Thank you CMYK.
You see, this fish is not any fish but a very special one: a merludine. Unfortunately, you can’t see the part of the body that is hidden in the water. He is able to bend his abdomen swiftly when he sees a prey. Sometimes his back shows a deep curvature which doesn’t last long but is sometimes enough to cause him severe back pain. This is the reason why merludines often end their lives floating beneath the surface, unable to move enough to sustain their existence.
I have made a rough SBS to explain you how this nature’s oddity works.

Thanks Filantrop.

Thanks Skyangel