The word shoal has two meanings. The first one is a shallow bit of water that is near an ocean. Typically, this is where the beach would be exposed when there is low tide, but the sand would be under the water at high tide.
Another meaning for shoal, which should be the one you are referring to, is a large group of fish. Long story short, when you are comparing herd and shoal, you can choose shoal when you are talking about fish!
In this meaning, the word shoal functions as a collective noun for fish. When you are talking about fish in general or a specific species of the animal, such as bass, minnows, herring, pilchards, salmon, or even larger species like sharks, you can call them a shoal.
- The fishermen were lucky to see a shoal of fish right beneath their boat!
- The shoal of piranhas work together to bring down their prey, and all the fish swim around their food until they thoroughly confuse it.
For a herd, however, there is an entirely different set of animals that this collective noun is useful for. If these animals are all together, they are in a herd: Antelope, bison, cattle, cows, crane, deer, donkeys, elephants, goats, hogs, horses, kangaroos, oxen, pigs, rabbits, seals, sheep, swan, walruses, wasps, whales, zebras.
When we use herd as a noun, we use it to refer to a group of animals that are kept together or live together. For example, a sheep farmer raises and breeds domestic sheep so he/she can sell the sheep’s wool and meat. The farmer looks after the herd of sheep and protects the herd from other animals.
A herd is used to refer to a number of animals living together but on the other hand, a shoal refers to a group of fish that swim together. Fishermen will look for shoals of different types of fish to catch when fishing such as a shoal of salmon or a shoal of bream or a shoal of halibut.
Here are some examples of the two nouns being used:
- The herd of elephants moves toward the watering hole to hydrate themselves and take a relaxing bath.
- The ocean is full marine creatures such as whales, sharks, turtles, and shoals of tropical fish.
Adding a correction to the above response:
Crane, swans and wasps definitely do not take the collective noun herd. In this case we would use the nouns flock (for cranes and swans) and swarm (for wasps).
