The following ImageJ macro code makes use of the Maximum and Minimum filters in ImageJ to perform a morphological closing operation on the particles in your sample image, and it then uses the Particle Analyzer to create ROIs from those:
open("https://dl.dropboxusercontent.com/u/65873264/Sample.jpg");
run("Duplicate...", "title=[Temporary Copy]");
run("8-bit");
setAutoThreshold("Default");
run("Analyze Particles...", "size=100-Infinity show=Masks clear include in_situ");
run("Maximum...", "radius=70");
run("Minimum...", "radius=70");
run("Analyze Particles...", "size=100-Infinity clear add");
selectWindow("Sample.jpg");
roiManager("Show All with labels");
roiManager("Show All");