In the name of Allah, the most Beneficent, the most Merciful.
A fish that dies naturally without an external cause and begins to float on the surface of the water is unlawful to consume.
Saaiduna Jabir Ibn Abdullah Radiallahu Anhu narrates from the Prophet of Sallallahu Alahi Wasalam, “What the sea throws up and is left by the tide you may eat, but what dies in the sea and floats you must not eat.” (Sunan Abi Dawud p.176 v.2)
The tuna which is captured is genuinely captured from below the sea and not from those which are floating on the sea, even though there maybe a possibility of it.
With regards to your question it will be permissible to eat the tuna fish. It is essential that we do not have baseless misgivings about anything without any concrete proof. This may lead to the illness of waswasa.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham