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How to Assess a Reservoir and Catch More Fish

Published: 30th June 2021 | Author: Rob Edmunds

I accept that fishing reservoirs can be daunting. The sheer size and the body of water often intimidates anglers and many simply do not know where to start, and so they don’t even try. But, by doing so you are missing out on some truly fantastic sport.

I always simplify everything. Large reservoirs, such as Grafham, stock around 30,000+ fish per season, every 7 – 10 days 1,500 – 2,000 fish will be stocked usually in batches of 500 at certain points around the fishery. This means that there will be a huge amount of fresh stock fish in one area. Once you find them catching them will be easy.

Plus you also have the resident or grown on fish that—although not as plentiful—are still present in large numbers. This means that there are many more fish in a reservoir that have never seen a fly (and so are often easier to catch) than in  small still waters.

Reservoir fishing can be truly excellent.

Initial thoughts on fishing reservoirs

You should keep things simple. Fish have similar habits all over the world. They all respond to a food, temperature, predation and structure.

Fish do not want to waste energy. They want to feel “safe” and have a plentiful food supply literally in front of them. They do not want to be rising up and down in the water column or being constantly “on the fin.”


This is the single most important factor if you are to be successful fishing reservoirs. You can only catch if there are fish in the area. Your first step when visiting any water should be to do some background research. There is usually a map of the water posted on the fishery website, and recent catch reports. These reports can prove a valuable source of basic information, giving you details of the depths, structure, etc. Fishery reports will provide information regarding areas that are holding fish and methods that are working. I usually look at the previous seasons catch reports for the same fishery over that month to work out common areas and themes.

When fishing reservoirs, knowing where to go is key.


After “location” depth is the second most important factor in being successful when fishing reservoirs. I use the analogy of a hungry kid in a sweet shop who will eat everything at eye level. He won’t bend down to the bottom shelf—or climb to the top shelf—unless there’s something very special or no other sweets in the shop. Fish are exactly the same. You must present your flies at the correct depth in order to be successful. However, you can pull up fish with a big dry or floating fry if conditions are right.

Get the depth right and you’ll start to find fish.


Tackle makes a big difference and light rods with AFTM’s of  #3 – 5 make fishing very difficult from the banks of reservoirs. Often conditions are far from perfect, especially early season, and you need a stronger and stiffer rod to punch the line out and get some distance. I always overload my rods by one line weight. For example, a 7 weight rod would have a 8 weight line. This makes for easier casting as the rod is loaded quicker and less false casts are needed. If you want to make casting easier in one step, just go up a line size.

Going up a line size will help with casting.

Conditions and assessing the water

Always fish the conditions and choose your method accordingly. Strongly consider any recent (over the last 2 months) changes in water levels and how that may affect your fishing.

A sudden influx of “fresh” water ( run off or pumping in) coming in the reservoir will change the temperature by at least 2 or 3 degrees. It generally switches the fish off the feed or pushes them deeper. This sudden influx of water can also lead to the reservoir “colouring up” with visibility often reduced to just 2 or 3 feet.

Low visibility means that you should fish slowly as any pattern moved at speed just won’t be seen. It’s also advantageous to use a larger pattern than normal in a dark colour. This will ensure the fly has a good silhouette and will stand out, allowing the fish to see it more easily.

It’s also worth remembering that the fish can feel disturbances in the water down their lateral line. The booby is a great fly for this. It ticks all the boxes when faced with difficult conditions in that it can be fished slowly, at depth, it has movement to induce the take, a good silhouette, and it causes disturbance. 

If the water is clear with a visibility of 7ft or more, then you are able to fish any method you desire. Just remember to present your fly at the correct depth.

Saying goodby to a quality rainbow trout.

Tips for fishing reservoirs

  1.  Read the features and avoid coloured water. Look at the contours of the bank, this indicates the depth of water in front of you. Although it’s not easy on man-made reservoirs, fishery maps are usually available and will give accurate details about depths.
  2. Points channel food across them. They also give you access to a range in water depths and so different temperatures. This is always a good starting place. They usually have some weed growth as well, which will hold food and fish.
  3. Look for cormorants/grebes working the water, which will indicate fry. Look for features, i.e. marker buoys/pontoons drop offs etc. Features will always attract fish.
  4. You must fish the correct depth. It’s best early season to start off deep (as it’s coldest in the morning). A single fly gives a better presentation, sinks faster and makes the fish compete for a single food source. This leads to more positive takes. lengthen the leader from 12:00 onwards if the fishing slows down.
  5. Think critically. Your retrieve will pull the fly deeper when fishing a sinking line along the bottom into the right holding zone. In contrast, a weighted fly on a floating line is pulled out of the zone when stripped.
  6. Boobies create disturbance, which is felt down lateral line of fish. They also have big profile so are easily picked out in coloured water. They’re perfect for deep water fishing as they allow perfect depth control, create movement with a marabou tail and can be fished slowly.
  7. Waves and wind action can lead to missed takes. Put the rod tip 2 – 3” under the water surface so you’re in direct contact with your fly and the fish.
  8. Induce the take if you get a little tap. A pause or speed up should produce a more positive reaction…..entice the fish onto your fly.
  9. Fish location is essential you can’t catch what’s not there! A response will usually be instant…if you have had no follows or takes within 45 mins move. Always try and find the fish.
  10. Don’t wade initially. It’s cold and you will push the fish further out and can also colour up the water. First you should make 5 short casts to 15ft. Then the next 5 casts to 20ft and finally 5 casts at your maximum distance. Work the water methodically in terms of both distance and depth. For example, the first 5 casts let the fly sink for 5 seconds. The next, 10 seconds. Once you get a take, concentrate your efforts at this depth. 

If you enjoyed this piece from Rob, check out his others on the blog here.

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