This is a place name and it uses the kun'yomi. You haven't learned this particular kun'yomi for 代 yet, so here's a mnemonic to help you remember it:
When you go to Tashirojima, make sure you visit the old woman. You know what woman I'm talking about, the one who starred on Murder she wrote (しろ). She writes a lot about cats now, I guess.
The population of Tashirojima is less than one hundred.
There are no gas stations on Tashirojima.
The most famous "cat island" is the island Tashirojima in Japan's Miyagi prefecture.