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Q & A
Effects of Floods on Fish
Question
How do flood conditions affect trout fishing in Pennsylvania streams? Does flooding contribute to fish mortality?
Answer
Overall, it is important to remember that floods are natural events that will occur from time to time. Typically flood events cause some shift in the habitat on streams that were subject to flooding, so anglers should expect to see some changes in the streams they fish.

For example, some pools and runs will become shallower and fill in with gravel. Also, some trees that have been in and along the stream bank providing cover for many years may be dislodged. Conversely, flooding also tends to carve some new pools and runs where shallower areas existed prior to the flood and trees that have fallen along the shoreline may also create some new cover for trout.

The scouring of the gravel that occurs during floods may have a negative impact on the survival of young fish and macro invertebrate populations. However, this scouring action can also provide clean gravel for trout spawning and for future generations of macro invertebrates to re-colonize.

The extent that flooding contributes to fish mortality often depends upon the severity of the event, the timing of the flood, as well as the life stage of the trout at the time of the flood. Typically, adult trout would be expected to handle flooding better than younger trout, as during a flood most trout will seek refuge from the strong currents in places like backwaters, eddies and along the stream bank until the flood waters begin to recede.

During recent floods we may have lost some fish that became stranded in backwater pools as the floodwaters receded. However, on most waters we would not expect a high rate of fish loss from this flood. Conversely, flood events in February of 1984 and January of 1996 occurred at a time when most of our young wild trout were still in the sac-fry stage before they emerged from the gravel of the streambed. Subsequently, during both of these years we noticed a numerically weak year class of young-of-the-year trout in many of our streams that support wild trout.

Fortunately, wild trout populations tend to be quite resilient and numerically stronger year classes in the years that followed allowed many of these populations to rebound from these events. Generally wild trout fare better in floods that occur in the fall as most have already spawned. By no means is this to suggest that floods are a welcomed event, but in nature things often seem to have a way of balancing out.

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