Oxford lists three primary meanings for the word grow. The first two reflect what we often immediately think of when we think about growing:
grow (v.) to undergo natural development by increasing in size and changing physically
grow (v.) to become larger or greater over a period of time; to increase
It’s the third meaning, though, that provides the key to your apparent oxymoron:
grow (v.) to become, gradually or increasingly :
sharing our experiences we grew braver
So, if the number of fish are gradually becoming diminished, then a headline might say:
Tasty Fish Grow Smaller in Warming Ocean
More likely, though, is probably referring to the size of the fish, if the gradually-warming ocean is causing the fish to become smaller over time (or, perhaps more accurately, "causing the average size of adult fish to become smaller over time").