In a humid environment, with an abundance of fruit at certain times of the year, what is the best method for drying fruit slices for later use? I don't want to use any type of equipment that needs power/active heating (like an oven)... is it possible to simply slice the fruits and leave in the sun/air? 
Hard to answer concisely, because there are many variations in processing before the Sardine hits the canning retort (a sterilization pressure cooker). Recall that there are sardines "in oil" "in spring water," even in "hot sauce." Here is a video example.  Sardines are canned under both pressure and temperature in the canning retort. I believe the pressure factor has more impact in supressing what you call scum ** though temp prolly plays a factor as well.  Home brew example: I rarely find scum like formations in my pressure cooker. I do find congealed fat on cool down.. but congealed protein.. not so much.   ** Not sure if the scum you are thinking of is congealed fat, or protein. 