Funnel Design

Part of Fishing

One-way fish entry systems that let fish swim in but prevent them from finding the exit.

The Principle

Fish swim forward. They follow walls, current, and scent trails. When a fish enters a narrowing channel, it keeps pushing forward because turning around in a tight space is difficult and unnatural. A well-designed funnel exploits this behavior: the entrance is obvious and inviting, but from inside the holding area, the exit — the narrow throat of the funnel — is nearly invisible among the walls of the trap.

This is not a complex mechanism. It is geometry doing the work of a fisherman.

Anatomy of a Funnel

Every funnel has four parts:

1. Wings — the wide, flared opening that guides fish inward
2. Throat — the narrow passage that fish squeeze through
3. Holding chamber — the enclosed space where fish accumulate
4. Backstop — the far wall of the holding chamber that prevents fish from swimming straight through

Critical Measurements

Component	Dimension	Why It Matters
Wing spread	40-80 cm (16-32 in)	Wide enough to intercept fish following the wall
Wing angle	30-45 degrees from center	Too steep and fish bounce off; too shallow and they swim past
Throat width	3-8 cm (1-3 in)	Must fit target species but feel restrictive enough that fish don’t casually exit
Throat length	15-25 cm (6-10 in)	Longer throat = harder to find the exit from inside
Chamber depth	At least 50 cm (20 in) behind throat	Fish must clear the throat fully before they can orient

Materials and Construction

Woven Funnel (for basket traps)

This is the most common funnel type, built into the entrance of a basket trap.

Materials:

• 8-12 flexible green sticks, 40-60 cm long, 3-5 mm diameter (willow, hazel, or any bendable wood)
• Thin cordage or plant fiber for binding

Steps:

1. Form the sticks into a cone shape, with the wide end matching the diameter of your trap entrance (typically 20-30 cm)
2. The narrow end (throat) should taper to 3-8 cm depending on target fish size
3. Bind the sticks together at the narrow end with cordage — wrap tightly so the throat holds its shape
4. Splay the wide ends outward and tie them to the rim of the trap entrance
5. The cone should extend 15-25 cm into the trap interior
6. Fish push through the flexible tips at the narrow end; the tips spring back closed behind them

Throat Size Is Everything

If the throat is too tight, fish will not enter. If too loose, they will exit as fast as they enter. For mixed species, aim for 5 cm (2 inches). Adjust after your first catches — if you are catching only small fish, narrow the throat; if nothing enters, widen it.

Stone Funnel (for weirs)

Built at the gap of a fish weir, using rocks to create a permanent one-way channel.

Steps:

1. At the weir gap, extend two rows of rocks inward (downstream), angling toward each other
2. Each row should be 50-80 cm long
3. The entrance between the rows should match the weir gap width (30-50 cm)
4. The exit (throat) should narrow to 8-15 cm — wider than a basket funnel because stone funnels are less precise
5. Behind the throat, build a circular or semicircular stone pen at least 1 meter across
6. The pen walls should be as high as the weir walls

Enhancement: Place flat stones across the top of the throat section to create a short tunnel. Fish are even less likely to find the exit if it is dark and enclosed.

Stick Fence Funnel

A hybrid approach using driven stakes with woven panels.

1. Drive two rows of stakes into the streambed, angled inward
2. Weave flexible branches between the stakes to create solid walls
3. Leave the throat gap between the innermost stakes
4. This works well in soft-bottomed streams where rocks are scarce

Design Variations

Double Funnel

Two funnels in series. Fish enter the first funnel into a small intermediate chamber, then pass through a second funnel into the main holding area. Escape rate drops to nearly zero because fish would need to find two narrow exits in sequence. Use this for high-value or escape-prone species.

Layout:

[Wings] → [Throat 1] → [Small Chamber 30cm] → [Throat 2] → [Main Holding Area]

Side-Entry Funnel

Instead of placing the funnel at the end of a trap, position it on the side wall. Fish swimming along the wall of a weir or bank encounter the funnel opening and turn into it. This works when you cannot control the direction fish approach from.

Floor Funnel

For bottom-feeding species (catfish, carp), build the funnel opening flush with the streambed. The wings lie flat on the bottom, and the throat angles slightly upward. Bottom feeders follow the substrate and enter without ever needing to change depth.

Positioning the Funnel

The funnel entrance must face into the current or toward the direction fish travel. Fish almost never enter a funnel from the side.

In a stream: Point the funnel opening upstream. Fish swimming upstream push into the throat naturally.

In a weir: Place the funnel at the downstream side of the gap. Fish passing through the gap enter the funnel immediately.

In still water: Orient the funnel toward the bank or toward structure where fish patrol. Place bait inside the holding chamber to draw fish in.

Testing Your Funnel

Before relying on a funnel trap for food, test it:

1. Drop a stick or leaf into the entrance — does it flow through the throat smoothly?
2. Push a stick through the throat from inside — does it catch on the flexible tips or narrow walls?
3. If using a woven funnel, push your fist through from the outside (simulating a fish). The tips should flex inward. Pull your fist back — the tips should resist.
4. Look through the funnel from inside the holding chamber. If you can clearly see the exit, fish can find it. The throat should be angled, darkened, or obscured.

Common Failures

• Throat too short: Fish poke their head through, see the holding area, and back out. Extend the throat to at least 15 cm
• Throat too centered: Fish inside the chamber swim along the walls and eventually find a centered exit. Offset the throat to one side
• Wings too rigid: In woven funnels, stiff wings do not guide fish smoothly. Use green, flexible wood
• No bait: In still water, an empty trap catches almost nothing. Always bait the holding chamber

Key Takeaways

• The funnel works because fish swim forward through narrowing spaces but rarely reverse through them
• Throat width of 3-8 cm accommodates most freshwater species while preventing escape
• Longer throats (15-25 cm) dramatically reduce escape rates
• Double funnels nearly eliminate escapes for critical food-production scenarios
• Always test the funnel before deployment — the throat must flex open from outside but resist from inside