This seems to have solved the puzzle! Excerpt taken from CIC See Temporary Foreign Workers Guideline page 104 (Version 2012.01.11).
Occupational code
Given the absence of an occupational code in the NOC for post-doctoral fellows, please use the applicant‘s specialty. For example, an applicant in Earth Sciences could be coded 2113 as a geologist or 2115.2 as a soil scientist.
I have pasted a link to the document below.
http://www.cic.gc.ca/english/resources/manuals/fw/fw01-eng.pdf