[R] substract 2 data.frames

[Nicolas Gutierrez]

Hi!

I have 2 data.frames: "fish" and "popn":

 >fish

    xloc yloc id birth  size weight  energy gonad
20   15   15 54   -60 107.9   63.0 15952.9   8.0
21   15   15 32   -60 105.1   61.4 15538.8   7.8
91    4   43 96   -60 118.9   69.4 17573.2   8.8
71   32    4 64   -60 121.6   71.0 17976.0   9.0
34    2   64 20   -60 116.2   67.9 17173.0   8.6
95    6   20 58   -60 106.5   62.2 15738.3   7.9
97    7   81 89   -60 104.5   61.1 15449.6   7.7
94    5   30 74   -60 103.0   60.1 15217.1   7.6
45   26   28 92   -60  98.6   57.6 14566.2   7.3
59   29   37 42   -60  98.1   57.3 14492.0   7.2

 >popn

    xloc yloc id birth  size weight  energy gonad
20   15   15 54   -60 107.9   63.0 15952.9   8.0    #delete
21   15   15 32   -60 105.1   61.4 15538.8   7.8    #delete
90    4   43 28   -60  74.6   43.6 11027.3   5.5
91    4   43 96   -60 118.9   69.4 17573.2   8.8    #delete
71   32    4 64   -60 121.6   71.0 17976.0   9.0    #delete
34    2   64 20   -60 116.2   67.9 17173.0   8.6    #delete
95    6   20 58   -60 106.5   62.2 15738.3   7.9    #delete
97    7   81 89   -60 104.5   61.1 15449.6   7.7    #delete
94    5   30 74   -60 103.0   60.1 15217.1   7.6    #delete
45   26   28 92   -60  98.6   57.6 14566.2   7.3    #delete
59   29   37 42   -60  98.1   57.3 14492.0   7.2    #delete
44   25   82 59   -60  97.6   57.0 14427.3   7.2
57   29   15 46   -60  96.4   56.3 14252.7   7.1
61    3   80 48   -60  96.4   56.3 14243.3   7.1
86   38   67 39   -60  91.3   53.3 13493.6   6.7

And I need to delete the columns in "fish" to the data.frame "popn" as 
follows:

 > new.popn

    xloc yloc id birth  size weight  energy gonad
90    4   43 28   -60  74.6   43.6 11027.3   5.5
44   25   82 59   -60  97.6   57.0 14427.3   7.2
57   29   15 46   -60  96.4   56.3 14252.7   7.1
61    3   80 48   -60  96.4   56.3 14243.3   7.1
86   38   67 39   -60  91.3   53.3 13493.6   6.7

And I need this within loops, so I can't do it "manually". Any guidance??

THANKS!

Nico