In a long-awaited report, a panel of independent experts convened by the British government said both those theories were unlikely, and that it had been "unable to identify a clear and convincing single cause" for the deaths.
"We've ruled things out, but we've not been able to confidently rule things in...," panel member Tammy Horton, a researcher at the National Oceanography Centre, told an online press briefing.
It was "about as likely as not" that a pathogen new to UK waters caused the deaths, it said, though no new pathogen was detected.
Several of the factors it considered might in combination also have been a cause, it added.
British Environment Minister Therese Coffey said on Friday she would consider if further analysis by government scientists might establish a conclusive cause.