ramblings of a catskill fly fisher

A change in feeding behavior of Ashokan Reservoir rainbow trout

Posted 1/24/24

Rainbow trout were first stocked in tributaries of the Esopus Creek in 1883, taken from California stocks. 

As those fish matured and reproduced, offspring spread throughout the watershed. …

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ramblings of a catskill fly fisher

A change in feeding behavior of Ashokan Reservoir rainbow trout


Rainbow trout were first stocked in tributaries of the Esopus Creek in 1883, taken from California stocks. 

As those fish matured and reproduced, offspring spread throughout the watershed. Then as New York City’s thirst for drinking water increased, the Ashokan Reservoir was designed and completed in 1912. 

The upper basin of that reservoir, as it turned out, became the feeding ground for rainbows that spawned in the tributaries and later migrated downstream to feed and grow. 

After three years in the reservoir, those rainbows would return to the Esopus and its tributaries to spawn. 

While actual spawning takes place in late winter and early spring, there is movement of some pre-spawn rainbows in late fall. Those fish overwinter before beginning the spawning process a few months later—after which survivors return to the reservoir. 

Once the reservoir was completed and rainbows began to move between the Ashokan and the Esopus, the dynamics of the fishery changed dramatically. Before the reservoir, stream rainbows rarely grew to a large size. But once in the reservoir, rainbows fed on a variety of plankton and small emerald shiners, allowing a lot of lake fish to reach a size of 18 to 20 inches in length.

When the Shandaken Tunnel that connects Schoharie Reservoir to the Esopus Creek was completed, it provided a constant supply of sometimes turbid—but cold—water to the creek. That cold water allowed the Esopus Creek trout population to survive the normally warm and dry months of the summer. Unfortunately, the tunnel was not managed with the fishery in mind, in that it was completely opened or shut, causing issues with the trout population. 

That is until 1976, when the the Water Releases Legislation was passed by the state legislature and signed by the governor. Once the regulations from that legislation were implemented, flows in the Esopus were stabilized.

Stomach analysis of rainbows collected from the Esopus and Ashokan found that those fish fed on a variety of aquatic insects, Daphnia (a genus of small zooplankton) and copepods. In fact, Daphnia made up a significant proportion of the rainbow trout’s diet. 

Then several years ago alewife herring, which are commonly called “sawbellies,” were introduced into the Ashokan Reservoir. The introduction of that species dramatically changed the food chain and the species composition of the fish that historically thrived in the Ashokan Reservoir. Sawbellies are plankton feeders and in addition to zooplankton (including Daphnia), sawbellies eat any type of small organism they find, including the larvae of walleyed pike and emerald shiners. 

As a result, both the walleye and emerald shiner populations have changed dramatically. Sawbellies then became the primary forage food for the large and smallmouth bass, the remaining walleyes and certainly for the brown trout that grow very large in the lower basin of the reservoir. 

What about the rainbows? As far as biologists were able to determine from the samples of that species (collected by gill-net survey), their primary food remained plankton. That’s why biologists and anglers never saw rainbow trout much longer than 20 inches in the Ashokan. Small zooplankton just do not provide enough calories to grow really large trout. 

To complicate rainbow trout feeding, every time the upper basin of the reservoir became off-color from the red clay deposits in the Esopus/Schoharie watershed, those trout were not able to feed; rainbows are sight feeders.

I proved that one day several years ago while fishing the mouth of the Bush Kill Creek, which enters the Ashokan on the west shore. On that day, the reservoir was way off-color, much like coffee with a bit of cream. Two rainbows were feeding in the plume of clear water, flowing in from the Bush Kill. I hooked and landed one of those fish. It was about 18 inches long, thinner than a pickerel, and weighed about a pound and a half, if that. Way underweight for a trout of that size. Those fish could not find food in the turbid water of the reservoir, because they could not see it.

About a week ago, when I stopped in at the local hardware store, a friend who works there showed and forwarded a picture of a very large, very fat rainbow that was caught in the Ashokan. That fish measured 25 inches in length and weighed at least six pounds. Calls to biologist friends who had worked on the reservoir years before were both surprised and a bit miffed about what they saw in fish. Their conclusion: that fish was eating something other than zooplankton.

And while I can’t prove it, I can speculate that at some point during the last several years, Ashokan rainbows changed their feeding habits. My guess is that those fish are now foraging on sawbellies. That’s why we are suddenly seeing these large rainbows. It’s a known fact that when sawbellies are the primary food source, they are responsible for growing brown trout that are well over 10 pounds in the Catskill reservoirs, including the Ashokan, Rondout and Pepacton. 

In my view, sawbellies are the only reason that we are beginning to see such large rainbows enter the fishery. If this is indeed the case, anglers can look forward to fishing for some very large rainbow trout in the Ashokan Reservoir, now and in the future. 

feeding, behavior, change, ashokan, reservoir, rainbow, trout


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