That | means | a higher proportion of what is in the sea is being caught.这个是主语+谓语+宾语从句
A higher proportion of what is in the sea | is being caught.这个是主语+谓语,整体作为从句充当原句子的宾语.
a higher proportion of | what is in the sea 后者又是一个从句
what is in the sea 这里what就是主语