Basically, Pufferfish's hitbox is broken. They always have the "inflated" hitbox, This means that you cannot use them as a player detector, as the tripwire is always Activated.

How to reproduce, 
Put a Pufferfish in a 1x1 hole

Put a tripwire over it

Place redstone dust next to the block the tripwire hook is attached to

redstone is powered showing tripwire is triggered by pufferfish.