...A witness, expert or otherwise, should not be involved in giving his opinions on the matter or involved in internet forums about the matter for years before a murder trial. Stopping posting when asked to be a witness or during the trial shouldn't be sufficient. Shouldn't McFadyen recuse himself as an expert witness?
I see no reason that an expert witness should not peruse internet forums. I would be concerned with if he didn't. Because of his interest in diving, I would expect McFadyen to follow this forum just like so many other divers. If nothing else, I would expect that what we have said here and the analysis we have provided might help in solidifying an expert's thoughts or at least providing an analytical framework for an expert to use.
I see no reason why someone should withhold their comments because they may be engaged as an expert or have been engaged as an expert, EXCEPT to the extent that such comments MIGHT provide information with which he or she might later be attacked, e.g. inconsistent statements or confirmation bias..
Do keep in mind that experts are engaged by the prosecution or the defense and as such there will always be some degree of bias. It is not like the Court is engaging an unbiased expert to assist it.
An expert is always open to charges of bias and a good cross-examination is the best means of showing that. It is then up to the jury to decide if the degree of bias renders the expert's opinions unworthy of belief.
For example:
"Isn't it true that you are being paid to come here and give your opinion for the prosecution/defense?"
"Isn't it true that if your opinion was not favorable to the prosecution/defense, you would not be here?
"Isn't it true that if your opinion was not favorable to the prosecution/defense, you would not be getting paid to be here?"
"Isn't it true that in forming your opinions, you understood that if they weren't favorable to the prosecution/defense, the prosecution/defense would find someone else whose opinions were favorable?"
I understand that McFadyen is a rescue diver and has assisted in investigating two out of air/low on air accidents at his club. Beyond the number of dives, I see nothing that qualifies him as an expert witness and he has already shown his bias. I have been assisting in investigating fatalities for the last few years in our province and have access to non-public information, but I would not be considered an expert witness, and the only difference is the number of dives.
An expert is anyone with knowledge, experience or training beyond that of an ordinary layperson who can use that knowledge, experience or training to form an opinion that would help the jury in its quest to find the truth. As noted above, nothing requires an expert to be impartial or unbiased. The question is whether it makes the expert's opinion unworthy of being believed. That turns in large part on whether there is an adequate factual foundation for the expert's opinions and whether the expert has the requisite knowledge, experience or training to form a valid opinion based on that factual foundation.
IMHO, there are any number of people in this forum who would qualify as an expert and whose opinions would be admissible in court.