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What happens to fish when a stream dries up?

Dan Magoulick, associate professor of biological sciences in the J. William Fulbright College of Arts and Sciences, replies:

Streams can dry up due to seasonal drying or drought, and this happens in many areas around the world. In many regions, this drying is predicted to intensify due to global warming. What happens to fish as a result of this drying is dependent on many factors, including the geographical setting, drying duration and intensity, and what fish species are involved. If the stream dries completely, then most fish must either move to more permanent water or die. In some systems, for example in the southwestern United States, fish have evolved mechanisms to deal with complete stream drying, such as burrowing and aestivating, sort of like hibernation. In most other systems, fish that do not move to permanent water die when streams dry completely, due to extreme physical conditions prior to drying such as high temperatures and low dissolved oxygen; predation by other fish or terrestrial predators such as birds, mammals, or reptiles; or the drying itself. Fish that move to permanent water may survive or they may undergo mortality during migration. If they do survive, they may have reduced fitness. If fish make it to permanent water and survive, they may move back to the dry stream upon rewetting. Depending on the duration and intensity of drying, this may take days to months.

In situations where streams dry partially, or intermittently, some permanent aquatic habitats remain. For example, Ozark streams often dry such that riffles dry completely, but some pools may remain permanent. These permanent habitats can act as refuges for fish and other aquatic organisms. In fact, some aquatic organisms, such as crayfish, may survive under the substrate surface, even in completely dry streams — but that’s another story. As with completely dry streams, fish must be able to move to the permanent habitats to survive, unless they occur there already. In these refuge habitats fish may suffer mortality due to extreme physical conditions or predation. However, many fish species are adapted to these conditions and have physiological tolerance or behaviors, such as foraging in shaded areas, increased ventilation, or surface breathing, that lead to increased survival. Fish often experience crowding and increased competition in these refuge habitats that may lead to extinction for some species within the refuge. If refuge habitats become isolated, then fish cannot escape the increasingly harsh conditions. However, if some refuge habitats remain connected, then fish may be able to move among habitats, and this will affect fish population and community dynamics in drying streams. Refuge size, drying intensity and fish mobility will play the largest role in population persistence among the connected habitats.

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