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I am writing a DnD module where fishmen are invading island nations. The islands would naturally have adapted to this in some way as the raids have been happening for years. What method would these island nations use to detect underwater invaders?

I first thought of nets with floats and bells but these would give constant false positives from fish or tides.

Magic is an option, but needs to be a realistic and low level spell that a run of the mill spellcaster could accomplish (say 3rd level or below). I considered a water-breathing spell cast on a regular patrol but the underwater terrain is unlikely to be hospitable to a normal patrol and visibility would be too limited to be useful.

I much prefer a more generic non-DnD specific answer but would settle for something in the DnD world.

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    $\begingroup$ When you want a solution within the framework of DnD magic rules, you might want to ask on rpg.stackexchange.com $\endgroup$ – Philipp Jul 18 '17 at 20:18
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    $\begingroup$ I would prefer a non-magical method more technological. But I'd settle for the dnd rules method. $\endgroup$ – danielson317 Jul 18 '17 at 20:19
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    $\begingroup$ They could always just keep some sharks well fed with regular fish scraps. Not a problem to fishermen who are very, very careful not to fall out of their boats but much more of a problem to any invading fishmen. $\endgroup$ – bp. Jul 19 '17 at 11:49
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    $\begingroup$ "Everything was peacefull, and then the Water Nation attacked" $\endgroup$ – Mr.Burns Jul 19 '17 at 14:00
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    $\begingroup$ DnD lets you fairly trivially turn every person who dies in your island nation into a skeletal sentry, or set up elaborate underwater labyrinths, or brainwash an aboleth, or construct Turing machines out of magic mouths that are arbitrarily good at pattern recognition, or have one guy in the middle of the island listen so well that he can hear raids approaching, or leave explosive runes covering every grain of sand on the beaches, or or or. If you want DnD answers, make it an RPG question. $\endgroup$ – fectin - free Monica Jul 20 '17 at 3:47

17 Answers 17

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Marine watchdogs.

First option:

Build a net wall around the city in the water like torpedo nets. Inside the net give a cautious and jumpy sea dweller a home. The net has no warning function, it simply isolates the sea dwellers from predators so that they are moving freely around the net.

If something is inside their waters, the sea dwellers are fleeing into provided homes which can be conveniently watched. So an intrusion will be detected faithfully.

Second option:

Train marine animals to warn people or even track and attack intruders. In our world dolphins or sea lions are trained e.g. by the US Navy.

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  • $\begingroup$ I love both of these ideas. Trained sea animals and sculpted ecosystems are brilliant. $\endgroup$ – danielson317 Jul 19 '17 at 4:55
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    $\begingroup$ +1 for the last paragraph. The US Navy has trained seals to detect divers approaching docked ships. Essentially guard dogs that are native to the water. $\endgroup$ – Wayne Jul 19 '17 at 14:02
  • $\begingroup$ It would be pretty easy for fish people to cut holes in the net without being detected. $\endgroup$ – sphennings Jul 19 '17 at 17:03
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    $\begingroup$ So as far as sustainable, non-magical, low maintenance, high reward this is the best option that actually answers the question. Trained dolphins or sea lions along with underwater weather balloons that the animals can release if threats are seen for greater distance would be an excellent system. $\endgroup$ – danielson317 Jul 19 '17 at 17:13
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    $\begingroup$ @sphennings The nets are to protect the watch from predators (and keep them near the island). Holes would just let things in, startling the jumpy watch and alerting the islanders. $\endgroup$ – jaxad0127 Jul 19 '17 at 18:36
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Don't live directly next to the water.

You can still be close enough to the water to easily hop in your boat and go fishing. Just build your house a couple hundred meters inland from the high water mark. This is just a good idea anyway. This makes it harder for your house to be washed away during a storm surge. Then any fishpeople will have to come out of the water to threaten your village. They can then be spotted by guards or lookouts as normal.

If the attacks by raiding fishpeople are a regular occurrence you build a wall around your town and you treat them like any other raiders that come from the water.

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    $\begingroup$ Not sure if you mean don't live near the water or really, really, near the water. If you mean just near the water, there are too many good reasons for people to live near the shore despite the dangers, which is why we do it and have for millenia. If you mean leave a little buffer zone, then I suppose that would be an option. $\endgroup$ – ozone Jul 18 '17 at 21:54
  • $\begingroup$ @ozone I mean put at least a field between your residence and the shore. $\endgroup$ – sphennings Jul 19 '17 at 2:10
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    $\begingroup$ While leaving some space on land could be helpful it's hardly a replacement for an early warning system. A well built fortress will have watchtowers and signal towers that will see an invasion coming miles (hours) ahead of time. I'm looking for an amphibious version. As it's an island nation building miles from the coast is not always an option. $\endgroup$ – danielson317 Jul 19 '17 at 5:02
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    $\begingroup$ This was common practice in the past, e.g. Puerto Pollença, Majorca (IIRC the threat was pirates) $\endgroup$ – Chris H Jul 19 '17 at 8:07
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    $\begingroup$ Historically, most towns grew up alongside either a river or an ocean bay. Leaving the water means leaving the best means of transport and a ready supply of meat... $\endgroup$ – CaM Jul 19 '17 at 12:15
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Your idea of nets with bells was a decent idea, but as you say, fish and tides and such would ring the bells. Plus, an intelligent creature would cut the lines before an invasion.

I think you might want to consider a more active defense.

Caltrops for example would be useful in the shallows, when your attackers must transition from swimming to walking. A few spikes in the seaweeds will slow down the attackers.

If those caltrops are also sitting on bladders of air, then the pressure would pop the bags. The rising bubbles would be visible to anyone watching the water's surface.

Another approach is tarred spears. Fire-harden a wooden spear, then coat it in tar to waterproof it. If the spear is 10 feet long, bury it at a 45 degree angle pointing out to sea so that only 3 to 5 feet are above the seabed. Build a wall of these spears, leaving only your fishing boats' exit path clear. This wall will slow them down, but not stop them...

Strings or wires can be tied to the shafts below the sea floor (so the wires are not exposed) and the other ends tied to bells above the high tide line. When the creatures either impale themselves on the spears or rip the spears out of their way, the bells will ring.

Build a sea wall so that once they come ashore, they then have to climb a wall. This wall should be strong enough to handle storm surges and high winds. It should have gates, of course, but those become choke points at which you can slow their invasion. Of course, the wall also protects your village from actual storms, so double win there.

Defense in depth. Use all of the above (and any other ideas you come up with), not just any one idea.

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    $\begingroup$ I think that Air Bladder Idea may be the best I've heard yet. Add some squid ink for day and phospherous for night or something like that to it and it could really be an excellent warning system using non-leathal underwater mines. Magic to place but relatively low maintenance. $\endgroup$ – danielson317 Jul 18 '17 at 21:19
  • $\begingroup$ Air Bladders are a great idea, especially with ink. However caltrops and spears are of questionable usage. Merfolk are going to swim most of that distance. And while caltrops and spears will help in the shallows, spears can be broken or removed quietly and caltrops will be buried via natural sand movement. $\endgroup$ – user2259716 Jul 18 '17 at 21:38
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    $\begingroup$ I don't think caltrops would work because ocean tides would move them or bury them in short manner, plus they could corrode. $\endgroup$ – ozone Jul 18 '17 at 22:02
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    $\begingroup$ Given that by the time they have to transition to walking from swimming they are showing their heads above the water, I think we can spare ourselves of complex bladder detectors. Also, caltrops would block access to the coast for the locals. If you don't have access to the sea you are better off not building besides it. $\endgroup$ – Rekesoft Jul 19 '17 at 7:27
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Here is something that might be slightly strange, but could work for you.

Guard posts built on Glass bottom buoys.

You will have to set, as a precondition, very very clear waters as a starting point. Set a guard on each buoy so that they can use the glass floor to look out into the rest of the lagoon. This will help cope with wave chop distorting your line of sight.

For night duty, design a floating, glass bottomed lantern, maybe with a mirror to focus a beam out into the water.

Make sure the observation posts are out beyond where the waves begin to break. Regular boat patrols to help. In case of storms, bring everyone in from the outposts and put them in bunkers on dry land. It's not as good for observation, but you won't want humans on a little buoy during a squall.

All you are really doing is a standard guard patrol picket. You are just making it a bit easier to see the fishmen coming by smoothing the "surface" you are looking through to avoid problems with refraction, and adding some primitive underwater lights.

Medieval glass wasn't always of the highest quality, so you might have to use a little bit of magic on the glass during construction. After that, no contiuous scrying or spellwork required.

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    $\begingroup$ Very clever. A good option. $\endgroup$ – danielson317 Jul 18 '17 at 20:49
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    $\begingroup$ I was going to say that I don't know if it satisfies the OP's question, but I see he likes it. I would like to add that a glass bottom may not be enough, but rather a glass cylinder that the guard can climb down into to look all around. If the water is clear, then I don't see why just a tower looking down on the water wouldn't suffice and underwater lights for night time. None of these options work in murky waters. Still really cool suggestion. Kudos! $\endgroup$ – ozone Jul 18 '17 at 21:58
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    $\begingroup$ If you're doing this at a medieval tech level, you're going to need magic to make the glass. Pieces of glass larger than a few inches square are a very recent development. $\endgroup$ – Mark Jul 18 '17 at 23:54
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    $\begingroup$ It is helpful, but not absolutely, necessary to have huge panes of glass. If the panes are small it just means the guard's face will have to be close to the pane in order to have an unobstructed view in that direction, and you would want multiple viewing ports to cover all the directions in which it is necessary to watch. $\endgroup$ – David K Jul 19 '17 at 11:44
  • $\begingroup$ @DavidK You could use a periscope-like arrangement. A lense at each end, a tube you can spin. Then you could scan the area. The glass and mirrors involved would be potentially beyond the relevant tech levels though :( $\endgroup$ – Tim B Jul 20 '17 at 20:01
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The Alarm spell is a 1st level spell that both Wizards and Rangers can cast. It's intended purpose is to notify you someone is where they shouldn't be.

Unlike mundane means, you can freely specify things like, "Fish don't trigger it."

As it's only a 1st level spell in theory even a scrub would be able to learn how to cast it with a bit of learning/practice. Rangers don't have schools and are usually self-taught. It would not be impossible for the local wise-man or grizzled hunter to have this trick up their sleeves and the position probably would even be a traditional role in the village if this sort of thing happened regularly.

Feats like Magic Initiate and Ritual Caster in particular make it plausible for even someone not well-versed in the arcane to know the spell if they study enough. You need not be a learned Wizard or a world-wise Ranger if it's just something that gets taught along a family line for example.

I realize you wanted tech but I didn't see the signature "Ward this area" spell listed in the answers so I thought I would mention it.

Edit: Upon further review, the Alarm spell itself will not help you. However, you can make spells of your own and if you have an entire nation's worth of resources and a reason to create such a thing, making a larger-scale version is not impossible or even improbable in the slightest. It does require you to dive out of the rulebook but such a thing is not impossible or even frowned upon. Part of the core premise of Wizards is unearthing and creating new spells to meet needs.

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  • $\begingroup$ Alarm spell could be used in conjunction with some other tools but the notification range is only 1 mile and the spell has to be re-cast every 8 hours. This makes the spell useful but not sustainable in the scenario described. Still +1 for moving in the right direction. $\endgroup$ – danielson317 Jul 19 '17 at 17:16
  • $\begingroup$ How is the spell not sustainable? When ritually cast it has no cost whatsoever beyond time. OP mentioned a 3rd level caster to boot- which I didn't originally see. $\endgroup$ – Friendlysociopath Jul 19 '17 at 20:23
  • $\begingroup$ Oh, I went back up and looked at the comments, coast-wide defenses are fairly hard to do. I envisioned a small fishing village- not an entire island. $\endgroup$ – Friendlysociopath Jul 19 '17 at 20:31
  • $\begingroup$ Still not unworkable. If it's taught to a family line, there would be multiple casters. They could spread out to cover the coast and trade off watches. $\endgroup$ – Michael Jul 24 '17 at 15:13
  • $\begingroup$ I maintain the Rangers are a distinct possibility even without the family line. You only require 2 levels in it to be able to learn Alarm plus an additional spell. Fluff-wise I believe city guards used to be 2nd level fighters so it's not remotely impossible to train your coast guard in these methods. $\endgroup$ – Friendlysociopath Jul 24 '17 at 18:15
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Early warning underwater acoustic mirrors.

http://www.andrewgrantham.co.uk/soundmirrors/

enter image description here

These amazing and amazingly huge parabolic acoustic mirrors were built in England (Germany too, I think) because they could amplify the sound of an airplane in the distance. Radar made them obsolete, but they are still there. Check out the linked page.

The bigger they are, the lower the frequency they can detect. From https://en.wikipedia.org/wiki/Parabolic_microphone

This is because, from the Rayleigh criterion, parabolic dishes can only focus waves with a wavelength much smaller than the diameter of their aperture. The wavelength of sound waves at the low end of human hearing (20 Hz) is about 17 metres (56 feet); focusing them would require a dish much larger than this.

I envision these things underwater. It is D&D so maybe they could be made from giant shells. The people in the 1930s who built the dishes shown had electric microphones, but one can transmit a sound signal along a taut string, in the manner of a tin can phone. enter image description here

Listeners topside would listen via a taut string running from a pickup at the focal point of the parabolic acoustic mirror back (possibly through a hole in the mirror) up to the listener. I envision the listener with his head up inside a fixed bell-like helmet, with multiple strings running to it.

enter image description here

In case you wondered: the listeners are listening for fishmen sounds. They might hear a lot of other interesting underwater things too.

Once they heard fishmen sounds, in addition to sounding an alarm they could mute one incoming wire at a time and by a process of elimination determine which direction the sound was coming from.

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    $\begingroup$ This is a decent way to hear other things. Storms. Boats. Where the schools of fish are, so you can tell your fishing fleets where to go. Tsunami?. A single submerged tube at a given depth and location would make a sound when tides were coming in or out, or when the tsunami was coming in. There is a very fun trick in inverse modeling where a sonar "ping" is shot into a seabed and the respose is recorded, then the response is time-reversed and broadcast, and all the 'non-average features' in the bed emit a ping. It doesn't have to be spherical, just coupled. $\endgroup$ – EngrStudent - Reinstate Monica Jul 20 '17 at 16:11
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    $\begingroup$ Another trick in inverse modeling is to put a perfectly clean signal through a wire, and record the distorted output. With those two pieces of data, you can re-engineer the input or output side of the wire so they undo the distortion of the wire. In the case of the village, you could clean up sounds from far away, and you could transform emitted sounds so that at far away they are less distorted. $\endgroup$ – EngrStudent - Reinstate Monica Jul 20 '17 at 16:24
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How real cities handled their ports

Ancient cities tended not to be located on the water, but rather a few miles inland with a client village on the water. This way, they had the utility of ocean access, while still being protected. If the village was lost, no big deal.

As cities got larger they tended to connect the sea-side village with the city via walls. This new and larger city was surrounded by walls on all sides, but often had an inner wall between the port and the actual city. This allowed for greater protection against seaside attacks. Not to mention the port itself may be walled off into sections. These sections might mean that there are 2 harbor areas or the open area only holds ships. All houses and warehouses being behind walls in the 2nd harbor area.

Further improvements were the usage of a sea wall, a wall that literally is standing in the water, so besides the entry to the port it is completely protected. Even the entry typically had a chain possibly with logs that was strung across it. These methods protected the main city, and while the port was not totally safe it was reasonably safe, at least against ships.

Your World

In your world, biology and the usage of magic skews things a little, so while the above would still be used you potentially need to add a few additional features. The levels of precaution being reliant on how much money and effort you are willing to invest, and whether the burning of a part of the port concerns you that much. Also, whether these merfolk carry weapons and armor or are simply aquatic naked people.

Detection

Mundane

At night a large chain net is spread across the opening of the port. It might be possible for someone to squeeze through but they would not be able to carry much if any equipment.

With glass: Have a few buoys with bells that are towed right inside and outside the entrance to the harbor. Divers with diving helmets or a diving sphere sit at the bottom and ring the bell if they see attackers.

Without glass: Build a few thin towers, basically thick poles, with a wooden tube carrying air to the bottom of the harbor. At the top of these towers are large bells. Divers suck on the tube and watch the entrance. If they see attackers they pull on the rope and ring the bell.

Buoys are more flexible, but ring faintly. Being rung will be louder than waves though. Towers cost more, are inflexible, but will not ring from the tide.

Magical

Same as above, but with water breathing. Maybe have "detect life" active.

Neutralization

Whether your solution is mundane or magical, you WILL need more towers and many more guards. Most ports would have towers, but most would focus outwards. You need towers facing inwards. They only needed a few guards as ships were seen from far away and the chain at night kept them out. Now you need many more guards, as people are harder to spot and keep out than ships.

You need towers to face inwards and have archers or mages constantly watching the harbor. If merpeople attack they need to immediately start firing. Depending on how heavily armed the merfolk are, you may need to sacrifice the particular harbor segment.

Throw pots of pitch, oil, or greek fire onto the water and light the surface. The water will catch fire either driving the merfolk onto a narrow dock or piece of land. The small area and many towers will create a killing field. Alternatively, they will have to retreat as the water will heat and cause burns and possible oxygen problems.

Conclusion

Unless the city messes up, is heavily outnumbered, the merfolk are somehow heavily armed (unlikely), or the merfolk can throw around a ton of magic or explosives, the city will be safe. The port and ships may be lost or raided, but the merfolk will pay dearly for it (seriously they will die).

However the cost of employing so many guards will mean that only cities can do this, as villages simply do not have enough people. So maybe back to the split coastal and interior villages.

Much more effective and safer for merfolk to raid small villages who cannot afford to designate standing guards, regardless of detection. Merfolk could also swim up river to unsuspecting villages.

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The same way they detect schools of fish, a person on a tall cliff or tower looking down, the more vertical their line of sight the better and farther they will be able to see into the water.

enter image description here

From above you can see well, the ocean around islands is quite clear. Your big danger times would be at night. Here is an image looking down on a bay from a cliff, note how you can see everything on the ocean floor.

enter image description here

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In the realm of simple magic: Great White Shark patrols.

The magic is to give the sharks a predisposition for patrolling the shore; they can go off to hunt for food, but the magic means they just never get tired of hunting the shore near the city, even if they never catch anything there.

Similar low level magic to attract them into cages and calm them, where they can be enchanted, perhaps even attaching an amulet to them or something like that. It should not be much more difficult than an infatuation spell. If you attach the amulet, than simple touch magic to a sister amulet can call the sharks in an emergency, or send them off to hunt elsewhere during the day so your own fishermen can get their jobs done.

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  • $\begingroup$ I actually did think about this. But magic items are very rare so it would have to be an actual spell. Spells on animals only last a day or so and require energy so one spellcaster would be dedicated to managing 8ish animals every day and be worthless in an actual fight. It's a realistic approach though. A dolphin or shark could cover 100x what a human with water-breathing could. $\endgroup$ – danielson317 Jul 18 '17 at 21:48
  • $\begingroup$ Well, not worthless: When an "actual fight" breaks out, the sharks will have already done their job, no point in continuing the patrol. Whatever else the spellcaster can do in a fight, he might as well be doing. The sharks did whatever they could do, and tomorrow is another day, he can go back to fish enchantment, if he (and the kingdom) survived the attack. $\endgroup$ – Amadeus-Reinstate-Monica Jul 18 '17 at 22:48
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    $\begingroup$ +1, but do you need magic to get sharks to patrol an area? Chum the waters on a regular basis with dummies that look like your fishmen that are stuffed with lots of delicious, bloody chopped fish. They odds are against them attacking the fishmen on sight -- the sharks might not be hungry, there might not be blood in the water, etc -- but if the fishmen are attacked by sharks or are aware of your shark attraction/training, it could give them pause. $\endgroup$ – Wayne Jul 19 '17 at 14:05
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    $\begingroup$ @Wayne Plus, answers the OP objection to devoting a spellcaster to the job. In fact, if the village fishes for food on a regular basis; have the scrap of the fish (guts, heads, organs, skeletons) collected by law for just this purpose. Then the only thing you need spellcasters for would be to dispel sharks in an emergency; e.g. men overboard, a sinking ship, or needing to be in the water to effect repairs or any other reason. Good idea! $\endgroup$ – Amadeus-Reinstate-Monica Jul 19 '17 at 14:39
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The society could employ trained Seals for monitoring the shore lines and deeper oceans for imminent attacks. We know that seals can be trained to perform stunts, ball games. I am sure they can also be trained to swim back to shore to inform a trainer of fish-people presence. After all seals are dogs of the sea.

Training of some sort of seafaring animal to alert of danger. It does not have to be seals. It could also be dolphins as they are also intelligent animals.

On Land we currently use dogs for similar purposes in our gardens and the such like.

As seals are important/as important as dogs in this society they would also selectively breed them to obtain traits they desire i.e. size/power/intelligence/speed.

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    $\begingroup$ I was going to suggest trained dolphins too. Dolphins are clever and love being around people. People can train them to act as life-dolphins and to do simple tasks. $\endgroup$ – RedSonja Jul 19 '17 at 13:16
  • $\begingroup$ A female former member of the Naval dolphin team out of USSR implied there were (ahem) social bonding things that had to be done in order to get the dolphins to stick around, especially against enemies or in militarized localities. The same was not required of seals. $\endgroup$ – EngrStudent - Reinstate Monica Jul 20 '17 at 16:17
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    $\begingroup$ That was my thought. The US Navy uses both porpoises and sea lions for harbor guards against divers. A Navy SEAL (the human variety) described a training mission where he stayed close to the docks to avoid the porpoises, and a sea lion nailed him. It was just training... but they do bite hard. $\endgroup$ – tj1000 Jul 20 '17 at 19:27
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You can defend your islands utilizing Brine pools or Carbon Dioxide lakes. These two naturally occurring underwater geographic formations that are heavily populated by extremophiles.

Brine pools are known for killing fish and crabs that go into them due to their salinity(3x to 8x higher than the ocean). These Brine pools are higher density than the surrounding ocean and swimmers would most likely be pushed out if on a normal swim. However if it were an armed raider walking along the floor the metal he is carrying would cause him to sink in.

A Brine pool with dead crabs

Carbon Dioxide lakes happen at much deeper locations in the ocean than Brine pools. The depths typically needed are 10,000 ft below sea level as the temperature is low enough that liquid carbon dioxide is denser than salt water and can be collected in pools. There are other ways to make them, like volcanic underwater geysers that release CO2 into underwater holes. Fish take in oxygen rich water in order to breath, and I would imagine spells that allow breathing under water do the same thing, using CO2 lakes the raiders would suffocate underwater, just like they not able to breath with a spell.

You would need to hand-wave some of this in order for them to form close to an island but both would work well for defending against underwater attacks. As added bonuses, neither should interrupt ship travel, so normal traders should never even notice it there. Also, due to lack of oxygen Iron weapons dropped by the raiders into either should stay in relatively good condition for collection later.

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  • $\begingroup$ I'd never heard of CO2 lakes under the ocean, that's really interesting. Google found an example though: news.nationalgeographic.com/news/2006/08/… $\endgroup$ – Tim B Jul 20 '17 at 20:34
  • $\begingroup$ These seem useful for wartime and under different circumstances. However killing all the fish on an island would be killing all the people too. You would be doing the fishmen's job for them. Clever bug not realistic to create or sustainable long term as it would destroy the ecosystem that feeds the island. $\endgroup$ – danielson317 Jul 24 '17 at 14:00
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Walls and Bells

A medieval population will be mostly composed of farmers/fishermen/... that's a lot of people scattered throughout during day time who, if not warned early enough, will be caught by the invaders.

The typical medieval strategy for protecting those people is to have a fortified place where people seek refuge in when the alarms bells are rung.

The problem, of course, is to be warned in time.

Note: a corollary is that should people sleep behind the walls, there is no need for early warning at night; thus the solution need not be concerned about illuminating the night, which would be a real hard problem.

This answer will therefore focus on a simple strategy: force the enemy to be visible early, delay it to give time to people to run to cover.

Seawall

Watching a large area, such as the shore of a whole island, is highly impractical.

If you have cliffs, that's great, otherwise I advise building a Seawall such as Pondicherry's all around the island, minus a few select passages for harbors.

This wall need not be smooth, or elaborated, it's fine to just pile on somewhat large stones. It's also fine if they are not too stable. They should however have their feet in the water, or close to, as the goal is to prevent the invader from getting an easy toehold on the island. Climbing rocks while waves keep smashing you against them is not an enjoyable experience. Bonus point if algae make them slippery.

Now, you can build watchtowers at regular intervals along the seawall, and you can also place a variety of devices on the wall to further slow down a foe: for example, transforming some holes between the rocks into inhospitable places with caltrops and other such implements. And it's fine if they rust due to the salt, just replace them once upon a time.

Harbors

This wall will have holes, for ships have to pass.

Each such channel will need watchtowers and an iron net that is raised until a boat tries to pass.

Atoll

If this does not provide an early enough warning, the next level of protection would be an atoll surrounding a lagoon. The goal is simple: force the fishmen to pop above water even if for a short period of time, so they can easily be spotted from the shore.

Note that it only really matters during daytime, as explained above, so there is no need to have torches or anything on the atoll. It's even fine if it's partially submerged as long as it's shallow enough.

If there's no natural atoll to build upon, this might be a challenge though. Sinking tree trunks, mostly upright, could build such a barrier without having to carry stones to sea. Try to aim for shallower areas.

Of course, a few channels will need to remain open if ships are intended to go at large; apply the same solution than the one for channels in the seawall: watchtowers and iron nets.

Note: those watching from there, and fishing/traveling in the great sea, would be at greatest risk; they would also be in the best position to raise warnings early.

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surround your island of aquafarms full of fish afraid of underwater fishmen. if the fish jump out of water, that means the fishmen are near them. U could use watchtower to monitor them..

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    $\begingroup$ Welcome to WorldBuilding! Could you expand on this answer a little? At the moment this answer is very short and unclear and will probably end up getting deleted. $\endgroup$ – F1Krazy Jul 19 '17 at 14:03
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A reef surrounding your island would make it hard for intruders to cross unless they knew where it was, and used the proper direction to enter, to avoid splintering their boats.

The shallow water over the reef means they could stand on it and not even cover halfway to their knees. Sharp spikes and poison/bactiria make this undesirable. You would also be able to see them crossing on the surface. Maybe some of the coral is bio-luminescent, so even during a moonless cloudy night, they would be illuminated from below, and silhouettes as they approched shore.

Poison/electric Eels and stonefish could inhabit the reef.

Sandbars could randomly come and go, only the locals know them well enough for navigation. Again, crossing on the surface makes you visable.

An atol could surround your island. (A round hollow island) So no boats or even fish can enter/leave except in known guarded locations. Your villages could be on the center island. The outer island, the atol, would be covered in guard towers. Boats would have to come ashore, and be carried to the water again across solid ground, troops would have to walk. All visable in the watch-fires. (Flamable fish-oils?) This would have similar protections that a moat would have on a dry-land castle.

Also, a wall surrounding the edge of your main island, with boat-gates near the openings in the atol for convenience, OR... far around the oposite side so you have to swim, or float around the outside walls, rocks and buring oil dropping on you, to get to the next gate to come in further. Kind of like an aquatic version of Minus Tirith, for all the Lord of the Rings fans out there. Multiple rings with few gates means the enemy will spend A LONG TIME floating below your walls unable to reach up and hurt you.

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Two ideas: fish oil, and fishing nets.


One component of visual detection could be fish oil. Or olive oil.

While glass-bottomed boats/buoys have been suggested, the problem with glass is that it requires a lot of glass.

When I was a child in Corfu, the Greek fishermen wanting to see into the water would pour a tiny splash - no more than a teaspoon - of olive oil onto the ocean surface (Olive oil being the universal panacea for everything in Corfu, used for everything from treating skin ailments to lighting fires, treating wood, a lubricating machinery, preserving food, rustproofing, frying, you name it). This would quickly spread out, to form a molecule-thick layer atop the water, completely changing the optical behavior of the water surface nearby, so that rather than just the reflective, sparkling ripples, you could suddenly see clear to the bottom.

Having it contained by a hoop would improve the efficiency, but it holds itself together pretty well anyway, remaining for minutes at a time, at least in the relatively calm waters of the Ionian sea.

Basically it just reduces the reflectivity and fractal choppiness of the water surface. I think the layer of surface-tension on the water is very refractive, and the oil prevents this, even in the tiniest of doses.

It works surprisingly well, like a magic spell that gives you an instant window into the water to see the glittering diced-rainbow of the fish swirling below.

[To be absolutely clear: this is a tiny localized film around the spotter, who would need no more than a couple of flasks per DAY, not an environmental disaster where the sea-life will need cleaning and oil will need containing within booms and so on.]


That said - any form of underwater detection is doomed to not be as good as overwater detection. Guard buoys or towers, and balloons in the shallows as suggested by another answer, will give you, at best, a few extra seconds, as they come in from the shallows. Compare this to the hours you'd get using overwater sight to see approaching ships.

I'd recommend three layers of netting around the entire island, or in an arc around populated villages/towns. Inner and outer layers should have diagonal mesh, and floats somewhat below the water level, but with flags projecting up above the water, such that if the mesh is cut, they bob up, but they cannot be removed downwards without being noticed.

The middle net (protected from fish by the other two layers) should have bells on, and be covered with fishhooks, sea urchin spines, etc to snag on people trying to sneak through - if jangled, then people are busting through fast, rather than cautiously.

Even this will give you perhaps 30 seconds' extra warning, a minute at best.

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  • $\begingroup$ One problem with the oil alternative is it kills fish. As this is an island fish are a primary source of food and income. While small amounts would do very little observing an entire island would be both hazardous to the local ecosystem and expensive to maintain. $\endgroup$ – danielson317 Jul 24 '17 at 13:57
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    $\begingroup$ @danielson317 - Eep, "kills fish"? I must have phrased this badly, then, as you must be imagining them freighting in multiple tankerfuls of thick, viscous crude oil, and just emptying them wholesale onto the island's coasts... not pouring a couple of drops of clear, refined oil onto the surface of the sea, forming a molecule-thick layer which completely changes the optical properties of the nearby surface. I'll update the answer to clarify, thank you. $\endgroup$ – Dewi Morgan Jul 24 '17 at 14:34
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Looking at the magical ways:

Drop "special" items in the water that attract and enrage sharks. Drop them in circular paths like fences.

Teach the locals "shark friendship" and/or "undetectable to shark" (not just invisable....)

Also:

Illusion of a fake village... Running, screaming villagers. Phantasmal village guards with spears/crossbows. The villagers use it as a tv/soap opera everyone adding their own ideas.... or avoiding it's bore and drugery.

The real village is over that hill right there... The path to get there is behind that rock illusion. Oh yeah.... and an illusionary shield that looks like just another mountain.

And if someone get past? Well... invisability on buildings when seen from outside town... you have to touch the inner sidewalks before you can see them.

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Are their sea-elves in your world? A friendship with them would be benificial.

Could they owe you a favor from a long ago war? You saved them? They hurt you, and are making reperations? Maybe just a few of them? A dozzen?

Maybe your island has something they want? A natural resource? They regularly send trading parties, with security to protect their "treasure" The Fishermen could also be after this resource, The elves HATE the fishermen and their raiding parties.

The elves protect you because they cannot refine the resource themselves, it has to be you? Maybe it's poisonous to them in its natural state, they rely on you. Maybe it's poisonous to you too, and you are being used as cheap labor? Maybe they let you manage it because the fishermen treated them badly in the past, its the right thing to do?

Maybe they are protecting SOMETHING ELSE associated with the island, some native fish or dolphin who ONLY feed at your shore or reef.

Are the fishermen sea-elves themselves? This could be a rival tribe, race, or species.

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