For this purpose, no. But otherwise yes, they are both used with other different meanings. In fact, this is the only case where they mean the same.
Ca can also be used as then, as you can see in my other comment. It can also be used to mean like: el este ca mine (he is like me), animale sălbatice ca urșii (wild animals like bears), rapid ca pantera (fast like the panther). It can also be used to mean as a: ca mamă... (as a mother...). And a few similar other scenarios among some expressions.
Decât has another meaning similar to doar or numai, that is to mean only, but must be used in a negation. So, while you would use doar like this: am doar un măr (I have only one apple), you would use decât as follows: nu am decât un măr (same meaning).
I beg to disagree. According to what I have heard, learnt and read, "ca" is used to make a comparison of equality, and "decât", of inequality.
Examples: Eu sunt mai înaltă decât ea. Era înaltă ca mine.
"I am taller than her. She is as tall as me"
As seen in the example above, the sentence "El are mai puține rațe ca bărbatul" fits the logic of the first example where the word "decât" is the one used.