How would an underwater race control the breeding for planktonic larvae species?

Question

In order to domesticate wild animals, you must necessarily complete a number of criteria (docility, can't be picky eaters, conforms to a social hierarchy, etc), but above all the most important criteria is the reproductive control step which separates taming from domestication. Most domestication candidates, like fish, crustaceans, mollusks, sessile creatures, etc, have tiny planktonic larvae that are mobile, freely swimming in the water column. Even demersal eggs where the parents watch over the eggs are, once hatched, tiny pelagic independent plankton that disperse away. That means that, once hatched, the larvae are extremely hard to see (or sense) and are difficult to catch, and it also means controlling wanted & unwanted traits is also difficult which makes true domestication impossible since they'll always be wild.

Quite aside from the difficulty of caging fish, broadcast spawning is easy as heck at sea (which applies as much to agriculture as pastoralism). How would a sapient aquatic race control the reproduction of a tamed wild animal that has planktonic pelagic larvae?

Assume neolithic era technology for these reef dwelling sapients. They use long strands of seaweed for rope, and corals (instead of stone) for building material and making cutting tools & spears. The rope could be used to haul larger quantities of stuff than one could carry, similar to how we use the wheel to push stuff around. It could also be further weaved into baskets and nets, and other similar items.

They use animal bladders filled with low density oil or gas for balloons to lift heavy objects. Oil is very common in underwater creatures and is incredibly easy to harvest in a primitive way (leave dead fish in container, oil is squeezed out & automatically floats upwards), parallel to paleolithic humans creating alcohol very primitively.

Answer 1

If you have a shore, you can create an isolated region of ocean. Chase some fish into a narrow bay, then dig a berm all the way up to the high tide to lock them into the bay. At high tide you can jump through the air over the berm and do whatever you need to do in your captive fish population. It would probably take an unreasonable amount of work to keep the berm from deteriorating, though.

For what it's worth: there's absolutely nothing wrong with having fish playing coral saxophones while a crab sings and plays clams like steel drums - or any other technological impossibilities required to make life better down where it's wetter, under the sea.

However, any vaguely realistic underwater civilization without a dry-land manufacturing base has zero access to glue, wicker, clay, fire, rope, thread, leather, oil, lime, salt, woodworking, and felt. I'm pretty sure that rules out every technology that isn't a particular shape of rock or earth.

Answer 2

Heat

Sea life often resides in pretty uniform temperature. Where the temperature on land varies from freezing to scorching, the sea is relatively unaffected. That makes the sea life often grow in specific zones for light and heat. This is one reason why people are afraid of climate change. On land we are quite adept at losing heat. But less than a degree difference in some deeper layers of the ocean can put the whole ecosystem out of whack.

We might use this to our advantage. If we can isolate some part of the ocean, for example via currents around islands, we can possibly affect the temperature at some of these places. It isn't easy, but humans have done weirder things for less.

Let's say you want to reduce their spawning. We put something on the surface of the icean, or one of the deeper layers, blocking the sun. Though minute on many levels, this small difference can change the reproductive rate. Want more? Remove the sunblockers.

As an alternative you might be able to somewhat redirect the current to lower or higher layers using the terrain, getting the same differences in temperature.

The idea is fraught with problems. New currents can bring new heat, or the amount if effort is monumental. I would argue that this arm of currents rarely refreshes, or actually has no current at all, keeping the plankton in the area. Secondly you have the power of suspension of disbelief. Too few people have enough knowledge to say it's bonkers. If you explain it is this way, it is this way. No one questions how wildlife or plantlife in A Song of Ice and Fire (Game of Thrones) survive years of winter, while humans themselves barely survive with stocked storages. Because it isn't mentioned, people do not even question it.

Answer 3

Basically, the current is your farm, downstream is the production spot and the harvest spot. Upstream is the seeding and before the seeding is the breeding.

And that is done, by having filter feeders bred to go for unwanted traits. Like a whale that drifts through the current, eating the initial stages and excreting or rejecting the wanted end-product, that then multiplies using the diggested unwanted byproduct and other elements dug up by some worms at the ocean floor in huge dustswirls rich in iron and other base elements.

Of course - there are parasites, and the occasional wolf of the seas going for the breeding filter feeders.

Answer 4

As you specified "conforms to a social hierarchy" is one of the criteria for domesticatable species. For aquatic species this probably must be further restricted to "forms schools" with "reproduces in a fixed area annually" added in order to keep them manageable without implausibly huge nets enclosing vast oceanic volumes.

Once the sapient seafolk are dealing with schools of fish, things get simpler. Drive off (or kill and eat) all undomesticated schools coming anywhere near your breeding areas. Cull the domesticated schools of individuals with undesirable traits before they reach reproductive age, so that only desirable individuals survive to breed.

The method is imperfect, as larvae and eggs may drift in from uncontrolled areas (actually volumes), but the majority of reproduction in the controlled area will be from unculled domesticated individuals. Repeat over many generations while maintaining control over the largest possible area, directly or in cooperation with neighbouring polities. Assuming that hardiness is one of the traits being bred for, eventually the domesticated species should become dominant. Culling will still be required to eliminate interlopers but it will be required in any case to eliminate sports with undesirable traits.