It looks like orders through EDGX and EDGA exchanges together account for a good chunk of the daily traded volume in NQ.
Since both EDGA and EDGX release both daily short numbers and daily traded volume on their website, they represent the most accurate proxy to total shares sold short each day as a percentage of traded volume.
For instance today (2/19/2014) total NQ traded volume was 2,081,208
EDGX short volume was 241,561 of 424,598 shares traded and
EDGA short volume was 22,637 of 38,437 shares traded.
So approximately 22% of NQ shares traded went through EDGA & EDGX and of those shares approximately 57% were sold short (which I would infer is representative of total shares sold short today).
Obviously 57% daily short is a massive number, but seems to be inline with what we're seeing in terms of the price action i.e. a massive effort by shorts to contain the share price below the 18.50 levels.
If this carries on, I suspect we will see an even bigger increase in the official short numbers when the next NASDAQ report for mid-Feb comes out.

