Technically, rheme (or comment) is the new information you add to already established theme (or topic).
You can't tell what is new and what is not from grammar alone.
This sentence, being a news headline, is supposed to deliver some news, tell us some facts we didn't know yet.
It's a well known fact that there is a city called Chelyabinsk in Russia. No surprise it, being a Russian city, harbors a few puddles. It's mildly interesting that someone would want to use a net to catch something in a Chelyabinsk puddle. But the real punchline (and the news) is this something being no less than a beaver.
As Nikolay Ershov mentioned above, normally the comment goes last in a Russian sentence. So, being properly constructed, the sentence would read
В Челябинске из огромной лужи сачком выловили бобра
But again, the functional division is a pure semantic thing.
Imagine this fictional series of headlines:
В Московском зоопарке родился шестиногий бобёр.
Шестиногий бобёр любит купаться в лужах.
Для ухода за бобром из Московского зоопарка был разработан специальный сачок.
Шестиногий бобёр сбежал из Московского зоопарка.
Третий день ведутся поиски бобра.
Бобра выловили сачком из огромной лужи в Челябинске.
In this example, Chelyabinsk would have been the comment, as the fictional beaver and his habits are known to us already, and the news is that he made all the way to Urals from Moscow.