From OU WEBSITE:
http://www.ouradio.org/index.php/ouradio/comment/9742/
Also see below:
OU Policy on Worms in Fish – 29 Adar 5770 – March 15, 2010
By Rabbi Chaim Goldberg
Recently there has been considerable discussion about worms in fish. Some Rabbonim understand the halacha (as formulated in Shulchan Aruch) to be that worms which hatch in the flesh are permitted, while worms that are swallowed whole and enter the viscera are prohibited.
Both of the worms migrate to the flesh. Based on this understanding these Rabbonim have stated that today, worms in fish are prohibited. This is because scientists maintain that contemporary worms known as Anisakis enter the flesh through the viscera and then migrate to the flesh. This is not a new issue and the question has been raised repeatedly in recent years. The OU has reviewed this matter and found it not to be an issue.
[Chulin 67B]
Rav Belsky further feels that it is irrelevant whether the tolaas entered from the viscera or from some other way, whether it happened when the fish was alive or after it died. As proof, he notes that S.A. (ibid) says that tolayim which come after the death of the fish are permitted. Rav Belsky felt these tolayim must have come from the viscera, because there was no other reasonable source for tolayim entering a fish after death and yet they are permitted.
Some are concerned that the tolayim found in the flesh are actually the forbidden tolayim originally found in the viscera (Shulchan Aruch forbids the tolayim found in the viscera). Rav Belsky felt this claim is not based on any significant research. Rav Belsky felt that his own inquiries from qualified experts indicate that the opposite is true, and that the tolayim in question are found in the flesh while it was alive. Furthermore, Rav Belsky feels even tolayim entering the flesh from the viscera would be permitted as per above.
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