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Merrill Lake Hex Hatch

Updated: Oct 5, 2021

Bill Matthews Outdoors
Merrill Lake Coastal Cutthroat on a Hex fly.

Location: Merrill Lake, Washington

Date: June 30, 2019

Event: Yellow Hex Hatch

Rating: 5 Bigfoots

Merrill Lake Hex Hatch

Beautiful Merrill Lake

Merrill Lake is a fly fishing only lake located in SW Washington near the town of Cougar. Here is the description from the Washington Department of Fish & Wildlife:

Merrill Lake was formed when lava flows from Mt. St. Helens blocked the stream valley. In the early 1930's the lake was planted with Eastern Brook Trout, Rainbow Trout and Yelowstone Cutthroat. Brown Trout were introduced in 1987 and have proved to be very successful.

This fly-fishing, catch-and-release-only lake is located north of Cougar near Mount St. Helens. It is open to fishing year-round, but action is usually slow and often inaccessible during the winter months due to snow. Merrill contains Coastal Cutthroat, Brown Trout to several pounds and some Rainbow Trout.

The lake was stocked in 2018 with catchable Rainbow Trout.

Two-pole fishing is NOT allowed

Special fishing restrictions: Fly fishing only for this lake

Shoreline access: Good Shore access is available as the water level drops during the spring.

The two Hex flies I used along with the balanced leach I fished under indicator.

There is good fly fishing on Merrill Lake almost year round, but it is known for its Hex hatch in late June and early July. I have fished this lake before, but never during the Hex Hatch so I was excited to see what it was all about. The Hex hatch starts right as darkness settles in on the lake. I wanted to get there early so I got to the lake about 3:00pm to get my bearings and see if I could catch some fish before the hatch started. I fished a balanced leach under an indicator for the first couple of hours. The bites were pretty consistent and I landed 5 or 6 Rainbows and Coastal Cutthroat Trout in the 12 - 15” range.

Around 6 pm I noticed that a couple of fish were striking my pink indicator. I decided that it was time to try out a dry fly. I switched to my rod with the Hex pattern tied on it. The bite was not red hot, but the fish that hit my fly absolutely attacked it. I think they were just trying to slap it instead of actually eating it. I was surprised that this fly was working with no visible hatch going on, but I figured it meant that the fish were very keyed in on the hex from the previous night. It was a bit frustrating because most of the fish were not actually eating the fly, so I only landed about one for ever 10 - 12 strikes - but it was fun to watch.

The Trout will absolutely drown and destroy your fly!

As darkness approached the bite totally turned off. I am guessing the fish were eating the emergers under the water. I suffered through an intense mosquito attack for about 30 minutes, but thankfully it subsided.

Around 9:15 pm the water started rippling , and the big Hexagenia Mayflies appeared all around me. The Mayflies were everywhere and the fish were having a field day eating them.

Here is a short videos of the Hex action

When the hatch started, my fly was attacked on almost every cast, and although I did not hook them all, my catch ratio went way up. I caught three cutthroat in about 30 minutes. I wish that I had taken the time to tie a Hex fly on my other rod because my fly was so drowned by all the action that I had to take time to dry it and eventually had to change it all together. Having a second rod ready to go would have really helped. It was an awesome 30 minutes of fishing, but the action really died off after that. I have heard that you can continue to catch them well after dark, but I had no such luck.

It was a really cool thing to see these big beautiful Mayflies emerging out of the darkness. The trout were totally keyed in on them and it was quite a spectacle to watch. I love experiences like this and I would highly recommend it. I hope to be back next year to enjoy this again!

Here are some tips I would suggest for this trip:

1. start early because there is a good nymph bite, and you can get action on dry flies too.

2. Bring mosquito repellent - they can be terrible

3. Bring two rods rigged up with dries for when the hatch starts - these fish absolutely attack your fly and it will get drowned and mangled. It is handy to have another rod rigged and ready to go because the hatch does not last long. If you are fumbling around trying to fix your fly you are going to miss some of the action.

4. Bring a head lamp - it will be dark when you get off the water

5. The hatch seemed to be happening all over the lake, so I don't think it matters what part of the lake you decide to fish, as long as you are ready to go when it starts.

6. There is a restriction on motors on Merrill Lake, I believe it is electric motor only. Most anglers I saw were in drift boats, float tubes and kayaks.


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