How to Fish Custom Islands: Map Loader Mods and Packs

Updated 2026-09-02 · Steam How to Fish

There is no single "how to fish custom islands mod" — there are two separate loaders, and the island pack you want decides which one you install. YoxiUI's CustomIslandsLoader (v1.5.0, 4,183 downloads, updated 1 September 2026) is the one the big island packs build against. Rompedev's HTFMapLoader (v0.2.8, 3,595 downloads, updated 22 August 2026) is a separate loader whose reference map is Rival Isles. A third mod, DevIslandUnlocked, is not a loader at all: it just switches on an island the game already ships with. All three need BepInEx, and none of them replace each other.

The two loaders side by side

Both do the same job — read an island out of a Unity AssetBundle and make the game treat it as a real place with radar, navigation and spawn points — but they use different folders and package formats, so a map built for one does not load in the other.

Loader	Author	Version	Downloads	Updated	BepInEx dependency
CustomIslandsLoader	YoxiUI	1.5.0	4,183	2026-09-01	BepInExPack 5.4.2100
HTFMapLoader	Rompedev	0.2.8	3,595	2026-08-22	BepInExPack 5.4.2305

Download counts are the Thunderstore figures as of 2 September 2026. Installing both at once is not something either README covers, so treat it as untested.

CustomIslandsLoader: replacement islands and added islands

The loader's README splits custom islands into two kinds, and the difference matters more than anything else on this page.

Replacement islands stand in for one of the game's five. The map's own json says which one it replaces, and the island borrows that island's NPCs, shops and progression, so everything works the way the original did.

Added islands are brand new places out in open water. They replace nothing, appear on the radar in their own colour, and carry their own NPCs, shops and wildlife. Per the README, they do not touch your progression: reaching one needs nothing unlocked and writes nothing to your save's island unlocks.

An island is always two files sharing one name, and the folder depends on the kind:

Island kind	Files	Folder
Replacement	`somemap.bundle` plus `somemap.json`	`BepInEx/plugins/HTFMaps/`
Added	`somemap.island` plus `somemap.json`	`BepInEx/plugins/HTFExtraIslands/`

If you install an island pack from Thunderstore with a mod manager, it lands in the right folder by itself and there is nothing to configure — the json already carries which island it replaces or where in the world it sits.

Turning one island back to the original

You do not have to delete files. The loader's config has an on/off switch per island:

[Maps]
UseCustomIsland1 = true
UseCustomIsland5 = true

Set one to false and that island loads the original next launch, with the map files still installed. Mod manager users edit this in the manager's built-in config editor; manual installs edit BepInEx/config/yoxi.htf.maploader.cfg. This applies to replacement islands only.

One quirk before you try a manual install: the CustomIslandsLoader README's manual steps tell you to copy HTFMapLoader.dll into BepInEx/plugins/, the same filename Rompedev's separate loader uses. The mod manager route avoids the ambiguity, and it is the one the README recommends first.

Co-op rules

Everyone in the lobby needs the same loader and the same islands. The game syncs which island is loaded as a number, so a player missing the files either stands in different scenery than the rest of the crew or cannot follow you to an added island at all. See co-op for how lobbies work without mods.

HTFMapLoader and Rival Isles

HTFMapLoader exists so a custom map can ship as its own Thunderstore package instead of every map needing its own BepInEx plugin. Version 0.2.8 scans installed plugin packages recursively for BepInEx/plugins/**/Maps/**/map.json, and still reads the older layout where a Maps folder sits beside the DLL.

Its README lists requirements of How to Fish 1.0.5, BepInEx 5 / Mono x64 and Windows x64, and supports automatic or manual placement, conflict avoidance against existing island navigation positions, native-style navigation triggers, per-map radar colours, IslandHolder, PlayerSpawnPoint and BoatSpawnPoint markers, runtime repair of common island objects, and optional seagull and clam spawners.

Rival Isles (v1.0.0, 4,183 downloads, 22 August 2026) is the reference map that proves the point: a small PvP-style test arena shipped as a fully separate package the loader finds on its own. Its declared config is map ID rival_isles, automatic placement and radar colour #FF4FA3; the README notes wildlife is disabled on the arena and the internal Unity scene is still named HTFE_Island9. It depends on HTFMapLoader 0.2.8 exactly, so it will not run on the YoxiUI loader.

DevIslandUnlocked and the secret developer island

The game already contains a hidden DevIsland; it simply is not part of the normal island navigation flow. DevIslandUnlocked (v1.1.0, 6,316 downloads, 22 August 2026) registers it as a sixth navigable island with a cyan native-style radar marker that follows the game's normal boat radar visibility.

The README is explicit that it does not move DevIsland, its meshes, lights, colliders, SpawnManager or IslandHolder — the content is baked for its original world coordinates, so only the navigation trigger and radar entry are added, and the normal five islands are untouched. Settings appear in BepInEx/config/com.howtofish.devislandunlocked.cfg after one launch: Enabled, ShowDevIslandDot and VerboseLogging. Version 1.1.0 was a compatibility update for 1.0.5.

The mod-free route is a save-file edit players documented in a Steam guide: set SpawnedIsland to 5 in the save txt under the Dazed Games save folder, then load it. That is player-reported, not an official feature, and patch 1.0.11 on 1 September 2026 added corruption checks and automatic backups for save files. The full walkthrough is on secret developer island.

Island packs available now

Both packs below are built for CustomIslandsLoader, not HTFMapLoader.

Pack	Author	Version	Downloads	Updated	What it contains
The_Island_Project	Ayinz	1.4.2	3,106	2026-09-02	Hand-built replacements: Fenwick Rock, Hollowpine Island, Isla Marisol, Lighthouse Cay, Kilaupono Island
BetterIslands	YoxiUI	1.2.0	2,214	2026-08-30	Hand-built islands 1, 2 and 3 with working NPCs, shops and their own day-night sky

Read The Island Project's page before installing: its own description says islands 4 and 5 are under maintenance and their revamps were pulled from the current update, so you get three reworked islands and the vanilla versions of the other two. Both packs list BepInExPack 5.4.2100 and CustomIslandsLoader 1.0.0 as dependencies.

The CustomIslandsLoader README also points at a MoreIslands pack of brand new open-water places, but that package is not in the Thunderstore listing we pulled on 2 September 2026, so treat it as not confirmed yet.

Companion mods and building your own

Mod	Version	Downloads	Why it matters here
Minimap	1.1.0	704	Live corner minimap that renders the world under you and works with custom islands out of the box
LivingSkies	1.0.7	2,441	Day-night cycle and storms; the loader README says it applies over custom islands too
CustomIslands_MapmakerKit	1.0.3	327	Unity project for building your own islands — download it manually, it is not a runtime mod

CustomIslandsLoader used to have a full-screen chart on the M key; the README says it was dropped because the game already has a radar and Minimap does the job better. The Mapmaker Kit ships no game content — everything it builds from is imported once from your own installed copy during setup.

To build for HTFMapLoader instead, its package includes docs/MAP_CREATION_GUIDE.md, docs/MAP_JSON_REFERENCE.md, docs/THUNDERSTORE_MAP_PACKAGE.md, two example json files and a HTFMapBundleBuilder.cs editor script. It asks for Unity 6000.4.4f1 on the Universal 3D (URP) template, an IslandHolder root at default transform values, colliders on walkable geometry, and both spawn markers.

Modpacks and compatibility

Only one Thunderstore package currently carries the Modpacks category: Enhanced_HowToFish by Linux_Squad (v1.0.8, 7,530 downloads, updated 29 August 2026). It is a quality-of-life bundle and contains no island loader and no island packs, so installing it does not give you custom islands. The full package list is on all mods list.

Both loaders declare BepInEx as a dependency, so a mod manager pulls it in. Manual installers need BepInEx 5 x64 for Mono, run the game once to generate config, then drop the DLL in BepInEx/plugins/.
Version pinning is strict on the Rompedev side: Rival Isles depends on HTFMapLoader 0.2.8 specifically.
HTFMapLoader's README targets game version 1.0.5. The game moved to 1.0.11 on 1 September 2026 and the loader has not been updated since 22 August 2026, so post-patch breakage is unsurprising — start at mods not working.
Custom islands change what an island looks like, not the game's own island order and unlocks. For the vanilla layout see islands, and for install basics see mods.

Sources and last check

Checked 2 September 2026. Versions, download counts, dependency strings, folder paths, config keys and feature lists come from the Thunderstore pages and READMEs for YoxiUI's CustomIslandsLoader, BetterIslands, Minimap, LivingSkies and Mapmaker Kit, Rompedev's HTFMapLoader, RivalIsles and DevIslandUnlocked, Ayinz's The Island Project, and Linux_Squad's Enhanced_HowToFish. The save-file route to the developer island is player-reported from the "Secret Developer Island" Steam Community guide and is not an official feature. Save-file corruption checks and backups come from the official 1.0.11 announcement dated 1 September 2026. Anything a README describes that no patch note confirms — co-op sync rules and progression safety for added islands included — is the mod author's own claim, not a developer statement.