diff --git a/src/simulator/crates/mc-ecology/src/dynamics.rs b/src/simulator/crates/mc-ecology/src/dynamics.rs
index a0107b8e..49492ee5 100644
--- a/src/simulator/crates/mc-ecology/src/dynamics.rs
+++ b/src/simulator/crates/mc-ecology/src/dynamics.rs
@@ -918,6 +918,7 @@ mod tests {
     #[test]
     fn predation_efficiency_sessile_prey_always_caught() {
         // A sessile prey (flee_speed=0) is always caught once detected.
+        // Chase probability should be 1.0 regardless of predator pursuit speed.
         let pred = wolf();
         let mut prey = Species::derive_from_traits(
             60,
@@ -934,11 +935,26 @@ mod tests {
         );
         prey.resolve_derived_stats();
         // flee_speed derived = 0 for Sessile
+        // Override concealment to isolate the chase=1.0 behaviour under test —
+        // otherwise a highly concealed sessile species drops detection below 0.5
+        // and obscures what we're actually measuring.
+        prey.evasion.concealment = 5;
 
         let eff = predation_efficiency(&pred, &prey, "ocean");
-        // chase = pursuit / (pursuit + 0) = 1.0
-        // efficiency = detection × 1.0 = detection
-        assert!(eff > 0.5, "Sessile prey should have high chase probability (flee=0), got {eff}");
+        // detection = 10 / (10 + 5) ≈ 0.667
+        // chase = 1.0 (sessile — flee_speed=0 forces chase to 1.0)
+        // efficiency ≈ 0.667
+        assert!(eff > 0.6, "Sessile prey with moderate concealment should give eff > 0.6, got {eff}");
+
+        // Also verify that a predator with pursuit=0 still catches sessile prey (chase=1.0),
+        // since neither can move so the prey remains reachable.
+        let mut still_pred = pred.clone();
+        still_pred.interaction.pursuit_speed = 0;
+        let eff_still = predation_efficiency(&still_pred, &prey, "ocean");
+        assert!(
+            eff_still > 0.6,
+            "Sessile prey should still be caught by stationary predator (chase=1.0), got {eff_still}",
+        );
     }
 
     #[test]