Fish have fins to protect them. Some fish are also poisonous.
Your question is very vague.
Are you asking how fish protect themselves? Or are you asking how humans protect and conserve fish?
Re-ask the question with clarification.
its protection is the slime on its body
Protection
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fish that give no protection to their young lay large numbers of eggs
for protection
Protection from predators.
Scales
For protection
possibly
-Remoras and sharks. The remora gets food and protection, the shark gets cleaned by the remoras. -Clown fish and anemones. The clown fish bring food to the anemone. The clown fish in turn get protection. They are not harmed by the stinging of the anemone.
Well , do you think if the fish has no head it can be complete ? Of course not , the function of head in fish it to give protection.
Protection,swim better, waterproof