Sentence:  Many years the mayfly <WSD>hatch</WSD> begins by the time the lake opens in April. Otherwise, expect strong <WSD>hatches</WSD> by mid-May. The <WSD>hatches</WSD> continue through midsummer. 
Meaning of the word marked with <WSD> tags:  The phenomenon, lasting 1–2 days, of large clouds of mayflies appearing in one location to mate, having reached maturity.