Short-lasting schools of anchovy, herring and hake after ocean thaws etc. : fish mechanics, speed?

Solution 1:

Anchovies, herrings, hake, and seahorses are all vermin, which means that they don't really travel and just sort of spawn where the game thinks they ought to live. So they didn't escape the map agains the drain effect, they just vanished back into the ether that formed them. No amount of draining would trap these tiny vermin in place. True creatures like carp or zombie giant whales would be susceptible to being pushed around by water; however, they would also need to migrate in from the edges of the map after the ocean thawed, rather than simply being spawned into existence.

As for school size, the massive schools description of herring is just flavor text; it has no effect on actual gameplay. The actual amount that spawn is controlled by the [CLUSTER_NUMBER] tag in the creature (or vermin) definition. Both anchovies and herrings have [CLUSTER_NUMBER:50:100] so they will spawn in similar-sized schools. Hake have [CLUSTER_NUMBER:25:50] so they will tend to spawn in smaller groups. It looks like Toady forgot to include a [CLUSTER_NUMBER] tag on the seahorse, so it gets the default of [CLUSTER_NUMBER:1:1], and spawns all by itself.

Why you suddenly saw a big school of each of these animals: probably because the ocean thawed all at once and the game said to itself "oh man, look at all this unpopulated water! I better get some vermin in there, quick!" I admit I'm guessing on this point.