The margin of error comes from the small size of the sample. They only tested 22 fish from random locations. The margin of error is therefore large. If they had collected and tested more samples, the margin of error would be smaller.
For an excellent overview of basic statistical sampling, look here:
http://www.custominsight.com/articles/random-sampling.asp This is good general knowledge for understanding polls, fish mislabeling, the number of green m&m's in a package, etc, etc, etc.