Is there a sentence fragment?
When it comes to seafood, Americans aren't always eating what they think they are. According to an FDA report based on DNA analysis of fish that are frequently misidentified.

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 - yes
There is a sentence fragment that does not express a complete thought.
When it comes to seafood, Americans aren't always eating what they think they are. According to an FDA report based on DNA analysis of fish that are frequently misidentified.
Here is one way to fix the sentence fragment:
When it comes to seafood, Americans aren't always eating what they think they are. According to an FDA report based on DNA analysis, an average of fifteen percent of fish are misidentified.