The Soak-Time Peak: Why a Longer Soak Does Not Always Catch More in a Baited Fish Trap

A widely repeated rule of thumb holds that a baited fish trap left in the water longer always holds more fish, so the best soak is the longest soak that weather and schedule allow. This study tests that claim with a simple dynamical model in which fish enter at a rate set by the remaining strength of the bait and by the space left in the trap, and leave through escapement and predation at a rate proportional to the number already inside. The claim does not survive the arithmetic. Under the baseline assumptions the catch rises to a peak of 19.43 fish at 18.04 hours and then falls: a trap hauled at 48 hours holds 12.86 fish, which is 31.9 percent fewer than it held at 24 hours and fewer than the 18.49 fish it already held at 12 hours. Halving the peak soak time costs 12.8 percent of the peak catch; doubling it costs 17.2 percent. The peak arrives earlier when bait decays faster, when escapement is higher, or when predators learn the trap, and later in cold water with slow bait decay (28.98 hours) or with a tight, low-loss funnel (25.08 hours). The paper gives a practical soak-time rule: haul near the computed peak for the conditions at hand, and treat the interval over which the model holds at least 95 percent of the peak catch — 11.94 to 26.34 hours in the baseline case — as the working window. This study is simulated, not measured.