Global warming looks like it will be a bigger problem for the world's fish species than scientists first thought: A new study shows that when fish are spawning or are embryos they are more vulnerable to hotter water.

With medium-level human-caused climate change expected by the end of the century, the world's oceans, rivers and lakes will be too hot for about 40% of the world's fish species in the spawning or embryonic life stages, according to a study in Thursday's journal Science. That means they could go extinct or be forced to change how and where they live and reproduce.

Until now, biolog...