Complex sentence: It might seem that prominent stripes on the back of mackerels provide camouflage against broken backgrounds, but that is not the case because mackerel live in midwater pelagic environments which have no background.
Split into simpler sentences: It might seem that the prominent stripes on the back of mackerels are there to provide camouflage against broken backgrounds. However, that is not the case because mackerel live in midwater pelagic environments which have no background.