The Catskill Fly Fishing
Center and Museum

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August 14, 2003 3:28 PM

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River Types: The Spring Hole

Acid Factory Pool

Trout have thrived since the glaciers retreated from North America and Asia some 12,000 years ago. Since then, these remarkabe, beautiful fish have occupied most available cold water niches, and evolved into 3 distinct genera, radiating into more than 23 recognized species, world-wide. (see: "Trout and Salmon of North America". Robert J. Behnke. The Free Press, New York) However, their aquatic environment is governed by conditions that would challenge even the most well-adapted freshwater animal. Trout have survived the ravages of floods, extreme reductions in flow rates, ice-outs, excessively high temperatures, variable food sources, predation, and competition from other fish species that feed on similar macro-invertebrates. In fact, trout are among the few aquatic life forms so hardy that they live wherever they are introduced, provided that the conditions are adequate. Only high temperatures, low pH (4.5 and lower), an absence of sufficient quantities of dissolved oxygen, or toxic pollutants (e.g., heavy metals such as arsenic, aluminum) discourage trout from colonizing a given aquatic habitat.

While it is true that some species are hardier than others, the above statements are valid for nearly all of them. Based on their resiliency in any given aquatic habitat, particularly freestone rivers, and their great adaptive capacities for thriving in new environments, it should not surprise us that when things get tough, trout usually prove to be tougher.

Consider fluctuating water temperature as a challenge to their adaptability. In summer, water temperatures in some freestone streams may reach up as high as 85°F for short periods, reducing the concentration of dissolved oxygen well below tolerance limits for all species of trout. Under these conditions, they do just what we would do, and seek out cooler environs. This means they move either to the mouth of a small feeder stream, or into a spring hole within the same river. Spring holes occur where the interface between permeable and non-permeable rock occurs at the level of the riverbed. The water in the porous rock (aquifer) traverses laterally, and by gravity feeds out into the main current where the aquifer intersects the river bottom. Spring water is typically around 45-52°F, and can dissolve more oxygen than the surrounding warmer water. In contrast, during winter months, spring holes attract fish because its waters are now warmer than the main river, which can be as low as 32.2°F.

Acid Factory Pool

It is in the summer that groundwater seeping into the stream saves many fish from suffocation. Almost every large pool on most freestone rivers has a spring in it, and during low water times when the temperature routinely exceeds 75°F, large aggregations of fish can often be observed there. However, while they avoid one hazard by remaining in the spring hole, they encounter several others; namely predation and starvation. Blue herons, mergansers, otters, eagles, ospreys, and the seemingly ever-present poacher find them easy prey at this highly vulnerable time in their lives. In order to feed, trout must wait for the cool of the night air to gradually lower the water temperature in the main stream. It is only then that they can venture forth to eat whatever food items remain in the stream. Dawn and rising water temperatures draw them back again to their refuge. Thus, if the time it takes the river to cool is not sufficiently long enough to allow the fish to forage freely, they rapidly loose weight and soon succumb to the ravages of starvation. During periods of drought, if high temperatures continue for longer than a week or two, springs flow slower, and massive trout kills can result. During this stressful time, larger fish survive by crowding out the weaker, smaller ones.

Acid Factory Pool

Angling in these places also poses a serious threat to all the fish, regardless of their size. Any fish hooked in a spring hole rushes into the surrounding warm currents in its attempt to get free, and is almost certain to eventually die of either heat exhaustion or starvation by depleting already limited fat reserve, even If it appears to be normal after being released. In fact, rivers should be closed during these times, a policy rarely implemented where such events are a regular occurrence.

Houses and campsites along the river that rely on wells as the sole source of water can lessen the flow of the aquifer to the stream. Commercial bottling of water collected from tapping into the river’s aquifer, or from a spring high up in its headwaters, places fish below in the main stem of the river at risk, and such activity could potentially reduce summer flows below tolerable limits. Protecting trout habitat includes understanding their biological needs. For the most part, it’s just plain common sense. Their health, as with ours, begins with an ample, constant supply of clean, cold water.

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