Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means decrease so the verb vocab version is to decrease. You can differentiate this from 減る because with this one, you are the one that is going to be doing the decreasing. The rush (らす) of decreasing something fills your veins.
The reading is the same as 減る, so if you know that reading you should know this one.
The plague reduced the number of blowfish to half its previous level.