Ultraman: S1 E10 - The Mysterious Dinosaur Base Ultraman
The fish were breeding abnormally at Kitayama lake, and SSSP headed there to to find out the cause of it. Apparently, it seems those fish eat foods that huge creatures have left behind and increase the number. And it found out that the identity of the gigantic creature was a dinosaur Jirahs that professor Nikaido brought back from Loch Ness 15 years ago. Because an environment around Kitayama lake was very similar to Loch Ness, and Jirahs was growing steadily.