Circle Detection/broken circles in Image

[mehtabpathan]

I am a newbie to ImageJ or any image processing software for that matter. I need to charaterize the diameters and volume fraction for the fibres as given in the image. Since the colour intensity within the fibres and background is the same, I cannot use thresholding for segmentation. I tried to detect edges and count the circles, but that isn't a success so far. I tried to convert to binary and fill circles but that is also unsuccessful. Could use some advice or alternate techniques for this problem.