Next time, cut two pieces of mono off line about 30 inches long, and tie a simple overhand knot in one for the girth. Lay the unknotted line along the length and cut it off...from tail to closed mouth. Take the line with the single knot and measure around the bass at the widest area...and cut off excess. Wrap those two lines and put in tackle box or pocket till you get home. Use a tape measure to lay line along to get a good reading. http://legacy.myfwc.com/fish/
Put the length in the box and click on the purple box, and it will convert to M.M. Put the girth in box and click on the purple box and it will convert to M.M. There are 3 formulas used . Look at the last one, by biologist Hal Schramm...I have found to be the most accurate. My son caught one on Toledo Bend in 3-05-16, and I caught one also. We weighted them at Fin and Feather Resort. Any weighed over 10 lbs, and you get a free replica. My son's weighed 11.1 lbs...was 25.0 inches long and 18.5 inches in girth. My weighted 11.40 lbs.ad was 23.5 inches long and 20.0 inches in girth. My son's bass was empty, and had already dropped he eggs, probably should have pushed 13 lbs or better, being 25 inches long...but did not have the girth.
Anyway..a fisherman can always get a measurement just by using their line.