Detect circles in image
1) Normalize your image to range $[0,255]$.

2) Select a threshold and threshold the image. For your image, what worked is: $\tau=[140-150]$.

3) Compute a Euclidean distance transform.

4) Apply watersheds segmentation.

If I apply this procedure, here is what I get:


Not perfect, but maybe a good start. The result looks similar to performing a Voronoi diagram on a smoothed/noise filtered version of the image. 

Now to each region, one could fit a circle, using any algorithm (edge extraction and fitting or Hough transform etc.) For now, I used a simple circle fitting to the edges:



Again, not the best, but could be a sufficient starting point.

I also think that you would find the following useful:


  Segmentation Methods for Light Microscope Image
  
  Detecting Spheric Particles