It's highly unlikely. Just do the math. If he has fished the rivers for 30 years straight without a break which is also unlikely he would have had to land 500 salmon every single season. If each season contains approximately 60 fishable days and he has fished non stop every fishable day, which I also doubt, he would have had to land over 8 salmon every single day he has fished for 30 years.
I fished Iceland's Midfjardara, which is one of Icelands best rivers. We fished it on its best weeks ever as per the camp management and the famous Ernest Schweibert who also fished that week. The anglers leaving the camp upon our arrival had very few fish due to a severe draught and then terrible rainstorms and wind for the last two days. When we arrived, due to the high water, the entire run of salmon which were holding in the Fiord for a month rushed into the river and filled the pools with fresh fish. I was one of the high hookers and took 32 salmon for the week which averages around 7 per day. You would have to do this and more for every year for thirty years and every day. Not possible.
I assume he has caught many more salmon than most people including myself but 15,000 is perhaps a big of a stretch.
I am at my camp on the Matane river and have been here since mid June. We have had a fabulous run of fish this year and it is my best year ever with 9 fish landed and 12 lost. Even though I am very pleased with these results I am no where near achieving my 15000th fish.