ISAFJORDUR, Iceland — Before it became a “Game of Thrones” location, before Justin Bieber stalked the trails of Fjadrargljufur, and before hordes of tourists descended upon this small island nation, there were the fish.

“Fish,” said Gisli Palsson, a professor of anthropology at the University of Iceland, “made us rich .” The money Iceland earned from commercial fishing helped the island, which is about the size of Kentucky, become independent from Denmark in 1944.

But warming waters associated with climate change are causing some fish to seek cooler waters elsewhere, beyond the reach of Icelandic fishermen. Ocean temperatures around Iceland have increased between 1.8 and 3.6 degrees Fahrenheit over the past 20 years. For the past two seasons, Icelanders have not been able to harvest capelin, a type of smelt, as their numbers plummeted. The warmer waters mean that as some fish leave, causing financial disruption, other fish species arrive, triggering geopolitical conflicts.

Worldwide , research shows the oceans are simmering. Since the middle of last century, the oceans have absorbed more than 90 percent of the excess heat trapped by greenhouse gas emissions. To beat the heat, fish are moving toward cooler waters nearer the planet’s two poles.