A Sizing Rule for Fish Trap Funnel Throats: Throat Diameter and Cone Angle Against Entry and Escape

A trap funnel has to do two opposed things: admit the fish that are wanted and then refuse to let them back out. This study originates a sizing rule for the funnel entrance cone by modelling both duties from stated assumptions. Entry is modelled as a rising function of the ratio between throat diameter and the body depth of the target fish; escape over a 24-hour soak is modelled from the throat area a searching fish is presented with, the cone half-angle that guides it back to the throat, and the cone length it must traverse. A design is accepted when modelled entry probability is at least 0.80 and modelled escapement over 24 hours is at most 0.10. A throat equal to the body depth admits only half the target fish (entry probability 0.50), so the throat must exceed the depth: the recommended window runs from 1.18 to 1.40 times the body depth for every target size computed. The cone angle ceiling tightens sharply as the target fish grows: at the stated standard, a 60 mm fish tolerates angles up to 40 degrees, a 100 mm fish up to 27 degrees at its narrowest recommended throat, and a 150 mm fish only 12 degrees. Computed windows are tabulated for target body depths of 60, 80, 100, 120 and 150 mm. This study is simulated, not measured.