Vermont, September 1, 2011. Irene was in a bad mood when she deluged her way into the Connecticut River watershed. Based on information sources tracking river flows, the Saxtons River that went two feet above its all-time record flow and the main Connecticut River that witnessed flows rivaling the legendary 1927 flood are just two examples of the magnitude of the event. For anyone who witnessed the devastating power of the floodwaters firsthand it is hard to imagine how any fish survive.

If two feet of flowing water can move a vehicle, what happened to fish during this epic flood given that fish have float bladders within their bodies to maintain neutral buoyancy so a fish is virtually weightless in water, and they have streamlined shapes designed to minimize resistance to water, and are equipped with an epidermis that secretes slimy mucus causing less friction in water? All of this was likely not enough to protect them during these flood conditions.

Fish instinctively move away from increasing flows into areas where the speed of the water is not so strong. They find eddies caused by stream obstructions and quiet pockets along the stream edges that are protected from the main force of the current. For the most part fish have a tendency to swim close to the bottom. The velocity there is lower and calmer by a factor of 3 than the water at the surface, but this haven is reduced when a raging stream begins to move its own bottom downstream. Some fish actually migrate temporarily into the floodplain areas for the duration of the high water.

These are escape zones that all fish use but there unquestionably was damage to the fish population during this flood. Small fish in particular can be decimated during floods and probably most were washed away during Irene’s wrath. Numerous studies show that juvenile fish and earlier life stages are particularly vulnerable to heavy losses during extreme floods, especially in high-gradient streams like the tributaries to the Connecticut River. There have been no obvious fish kills because whatever the loss, it occurred underwater and any dead fish were swept away like everything else in the path of the flood waters.

Fish gill injury from exposure to high suspended sediment loads would be a source of injury to larger fish. Suspended sediment in streams can induce gill flaring (fish coughing) and decrease respiratory capabilities of fish through gill abrasion. The sediment particles act as sandpaper against the gills causing damage and making breathing difficult. Lacerated gill tissue provides an entry for toxic chemicals into the bloodstream either bound to sediment particles or in solution. Although it has been documented that thick stream sediment like the conditions in this flood can and do cause direct gill damage that may lead to death, no one has quantified the overall impact on any single fish population, mostly because flood events are so unique in location, water amount, timing and duration.

Yet for all the losses in the fish population they will repopulate the streams both through new fish being born and by fish migrating back into the stream. The caution to how quickly nature will take her course depends on how many young fish were lost and whether, how soon and in what numbers the invertebrate life returns. Entomologists agree floods reduce or destroy the insects and other aquatic invertebrates that are the food base for fish. The newly formed structure of the stream bottom and whether it is comprised of silt, gravel, cobble or boulders and the particular mix of each will determine if and when invertebrates will return to that river reach.

There is also the question of fish habitat itself. High flood waters deposit silt, sand, and gravel in pools that turn them into long flat useless reaches of habitat for fish. Shore side vegetation that offered food and shelter get washed away. Fish will adjust to the new watery landscape after the flooding wrought changes by finding new sheltering and feeding lies but unfortunately unless everything goes right, it could be a while before our streams are back to any kind of normal.

One field study reported no changes in an adult brook trout population immediately after a series of flood events despite intense channel alterations – that was the good news. The bad news was they found that after two years the standing crop of brook trout had been reduced by over 90 percent and showed no signs of recovery. They attributed this severe fish population drop to reduced habitat for and therefore reduced invertebrate populations and the destruction of suitable aquatic habitat including spawning habitat for fish in the study river.

So it is the long term effects of a flood like this one that are of real concern from a fishery perspective. Fortunately wild fish are resilient and tend to fare better in floods than their hatchery cousins especially if the flooding does not affect spawning or incubation of eggs. Irene missed most of the spawning seasons for our fish species. The next cycle is for brook and brown trout coming up this fall.

We now enter the wait and watch stage as we hope to see aquatic life in our rivers recover as quickly as we mammals hope to recover from the devastation to the land.

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David Deen is River Steward for the Connecticut River Watershed Council. CRWC has been a protector of the Connecticut River for more than half a century.