Although kosher fish are usually identified only by the presence of scales , the Orthodox Union has a long standing policy of accepting as kosher all reddish-pinkish fillets, even without a piece of skin by which the fillet can be identified. The basis for this policy is that there is no fish that has a reddish-pinkish flesh underneath its skin except salmon, trout and possibly some carp, which are all kosher fish.
The recent use of an artificial vitamin supplement, designed to redden the flesh of farm-raised salmon, has raised a question about whether the Orthodox Union can continue to assume that all reddish-pinkish fish are kosher. If this vitamin can be used to redden the flesh of kosher fish, could it not be used to redden the flesh of non-kosher fish as well? Based on my research, I hope in the following article to explain why this worry is an insufficient reason to discontinue accepting reddish-pinkish fillets as kosher.
The basis for our policy of accepting all reddish-pinkish fillets as kosher comes from a psak that Rabbi Yisroel Belsky, Shlita, received from Rabbi Moshe Feinstein, zt”l, that a fish fillet with a reddish-pinkish color could be accepted as a siman-muvhak of kashrus, if one could be reasonably certain that no non-kosher reddish-pinkish fleshed fish exists in nature. Rabbi Belsky maintains that the matter has been sufficiently investigated and no non-kosher red-fleshed fish exists.
Rabbi Feinstein’s psak, however, could only apply to fish whose flesh are naturally red, such as wild salmon and trout. The flesh of the farmed varieties of these fish would (if not for supplementation) be a sickly pale-white. The reason for farmed salmon’s natural absence of redness is a lack of astaxanthin, an anti-oxidant that wild salmon and trout absorb from their diet of lobster, shrimp, krill, plankton and algae. Farmed salmon’s diet lacks the aforementioned delicacies, and must be fed an artificial astaxanthin, such as Carophyll-pink manufactured by Hoffman-La Roche, in order for them to mirror the color of wild fish. It is the color altering property of this nutrient that is causing concern in the kosher community. If astaxanthin can alter the color of the flesh of farmed salmon and trout, could it not alter the color of the flesh of non-kosher fish as well?