Further to what Ken has already mentioned, you can purchase the fishing license from the local chai wala in Dadu Pur. You can also take the old road from Paonta Saheb to Masuri via. Dak Patthar, katapatthar towards kampti Fall and as you come to Yamuna bridge, go down and as you go down strem of the river, you will find a big rock jutting out in the river. Try casting either side of the rock and you may hit a big one. Do take care while fishing that spot as it is very treacherous and the water from where you will be spinning is very deep. I do not think, this is the best time to go fishing there. In case you can not do any fishing any where, you can always try bait fishing on the right side of Gurudwara steps.
Best of luck.