is essential for populations to persist. Movements may be between areas of shallow and deeper water or between the water's edge and mid-stream areas. Animal movements may be downstream (intentionally or unintentionally) or upstream. For many organisms inhabiting small streams, lateral movements or movement between surface and deeper water within the stream channel are severely constrained. Under these circumstances upstream and downstream movements are all the more important for aquatic organisms. Also important are movements between the stream channel and adjacent floodplains, as well as upstream and downstream through floodplains and riparian areas. For rivers with large floodplains these movements are especially important.

Some organisms are capable of moving only relatively short distances unless displaced by floods or when attached to other animals or woody debris. Others – such as migratory fish – are strong swimmers with the capacity for long-distance movements and the ability to move upstream against strong currents. In between are a whole host of species: some with the capacity for strong bursts of swimming but with a tendency to stay put, and others – some crayfish for example – that are capable of long-distance movements but typically crawl rather than swim.

Many weak swimmers and crawling species take advantage of boundary zones along bank edges and the stream bottom where water velocities are much lower than in the water column. Under natural conditions, movement by some stream organisms depends on the diversity of channel structure and hydraulics typically found in natural streams. This diversity creates alternate pathways throughout the channel bed and along the bank line; if any point in the channel is a barrier (e.g. high velocity, hydraulic jump), other less strenuous pathways are generally available. Maintenance of unfragmented stream bottom and bank edge habitats is the best strategy for maintaining continuous and interconnected populations for a variety of weak swimming species.

Fish

For fish, swimming ability is highly variable among species. While terms related to swimming ability do not have standardized meaning, most researchers use three categories to describe swimming ability. These include:

burst speed: relatively high speeds that can be maintained for only a few seconds
prolonged swimming speed: includes the range of speeds between burst and sustained
sustained speed: speeds that can be maintained for long periods without fatigue.

Swimming speeds are significant factors when one considers the ability of animals to move through river and stream ecosystems. Burst speed is most relevant for physical barriers or short sections of relatively high water velocity. Prolonged speed is important for crossing longer sections of high water velocity. Long-distance movements of migratory fish and the ability of fish to maintain position in the stream channel for long periods of time depend on the sustained speed of fish.

The danger in using data on the swimming abilities of fish to design river and stream crossings is that we have the most information about strong swimmers (migratory fish) and know very little about the majority of fish species, especially small fish (including juveniles). We know even less about the swimming abilities of non-fish species that inhabit rivers and streams. Species-specific design is also hampered by a lack of information about how turbulence affects animal passage. Theoretically we can reduce velocities within the crossing structure by increasing roughness, such as through the use of baffles. However, increasing roughness also increases turbulence and we have very little information about the ability of various organisms to move through areas of high turbulence.

Other Aquatic Vertebrates

There are a number of relatively large aquatic animals that inhabit streams and rivers but are rarely considered in terms of barriers to movement. Much of the U.S. supports large species of aquatic salamanders (species that rarely or never venture forth on land). Mudpuppies, waterdogs, hellbenders, sirens, and amphiumas are salamanders that are fully aquatic and range in adult size from about a foot to over three feet in length. The Oklahoma salamander and the Pacific giant salamanders of the west coast are other aquatic salamanders that are vulnerable to movement barriers.

Significant portions of the U.S. support softshell and musk turtles, aquatic reptiles that rarely travel overland. Movements of spiny softshell turtles are almost exclusively aquatic (with the exception of nesting and basking). In Arkansas, these turtles moved on 85 percent of the days they were tracked with average daily movements of 403 - 465 ft/day. Some individuals moved more than 2,970 ft/day. Annual home range length for these animals averaged between 4,620 and 5,775 ft (Plummer, et al. 1997, Chelonian Conservation and Biology. 2(4):514-520).

Little is known about the swimming abilities of amphibians and reptiles but it is believed that they are not strong swimmers (relative to migratory fish). Many species may rely more on crawling than swimming. Yet movement and population continuity is essential to the survival of their populations. When moving upstream, aquatic amphibians and turtles probably seek out lower velocity sections of streams and take advantage of boundary layers (low-velocity zones) along the stream bottom and bank edges. Some salamanders may require relatively continuous cover on the stream bottom, moving from rock to rock in order to reduce exposure to predators or high velocities.

Invertebrates

Although some crayfish can travel overland, many species are fully aquatic. Some have been documented moving long distances within streams and all probably depend on smaller scale movements to maintain continuous and interconnected populations. In headwater stream systems of the Ozarks and southern Appalachians, crayfish are dominant components of these ecosystems, rivaling aquatic insects in importance. Many headwater populations have been isolated long enough (due to natural conditions) to become separate species. In these regions of the U.S., headwater streams support many rare crayfish with very limited distribution. Further population fragmentation could imperil entire species of crayfish.

As a group, the most vulnerable animal species in the U.S. are freshwater mussels. Over 70 percent of the 297 species native to the U.S. and Canada are endangered, threatened, or of special concern (Williams, et al. 1993, Fisheries 18(9):6-22). Although adult mussels have a very limited capacity for movement, dispersal typically occurs when larvae (glochidia) attach themselves to host fish or salamanders. Therefore, survival and persistence of freshwater mussel populations is dependent on the capacity of host fish to move through river and stream systems. Many endangered mussels depend on small, sedentary host fish that are typically weak swimmers and therefore highly vulnerable to movement barriers.

River and stream ecosystems contain many other species for which we know little except that they probably have limited capacities for movement. These include worms, flatworms, leeches, mites, amphipods, isopods, and snails. Collectively, these often overlooked taxa account for a significant amount of the biomass and diversity of river and stream ecosystems. For most, swimming ability is less relevant than the ability to move through streambed substrates. Although large numbers of invertebrates can often be supported in relatively small areas, appropriate habitats may be patchy and dynamic. In these situations, a regional population is generally maintained through cycles of local extinction and colonization in response to changes in habitat conditions. Scour and deposition related to flooding or changes in stream hydraulics (e.g. debris dams and deflectors) may destroy habitat in some areas while creating suitable habitat in others. It is unclear how these organisms move upstream any significant distance. It is safe to assume that some mechanism must exist or else populations would continually shift downstream as upstream populations are lost to local extinctions. One possible mechanism for such movements is when small organisms or eggs are transported by larger animals, perhaps in association with adhered sediment or debris.