Usually the Thermocline will drop straight or nearly straight down for 10s if not 100s of meters. It is saying that temperature is more or less steady through this range of depths. Then it will break sharply to your left and in doing so identify a layer in the sub-surface where temperature drops fast. The area above this shelf is called the Mixed Layer as water in this region easily mixes because of its similar physical properties.
But the sharp break in temperature creates something of a wall between these warmer, mixed layers above and colder stratified layers below. It’s a hugely important control zone for thermometric species like tuna and billfish. These species can then move between the zone below it preferred by their physical processes and the zone above it preferred by their prey. Knowing where the mixed layer lies and how the preferred temperatures of these species relate to that zone can provide another factor contributing to fishing success.
