diff --git a/api/src/lib/fishingEngine.ts b/api/src/lib/fishingEngine.ts
new file mode 100644
index 00000000..ef60c791
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+++ b/api/src/lib/fishingEngine.ts
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+/**
+ * Water Tile Fishing Simulation & Loot Catch Engine for OpenAO MMORPG.
+ * Simulates water salinity/depth classifications, bait modifiers, nocturnal species,
+ * and sunken treasure chest recovery with robust input validation.
+ */
+
+export type WaterTileType = "FRESHWATER_RIVER" | "COASTAL_OCEAN" | "DEEP_SEA" | "LAVA_LAKE";
+
+export interface FishingRod {
+    rodId: string;
+    name: string;
+    tier: number; // 1 to 4
+    bonusCatchPercent: number; // e.g. 0.05 = +5%
+}
+
+export interface FishingBait {
+    baitId: string;
+    name: string;
+    potency: number; // 1 to 3
+    attractsRare: boolean;
+}
+
+export interface FishSpecies {
+    speciesId: string;
+    name: string;
+    waterType: WaterTileType;
+    minSkill: number;
+    baseCatchWeight: number;
+    goldValue: number;
+    isNocturnalOnly?: boolean;
+    isTreasureChest?: boolean;
+}
+
+export interface FishAttemptParams {
+    fishingSkill: number; // 1 to 100
+    rod: FishingRod;
+    bait?: FishingBait;
+    waterType: WaterTileType;
+    isNightTime: boolean;
+    rng?: () => number;
+}
+
+export interface FishCatchResult {
+    success: boolean;
+    caughtFish?: FishSpecies;
+    skillExpGained: number;
+    reason?: string;
+}
+
+export const FISH_SPECIES_CATALOG: FishSpecies[] = [
+    { speciesId: "carp_river", name: "River Carp", waterType: "FRESHWATER_RIVER", minSkill: 1, baseCatchWeight: 60, goldValue: 5 },
+    { speciesId: "trout_river", name: "Rainbow Trout", waterType: "FRESHWATER_RIVER", minSkill: 20, baseCatchWeight: 30, goldValue: 18 },
+    { speciesId: "salmon_ocean", name: "Coastal Salmon", waterType: "COASTAL_OCEAN", minSkill: 35, baseCatchWeight: 45, goldValue: 35 },
+    { speciesId: "shadow_eel", name: "Shadow Eel", waterType: "COASTAL_OCEAN", minSkill: 50, baseCatchWeight: 25, goldValue: 75, isNocturnalOnly: true },
+    { speciesId: "kraken_tentacle", name: "Deep Kraken Tentacle", waterType: "DEEP_SEA", minSkill: 75, baseCatchWeight: 15, goldValue: 250 },
+    { speciesId: "magma_swordfish", name: "Magma Swordfish", waterType: "LAVA_LAKE", minSkill: 90, baseCatchWeight: 10, goldValue: 500 },
+    { speciesId: "sunken_treasure", name: "Sunken Treasure Chest", waterType: "DEEP_SEA", minSkill: 60, baseCatchWeight: 5, goldValue: 1000, isTreasureChest: true },
+];
+
+export class FishingEngine {
+    public static calculateCatchChance(
+        skill: number,
+        rodTier: number,
+        rodBonus: number,
+        baitPotency = 0
+    ): number {
+        const clampedSkill = Math.min(100, Math.max(1, skill));
+        const clampedRodTier = Math.min(4, Math.max(1, rodTier));
+        const clampedBait = Math.min(3, Math.max(0, baitPotency));
+
+        // Base formula: 30% + (skill / 100 * 45%) + (rodTier * 4%) + rodBonus + (bait * 5%)
+        const chance = 0.30 + (clampedSkill / 100) * 0.45 + clampedRodTier * 0.04 + rodBonus + clampedBait * 0.05;
+        return Math.min(0.95, Math.max(0.10, chance));
+    }
+
+    public static attemptFishing(params: FishAttemptParams): FishCatchResult {
+        const rng = params.rng || Math.random;
+        const catchChance = this.calculateCatchChance(
+            params.fishingSkill,
+            params.rod.tier,
+            params.rod.bonusCatchPercent,
+            params.bait?.potency ?? 0
+        );
+
+        if (rng() > catchChance) {
+            return {
+                success: false,
+                skillExpGained: 2,
+                reason: "The fish escaped the hook.",
+            };
+        }
+
+        const eligible = FISH_SPECIES_CATALOG.filter((fish) => {
+            if (fish.waterType !== params.waterType) return false;
+            if (fish.minSkill > params.fishingSkill) return false;
+            if (fish.isNocturnalOnly && !params.isNightTime) return false;
+            return true;
+        });
+
+        if (eligible.length === 0) {
+            return {
+                success: false,
+                skillExpGained: 1,
+                reason: "No fish of suitable skill inhabit these waters.",
+            };
+        }
+
+        let totalWeight = 0;
+        const weightedPool = eligible.map((fish) => {
+            let weight = fish.baseCatchWeight;
+            if (fish.isTreasureChest && params.bait?.attractsRare) {
+                weight *= 3.0;
+            }
+            totalWeight += weight;
+            return { fish, weight };
+        });
+
+        let roll = rng() * totalWeight;
+        let selectedFish: FishSpecies = eligible[0];
+
+        for (const item of weightedPool) {
+            if (roll < item.weight) {
+                selectedFish = item.fish;
+                break;
+            }
+            roll -= item.weight;
+        }
+
+        const exp = Math.max(5, Math.floor(selectedFish.minSkill * 1.5));
+
+        return {
+            success: true,
+            caughtFish: selectedFish,
+            skillExpGained: exp,
+        };
+    }
+}
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diff --git a/api/src/tests/fishingEngine.test.ts b/api/src/tests/fishingEngine.test.ts
new file mode 100644
index 00000000..5529b19a
--- /dev/null
+++ b/api/src/tests/fishingEngine.test.ts
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+import { describe, it, expect } from "vitest";
+import { FishingEngine, FishingRod, FishingBait } from "../lib/fishingEngine.js";
+
+describe("FishingEngine Habitat Classification and Catch Calculations", () => {
+    const basicRod: FishingRod = {
+        rodId: "rod_cane_01",
+        name: "Simple Wooden Rod",
+        tier: 1,
+        bonusCatchPercent: 0.0,
+    };
+
+    it("catches river fish with baseline skill and rod", () => {
+        const res = FishingEngine.attemptFishing({
+            fishingSkill: 15,
+            rod: basicRod,
+            waterType: "FRESHWATER_RIVER",
+            isNightTime: false,
+            rng: () => 0.05,
+        });
+
+        expect(res.success).toBe(true);
+        expect(res.caughtFish?.speciesId).toBe("carp_river");
+        expect(res.skillExpGained).toBeGreaterThan(0);
+    });
+
+    it("restricts nocturnal shadow eels to nighttime conditions", () => {
+        const dayRes = FishingEngine.attemptFishing({
+            fishingSkill: 60,
+            rod: basicRod,
+            waterType: "COASTAL_OCEAN",
+            isNightTime: false,
+            rng: () => 0.05,
+        });
+        expect(dayRes.caughtFish?.speciesId).toBe("salmon_ocean");
+
+        let callCount = 0;
+        const rolls = [0.05, 0.90];
+        const nightRes = FishingEngine.attemptFishing({
+            fishingSkill: 60,
+            rod: basicRod,
+            waterType: "COASTAL_OCEAN",
+            isNightTime: true,
+            rng: () => rolls[callCount++ % rolls.length],
+        });
+        expect(nightRes.success).toBe(true);
+        expect(nightRes.caughtFish?.speciesId).toBe("shadow_eel");
+    });
+
+    it("boosts rare sunken treasure chest probability with glowing shrimp bait", () => {
+        const rareBait: FishingBait = {
+            baitId: "bait_glow_shrimp",
+            name: "Luminescent Shrimp",
+            potency: 3,
+            attractsRare: true,
+        };
+
+        const res = FishingEngine.attemptFishing({
+            fishingSkill: 80,
+            rod: { ...basicRod, tier: 4, bonusCatchPercent: 0.15 },
+            bait: rareBait,
+            waterType: "DEEP_SEA",
+            isNightTime: true,
+            rng: () => 0.01,
+        });
+
+        expect(res.success).toBe(true);
+    });
+});
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