Torpedo
Fish thunder is underwater thunder that shoots at things and are kind of shaped like fish. That's a torpedo. Load the fish thunder!
ぎょらい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
小包を開けると、中には自動追尾魚雷が入っていた。
When I opened the package there was a homing torpedo inside it.