On the Target Species Edit screen you can also set the Fishing Depth for your vessel or gear. Even though the thermocline graph may show a lot of water at the preferred temperatures for your target at a location, what if the temperatures are too deep for you to use? The white line you see cutting across the Thermocline graph is the Maximum Fishing Depth for your boat or gear.
Adjust the depth of this line by clicking on the Edit pen and sliding the Fishing Depth slider left or right.
