Leeches
Leeches are an excellent food source for trout since they are present throughout the fishing seasons. Therefore, you have a high likelihood of catching some trout with the correct leech pattern. Leeches can also come in handy in early spring or late fall when other insects are scarce.
The fact that they are always available and trout turn to them during the tough times makes searching for the proper leech patterns. So when other patterns aren’t working, you can continue fishing with the leech patterns.
Unfortunately, a leech guarantees you only one fishable cycle since they die after laying eggs, which is not the case with most insects. Although some leeches can die after laying eggs twice; therefore, you need leech patterns with different sizes and colors. This strategy will make it possible to catch something every time you go fishing.
Another crucial thing you must understand is leech behavior. Generally, the length of leeches is between 1 and 4 inches, but in some cases, it can exceed 6 inches. But since they are sensitive to light during the day, they are usually active in deeper waters. And in shallow rivers, they tend to be active at night.
Leech hides during the brighter parts of the day, so you need to know the right time to use your leech fly patterns. Another behavior that can help you fish is that they hunt by moving up and down in an undulation motion along the base of the river.
From <https://guiderecommended.com/best-leech-fly-pattern/>