OG-53

I was confused by OG 53,  can someone can explain the hypothesis "sharply rising water temperatures must be involved in ...."?

Since the paragraph just mentioned the change of RANGE of temperatue, reducing from 50 to 6, never refer the temperature is up or down.

Why OG said that "for the hypothesis to be tenable it is important that the fish in stream in the River area that retain a wide temp difference have not lose their ability to reproduce "

Thanks in advance.