Possibility of actually freezing the layers. This can be done by editing the last layer in the *.cfg file and adding the following parameter l.stopbackward = option_find_int_quiet(options, "stopbackward", 0);.
Improving YOLO for close together objects
- Increase resolution of network: https://github.com/AlexeyAB/darknet/blob/b585124e55fcd18a7511d3b2fdc6bc3db5368ef4/cfg/yolo-voc.2.0.cfg#L4
- And increase nms: https://github.com/AlexeyAB/darknet/blob/b585124e55fcd18a7511d3b2fdc6bc3db5368ef4/src/detector.c#L459
- Also if your images and objects on images both are big, then you will should add additional convolutional layers: https://github.com/AlexeyAB/darknet/blob/b585124e55fcd18a7511d3b2fdc6bc3db5368ef4/src/box.c#L279