What infrastructure would be needed to "farm" merfolk scales?

Question

In the world of my story, merfolk have been captured by humans to serve the cruel purpose of providing their colorful scales to make jewelry, chandeliers, etc, with their colorful scales. I'm not looking in this post specifically for how this would work biologically - I've mostly worked out though that it is not an extremely gory process - ie, the merfolk are not skinned alive and then left for dead. The scales come off with a sharp knife, and will regrow some months later.

More of what I'm interested in is getting feedback on the system that would need this to work. I'm in an early 19th century setting circa 1800 - 1850. I've figured so far that this is what I would need.

-for a group of around 30 captive mers, a cove near the ocean has had it's exit blocked off to hold the mers in. The mers require the water to be changed periodically, so there are channels controlled by sluices that let water in and out. Food is provided by dumping a porridge-like mixture into the water.

When a mer is ready to be scaled, they are lured over by the release of food, and then caught around the neck with a staff with a noose around the end. They are then pulled into a cage in the corner of the pool, where they are winched up, then bound. Any mers that attempt to stop this, or misbehave in general, are struck with long staffs. This is not always needed however, as even with the water being changed out periodically, it's lack of natural movement has resulted in a numbing effect on the minds of the mers. They can function but it is much more difficult.

The mer to be scaled is then brought to a person sized bowl, where they are secured at the wrists and around the end of the tail just above the fins. The mer is provided with a wooden dowel to bite down on (not out of kindness - it helps them stay more steady) and then the scaling begins.

A well practiced scaler can remove the scales with minimal blood, though some is always to be expected. Extra care is taken around the tips of the fins and the "folds" in the front and back that are not very visible, but have smaller scales than the normal thumbnail sized ones.

Following the scaling, the mer's now bare tail-skin is cleaned off to prevent infection, and then is released back into the pool.

So far as I have figured, my scalefarm at the start is able to be manned by two - the master and and apprentice. As the number of mers increases to thirty, a second apprentice is taken on. The apprentices sleep in a bunkhouse adjoining the master's quarters and office.

The farm itself has a pontoon bridge going around the whole edge of the pool to aid in watching the mers. the front of the farm and the seawall are blocked off by netted fences higher than a person.

Lastly, the farm has a small forge for maintaining tools and fixing mechanisms in the sluice gates.

So that is my outline of how all this works. Let me know how you think this could be improved, or how it would be ramped up for a larger operation (or downgraded for a smaller one).

This practice is legal, though there are regulations on it. This is in the broader context of the human nation and fractured merfolk city states being at war with each other for the past 80 years or so. As the only exposure most humans have to merfolk is seeing them sedated in farms or attacking human shipping, they are viewed mostly with contempt.

Answer 1

There are some issues with this. Each issue and potential solutions to it are outlined below;

Sluice Security Risks

The mers require the water to be changed periodically, so there are channels controlled by sluices that let water in and out.

These 'channels' are a security risk. As @legio1 said, if the channels are deep enough for quick, efficient water exchange, the water-breathing Mers will be able to operate with relative impunity to undermine the grates which prevent escape from the bottom of the bay, where humans can't reach. Make it shallow enough for the air-breathing workers to deal with escape attempts and there won’t be efficient water exchange.

Solutions to this can include passive defenses, such as spikes and such that keep the Mers out. Alternatively, you could have regular checks of the grates that ensure that the Mers aren't trying anything funny.

Nope, not falling for THAT again

When a Mer is ready to be scaled, they are lured over by the release of food, and then caught around the neck with a staff with a noose around the end.

It is not going to take long for them to figure out that the release of food could mean they get grabbed, locked in a cage, and de-scaled. Hence, they are going to stop coming over to the spot you want when you release the food. You can solve this by simply only releasing food at the spot with the cage, but then you have a situation where Mers are starving themselves to avoid being grabbed. Desperation would eventually drive them over to where you want, but deprivation of food may reduce scale quality, which is bad for profit margins.

One could fix this by outfitting the Mers with collars that are affixed to a long 'leash'. When it's time to de-scale a Mer, you use the leash to yank the Mer into the de-scaling cage. Mers caught screwing with their collar in any way or caught removing their collar are administered a beating via long staffs, and then have their collar replaced.

Violent tail-slapping

The Mer to be scaled is then brought to a person-sized bowl, where they are secured at the wrists and around the end of the tail just above the fins

The Mers here are going to violently thrash throughout this, securing just the tail and the wrists isn't going to be enough. Fish can hit quite hard just by flapping their tails, and this is going to apply doubly for the Mers here.

The solution? Have a stretch rack of sorts set up. The rack features restraints at the bottom for the tail, restraints near the middle for the waist, and more restraints at the top for the neck and wrists. The tail restraint can be adjusted so that the Mer's tail is stretched as far as it can go, minimizing its flexibility and thus preventing any attempts by the Mer to slap the de-scalers away with its tail.

Under this system, the Mer would be strapped to the rack after being winched up, given a dowel to bite down on, and then the de-scaling begins. Those who struggle too much would be beaten. Once the procedure is done, the Mer is given extra food if it didn't struggle, to help discourage excessive struggling.

Shortage of security

So far as I have figured, my scalefarm at the start is able to be manned by two - the master and and apprentice. As the number of mers increases to thirty, a second apprentice is taken on. The apprentices sleep in a bunkhouse adjoining the master's quarters and office.

2, maybe 3 apprentices is not enough. As mentioned by @legio1, Mer-leaders will be under constant pressure to do something about these horrifically inhumane torture-plexes. And that means that these farms are going to be at constant risk of attack.

As a result of this, heavier security is necessary. I think that a guard will be needed for every 3 or 2 Mers in captivity, in addition to the Master and Apprentices.

SCANDAL!!!

This is in the broader context of the human nation and fractured merfolk city states being at war with each other for the past 80 years or so. As the only exposure most humans have to merfolk is seeing them sedated in farms or attacking human shipping, they are viewed mostly with contempt.

This is by far the biggest problem. These things are sentient, and the populace knows it. And the populace is not going to be happy with having Mers who've done nothing wrong being tortured in the name of profit; that's a PR catastrophe of career-annihilating proportions just waiting to happen.

Fortunately, all you need to do to make this work is convince the population that they have done something wrong, and that's as simple as saying that this is a penal camp for enemy combatants. Of course, these Mers could just be innocents kidnapped off the ocean floor, but that could be hard to prove as the equipment and such you could use to identify them as such is removed and disposed of. The only people with any evidence that their innocent are other Mers, and they can be easily dismissed as charlatans trying to ensure war criminals get away scot-free. And human sympathizers who object could be dismissed as 'soft on war crimes'.

Answer 2

you need more security

I think there are several security issues that need to be patched.

water exchange sluices

As described, the water exchange sluices are a major security risk. Mer-leaders will be under constant pressure to do something about these torture factories. If the leaders of New Mer City can destroy one of these “farms” and free the captives, it would significantly boost their standing with the neighboring city-states of Mer Town and Mer-opolis. That could gain them some major allies in their war against the Mer Commonwealth.

Given that incentive, the sluices are going to be under constant attack from the outside.

Similarly, there are sapient people held captive and routinely tortured. They are going to be looking for weak points and those sluices are the path to freedom.

Defenses need a major upgrade. But that’s not going to be easy. Make the channels deep enough for quick, efficient water exchange and the water-breathing mers will operate with relative impunity to undermine the system from the bottom. Make it shallow enough for the air-breathing workers to deal with the mers and there won’t be efficient water exchange which will keep the gate open for hours every day. The only solution is going to be extra people to work security.

sapient captives

The means of getting the mers into the shaving rig is also lacking. The current plan is to lure them over with a release of food. Will that really work with a sapient species that knows what going over there means?

Supposedly captivity has led to a “numbing effect” but they can still function. If they can function, they can (and at least some will) resist.

anatomy

Finally, the way mers are secured for scaling is insufficient. The plan is to secure them with bindings at the wrist and tail. That’s not enough. Anyone who’s ever handled a live fish in a situation that it doesn’t like (e.g., out of the water) will have experienced just how flexible their spine is and how powerfully they can move their body. The same will apply to the mers only worse because they are big and they are sapient.

A full-grown mer that is angry and afraid will fight for its life. A single hard slap from that tail is likely to severely injure or even kill whatever sucker is tasked with tying it down.

scale

You specifically ask about scaling up and scaling down. Both present problems.

Scaling down is hard because this needs to make economic sense. Even a small farm will need several people at least to make this work safely. Is the price of scales enough to support five or six people with only 10 mers?

Scaling up presents an incentive problem. Obviously, a large operation can afford better security. But a large operation also makes a tempting target. Returning to the leaders of New Mer City, if they have to choose between hitting a little mom & pop torture farm to free a dozen or so mers or taking down a massive factory and freeing hundreds or even thousands, they’re going to go big. The pressure on facility security will be immense, probably on the scale of a small war. Seriously, how valuable are these scales again?

Answer 3

Plantation Slavery:

In the realm of brutality, there is no discrimination. Your enslavement should be a system of capture, enslavement and oppression that brutalizes the mer-folk enslaved but also damns the mer-folk themselves for involvement in exploiting their own people. Look to how humans have enslaved people over the ages, and you'll be halfway there. Your timeline corresponds well to the period of the slave trade on Earth. Exploiting the pain, pleasure, greed, hate and envy of mer-folk can do your work for you.

• Mer-slavers: Your mer-folk are not innocent, and do not hold all other mer-folk warmly in their hearts while singing kumbaya. War in Africa fueled the slave trade, and the slave trade fueled war in Africa. Mer-tribes capture and sell rival mer-folk into slavery. Preferably the slavers would be the mer-folk closest to the coasts where the slaves are kept. Escaped mer-folk are then captured by mer-slavers and sold back into slavery.
• Mer-hunters: Keeping an elephant to get ivory is expensive, but simply killing the elephant(or hacking off it's tusks in the wild) is easy. You don't have to enslave your mer-folk if they war with each other. The mer-folk know they can brutalize rival populations and raid into enemy territory, then strip enemies of scales and sell the scales to humans for weapons, drugs, or whatever trade items are valuable.
• Mer-plantations: Your mer-slavers may even run the system, either by running the plantations off-shore or working as overseers. Like the drug trade, simply by offering a good price for scales, the humans can wash their hands of the brutality. Or perhaps selected enslaved mermen are given the chance to NOT be harvested, be in charge of their fellow mer-folk, and have exclusive rights to mating the females. In any of these cases, the hard work is performed by willing partners to the slavery.
• Opium: Your slavers give their slaves opium before harvesting to reduce the pain. But as the slaves grow addicted to it, they look forward to the harvest to get their next fix. Eventually, they become willing participants in the system so they can get their next hit of opium (or heroin, depending on where your drug making is at).

All these ideas can be combined with each other and integrate into a sort of slave industry. A mere 30 slaves is a paltry number, and the same system may utilize mer-slaves as labor, with scale-harvesting used as punishment (akin to whippings).