Sea Chicken (シーチキン) is the can food of tuna (鮪/ Maguro). The reason it’s called Sea Chicken is that it tastes like the chicken’s breast meat (鶏のささ身/ Tori no Sasami). It is also a trade mark of Hagoromo Foods Group since 1958. The related products can be seen on Japanese rice balls (おにぎり/ Onigiri) to indicate the main flavor / ingredient.
So next time when you are traveling in Japan and see the word, (シーチキン), you know what’s in there.
P.S. Official website: http://www.hagoromofoods.co.jp/