Giant otter pack - 2D river foraging map

Search and encounter rate

A swimmer of speed v that captures anything within radius r sweeps a strip of width 2r. Among stationary prey at areal density ρ it meets prey at aρ with a = 2 r v (the "ideal gas" collision rate). The sim measures this with a blind otter in a periodic arena and compares it to the formula.

Handling time and Holling type II

After a catch the otter eats for h seconds and cannot hunt. The intake is then f = aρ / (1 + a h ρ), saturating at 1/h. Visual chasing (detection radius larger than the capture radius) raises the effective a above the blind-search value, so measured points lie above the dashed blind curve.

Group hunting

Fish flee along the sum of unit vectors pointing away from each nearby otter. One otter pushes a fish in a straight line; several otters from different sides make the vectors cancel, and the fish is pinned. This encirclement is emergent, not scripted; the cooperation slider raises the chance that a contact becomes a catch.

Fish recruitment

Fish follow logistic growth, births at r F and deaths at r F2/K, so the mean obeys dF/dt = rF(1 - F/K) - catches and F(t) = K / (1 + ((K - F0)/F0) e-rt) without otters.

Honest simplifications: a single 80 x 45 m reach, uniform fish, constant swim speeds, recruitment compressed in time so a run shows regrowth in minutes (real populations take months), mean fish mass 0.25 kg, a caiman that retreats from four or more otters is a stylised rule, and fish are dense and plentiful, so the intake per hunting hour is far above what a real otter (about 3 kg of fish a day) needs; real bouts are short. Giant otters really are social, hunt fish in cooperative groups and mob caimans; the numbers here are illustrative, not field data.

• Compare 2 and 8 otters at the same fish density: per-otter intake changes as they interfere.
• Raise capacity K and watch intake saturate near 1/h.
• Drop fish near the bank, switch on the caiman and see the pack mob it.