Salmon is generally considered by kashruth authorities to be distinctively identifiable as kosher from the appearance of the flesh. Nonetheless, I've noticed that the local Costco sells (side by side) certified kosher and uncertified, fresh, precut, packaged, farmed salmon. What, if any, are the halachic advantages of buying salmon certified as kosher, rather than cheaper and/or fresher uncertified fresh salmon, from Costco? Is there a halachic concern, even in the current allergen-conscious age, that a mass producer of salmon (such as the one supplying Costco) would be using equipment contaminated with nonkosher species such as shellfish?
R Sholem Fishbane, the Executive Director of the Associated Kashrus Organizations (AKO), an umbrella group of major kosher-certifying agencies and the and the Kashruth Administrator of the CRC kosher supervising agency spoke to the Headlines podcast about this.
At 1h43m here he explains salmon can be purchased without supervision. He says "at most, to be truly machmir you can do shifshuf (rubbing) of the place which was cut". He explains there is no need to remove a klipa of fish since there is no blia (absorption). He speaks of fresh salmon being cut in front of you with knives that were possibly used to cut non-kosher fish. Once you accept the ruling of the OU (which you quote in your question) that pink-looking fish is kosher, then I don't see a difference between freshly cut and packaged.
But don't take my word for it ...
OU Kosher writes
The factories which produce both wild and farmed salmon produce copious amount of fish every day, and even if there was a non-kosher product cut with the same knife, the non-kosher residue on the knife would be wiped off on the first few pieces of fish cut with that knife. Those fish would be mixed in with thousands (or tens of thousands) of other salmon, which would be perfectly clean of residue, and become nullified (batul). The Shulchan Aruch (YD 96:4) rules in the case of lemonade and salted fish that all of the product is permitted, despite the non-kosher knives involved. The Rama explains this is due to the large amount of product cut with the same knife.
The Sephardic Halacha Center writes
In the case of a large-scale factory that processes only kosher fish with skin (such as a facility that packages salmon), the Aharonim are lenient and do not require the utensils to be certified kosher, as the possibility that they were used for a non-kosher items is remote, and even in such a case the small amount of non-kosher residue would be nullified in the vast amount of kosher fish that are being processed.
What, if any, are the halachic advantages of buying salmon certified as kosher, rather than cheaper and/or fresher uncertified fresh salmon
There are only two simanim which the Torah endorses as being proper identifiers of kosher fish. Namely, fins and scales (סנפיר וקשקשת). And where proper scales are identified, fins are presumed (Niddah 51b). As a general rule, a fish fillet of unknown provenance which has had its skin with all of the identifying scales removed, is not treated as kosher. Without possession of the prescribed identifier (scales), its kosher status would not be ascertainable.
Interestingly, the Talmud (AZ 40a) does explore other characteristics by which a kosher species may be identified (shape of the head, bony skeleton, shape of the eggs), however the Shulhan Arukh omits any mention of these features following the Rambam who ruled according to the opinion in the Talmud that such secondary features are insufficient.
This leads to an interesting question however concerning species of fish such as salmon that are known to be kosher (i.e. possess fins and scales) and contain other additional identifiers that were not endorsed (pink/red flesh). That is to say, how is a fish fillet that does not possess the prescribed identifier (scales) but does possess other identifiers unique to the specific kosher species to be treated.
The Beth Yosef (YD 83:8) on a related topic introduces a new siman (identifier) for accepting the kashruth of roe. He states that red roe only come from kosher species, and therefore may be accepted as such, even in the absence of other identifiers. The Peri Hadash (sq 26) strongly disagrees with this, affirming that such a hazaqah (presumption) has no basis in Hazal and it may therefore not be relied upon.
Some Poseqim (R. Moshe Feinstein, and R. Yaaqov Kaminetsky) in reliance on this Beth Yosef have reasoned that where there is another identifier specific only to a kosher species, e.g. red flesh, then even where the scales have been removed from a fillet the color of the flesh alone may be relied upon to ascertain its kashruth.
This position however is a leniency and is not universally recognized. There are two primary grounds on which opposition has been expressed to reliance upon the pink/red coloration of salmon flesh as a siman.
A) It introduces new secondary simanim (identifiers) that were not anywhere endorsed by Hazal. Reliance upon secondary simanim is a bold move methodologically, one which the Peri Hadash took issue with in regards to the Beth Yosef’s position on red roe. If Hazal did not legislate any additional means by which to properly identify a kosher species of fish, what license have we to exceed the scope of that which they set down?
B) The particular secondary siman, of pink/red coloration is one that can be produced artificially in kosher fish. This has caused some to question its reliability as a siman. Those that rear farm raised salmon routinely introduce astaxanthin, an artificial carotene, to the feed in order so that the salmon flesh will produce a pink color. Only species that are capable of storing natural carotenes in their flesh are amenable to such artificial coloration. To date, non-kosher species of fish have not been shown to be amenable to this specific method of artificial coloration. Despite this, the sheer fact that the siman can be added is regarded by some as sufficient to be reluctant in utilizing this secondary siman. The fact that non-kosher species have not been shown to be amenable to such color change via this process (i.e. introducing artificial carotenes) does not mean that other processes utilizing non-carotenoid-based pigments cannot be shown to be effective in producing such color in the flesh of non-kosher species of fish (I am unaware of any research on this, presumably the market has deemed the former method sufficiently economically viable, particularly given that it is additionally integral to the health and vibrancy of the species). It is worth noting the testimony of Dayan Yonathan Binyomin Weiss (Chief Rabbi of Montreal) who wrote (as quoted in בית דוד - כשרות הדגים הקלופים here):
הוצגו לפנינו שלוש חתיכות דג אחד מהם היה סלמון טבעי השני סלמון שצבעו אותו בצבע ורוד והשלישי דג קאד שצבע בשרו המקורי הוא לבן לגמרי וצבעו אותו בצבע ורוד ובשום אופן לא היה אפשר להבדיל בין שלוש החתיכות
We were presented with three pieces of fish, one of which was a natural salmon, the second a salmon that had been dyed pink, and the third a cod whose original flesh color was completely white and they had dyed it pink, and it was in no way possible to differentiate between the three pieces
Though I do not know which species of “cod” (some of which are not kosher) R. Weiss referred to, again, the sheer fact that there can be a confusion between species based on added coloration is deemed by some as sufficient grounds to be hesitant.
Accordingly, when it comes to salmon, when there is a choice between a skinless fillet that has been supervised by an agency that does not rely on red flesh as a hazaqah (and thus certain to have had fins/scales) over a skinless fillet that has not been supervised (and thus no simanim recognized by Hazal), some may argue that the "halakhic advantage" goes to the former as it presents less halakhic impediments. Of course, it ought go without saying, that those that deem such reliance permissible (such as the OU in your link) have found these impediments to be sufficiently surmounted.
The two potential issues with uncertified fish are... the signs to identify kosher fish and the "knife" used to cut them up
- The signs are fins and scales though fish which already have scales will have fins as well. Thus to determine if a fish is kosher only the skin needs to be attached. Some hechsherim have alloud fish to be identified as kosher through other means and this seems like a questionable leniency considering the fraud which takes place in the food industry and the possibility for fish to be faked.
- The issue of the knife is the potential it was used for non kosher food before getting to the kosher fish. This is addressed in yoreh deah (I forget which siman right now) where a knife is considered cleaned when cutting through many of the same kosher item repeatedly. Regarding the initial cuts since this is all done together out of convenience (and also not for the purpose of making it botel or by a person to whom en mevatel issur letchatchila would apply to) those would become botel in the grand total of large amount of kosher fish cut up.