I haven't had a chance to write up my chaburos from the last few weeks. I spoke about some "controversial" topics, one recent anmd one from a few years ago. Parshas Shimini I spoke about the shailah surrounding the worms in the fish and Parshas Tazria Metzora I spoke about metzitzah b'peh.
I really didn't have anything to add to the question. I just presented the shailah and explained what the machlokes is about. Most of my information came from a friend of mine who works in kashrus who sent me several articles and teshuvos on the topic. Also, the OU has a write-up of their position (l'kula) on their website and last week's HaModia also had an article on the chumra position.
In summary, the gemara in Chullin 67B says bugs/parasites/worms found in the inside of the fish (the "viscera" i.e. stomach section) is assur and those found between the flesh and skin or in teh flesh are muttar. The gemara says the reason is cause they grew from within the fish. This is also how we pasken.
The problem is that we now know that all parasites come from the outside. It is the proverbial cycle of life. These parasites get eaten by krill which then get eaten by fish that we eat (like salmon) then these fish get eaten by seals and dolphins. The parasites lay their eggs in the seals/dolphins and the eggs get expelled with the watse and they hatch in the water and the cycle starts all over again. The question is, if we know that the worms come from the outside of the fish, maybe they should be assur.
The question was raised 25-30 years ago and the poskim of the time among them Rav Moshe Feinstein zt"l , Rav Shlomo Zalman Aurbach zt"l held if the worms are found in the flesh they are not assur and fish did not require bedikah. (Rav Elyashiv was also matir but I understand it was for different reasons then Rav Moshe and Rav Shlomo Zalmen. I am also not clear on where they disagreed). In order to reconcile Chazal with the science on eof two approaches were taken. 1) Either we ignore science and say Chazal were correct, therefore, if Chazal say bugs in flesh are muttar cause they grow there, then they are muttar. 2) Or a sevara was said that microscopic worms that can't be seen are muttar and since the worms were microscopic when they entered the fish, they never became assur. They only grew inside the fish. It should be pointed out that Rav Elyashiv doesn't hold of this sevara-he holds even if it originally was microscopic, but now you can see them, they would be assur. However, he still held the worms werer muttar 30 years ago.
Recently, the issue was raised again. What changed? The claim is that we can now see very clearly that one of the worms in the fish called Anasakis comes from the outside. The arguement is that the hetterim of 30 years ago only apply to parasites where we don't know for sure where they originally came from. Howeevr, if we know with 100% certainty they come from the outside, then even the poskim of yesteryear (i.e. Rav Moshe and Rav Shlomo Zalman) would have said they were assur. Furthermore, the sevara of microscopic worms doesn't apply since we can see these worms outside the fish. This is the opinion of the machmirim.
The meikilm hold that nothing has changed. The same parasite we have now is the same as 30 yeas ago and is the same as in teh times of Chazal. I spoke to someone who is familair with Rav Belsky's opinion and Rav Belsky is meikil. He holds that when Chazal told us worms found in the flesh are muttar, they are muttar. Chazal never distinguished where these worms originally came from, as long as they are found in the flesh it is muttar. He further feels that when Rav Moshe was aksed this shailah, he didn't even understand why it was a question. It is a davar pashut it is muttar and there is nothing to talk about.
That's a brief synopsis of the issue. My understanding is that the major national kashrus organizations are being meikil although there are definately local va'ads that are being machmir. It should also be noted that the HaModia had a list of fish (besides wild salmon) which are a problem according to the machmirim. One of the reasons some fish (like farm raised salmon) don't have this problem is that these fish don't eat krill and therefore they don't have these parasites in their system. (There could also be othe reasons but this is one of them).
As always ask your Rav.