Did Isabel Have an Impact on Our Fish Populations?

by Dean Fowler
(Who is Dean Fowler & what else has he written)

We all know that weather influences fish behavior, and thus, fishing success. But it also affects biological processes that ultimately determine the quality and size of fish populations, namely, reproduction, recruitment (the rate at which individuals reach a specific size, age, or life stage), growth, and mortality. These processes are affected by both long-term weather patterns (AKA climate), and short-term events such as storms.

The rains, winds, and storm surges brought by hurricanes can affect fish populations immediately and for years to come. They can cause flooding, shoreline and soil erosion, salinity intrusion in tidal areas, uprooting of aquatic vegetation, and loss of trees, all of which can have direct or indirect impacts on fish. Most impacts are very short-lived and last only hours or days. Some examples of these would be elevated water level, velocity, turbidity, and salinity, changes in temperature, dissolved oxygen, and feeding activity, and displacement of fish. Thankfully, in Virginia, hurricanes typically arrive well after most warmwater fish have finished spawning in the spring and before trout begin spawning in the fall. However, young fish can be susceptible to displacement and/or physical injury caused by flooding, loss of vegetative cover, and excessive salinity. This can make them more susceptible to predation and/or starvation.

The felling of trees can have both positive and negative impacts on fish. Loss of overhead cover generally causes stream temperatures to rise, which is detrimental to coldwater species like trout. As many of you know, the ability of water to retain oxygen declines as temperature increases, so loss of overhead cover can also reduce oxygen levels. Trees and the layer of leaf litter that they deposit on the ground are very important in preventing erosion of soils into streams and lakes, which reduces habitat quality for most fish species. On the positive side, trees that fall into the water provide cover for fish and habitat for aquatic insects (which increases food availability for fish). In streams, downed trees also help to reduce water velocities, which reduces scouring and erosion.

Now that I’ve talked, in general terms, about how hurricanes can affect fish, let’s look specifically at how Hurricane Isabel may have had an impact on Virginia’s fishery resources. In most areas, Isabel dropped much less rain than Hurricane Floyd did in 1999. As a result, most inland rivers, streams, and lakes experienced, at most, minor flooding. Little impact on fish populations would be expected at these locations. Probably the only impact of note might be a temporary increase in the food supply as terrestrial insects and other small forms of animal life were washed or blown into streams.

In coastal streams and rivers, however, the situation was quite different. Although the rains were nothing to sneeze at, the storm surge, up to 8 feet in places, was the main culprit behind the flooding in these water bodies. And the surging ocean and bay waters brought salt with them. In freshwater fish, maximum salinity tolerance varies both among life stages and species. Very young freshwater fish typically have a lower tolerance for abnormally high salinity than adults. Most fish species that live in tidal freshwater are adapted to salinity fluctuations and can tolerate salinities as high as 8 to 16 parts per thousand, which is 25-50% that of seawater. Some, like white perch, can actually live in pure seawater for a time. Although the storm surge probably brought much higher than normal salinity, it occurred for only a few hours so it probably had little direct impact on fish. Indirect effects, however, could take place via an impact on fish habitat. Plants such as water lilies are particularly susceptible to abnormally high salinity, and being exposed to it for as little as a few hours can kill them. Several of our coastal rivers have historically had abundant beds of lilies along their margins and in backwater areas. It’s not hard to see how the loss of these plants and the cover they provide could affect fish. We won’t know to what degree they’ve been damaged, if at all, until late-spring when they typically emerge.

Though Isabel didn’t carry as much rain as Floyd, its winds did pack a much bigger punch. Gusts up to 80+ miles per hour were responsible for felling massive numbers of trees throughout the eastern half of the State. In most locations, the numbers of trees lost were not sufficient to cause an increase in erosion that would result in a meaningful long-term increase in silt inflow into nearby water bodies. That’s good news, but there’s more. I’ve seen numerous cases where the amount of woody debris in a water body was increased substantially as a result of Isabel’s tree felling. As discussed earlier in this article, this can provide significant benefits to many forms of aquatic life, including fish. Given the massive numbers of trees that were felled by this hurricane, there are undoubtedly streams and swamps where fish populations may experience increased growth, and possibly even survival, due to this enhanced habitat.

On the downside, Isabel’s winds, rains, and storm surge combined to cause a significant amount of damage to shorelines. Not only did this liberate soil into the water, it left many areas with a relatively barren, more homogenous shoreline habitat that is less desirable for most fish species. The loss of near-shore aquatic plants and shoreline plants and trees makes these damaged shorelines even more susceptible to future wind and wave action. It will take a long time for many of these shorelines to re-stabilize. Until that happens, there will be some erosion of the shoreline soils into these streams and fish habitat quality will be impaired.

In an overall sense, it appears that our fish populations avoided significant damage from Isabel. In fact, there will be localized areas where fish populations may even benefit from the habitat alterations she caused, as well as areas where they’ll suffer. Things would have been different if Isabel had brought heavier rain and/or maintained the wind speeds that it had as it moved across the Atlantic. I’m thankful it didn’t!

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