Chum, the size of the stomach is characteristic of chums. They have a very acidic digestive system, the gelatinous pile of white stomach contents could be any thing from squid to jelly fish. It is all digested so fast that it is difficult to identify stomach contents unless they just ate something. They do eat jelly fish.

The best feature used to identify salmon, particularly immature, on the high seas is the tail. Each species is different. Chum have black between silver on the tail rays. You can see that in the first picture. and the picture below, a sport fishing gear (pink buzz bomb) caught chum on a Japanese research vessel in the Bering Sea. Hook and line is the preferred way to catch salmon that are going to be tagged and released. The research vessel crew all participate in catching salmon for tagging though most fish with a handline, a plastic bottle float, and red lures they tie themselves.