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 (see below).
The 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, z”l, received from Rabbi N’ Greenblatt in the name of Rabbi Moshe Feinstein, zt”l, that a fish fillet with a reddish-pinkish color could be accepted as a siman-muvhak (an absolute sign) 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.  It should be noted that some recent discussions about Dogfish Shark (also called salmon shark) are nothing new, and Rav Belsky was not concerned that this “looked like salmon”.
Rabbi Feinstein’s psak (ruling), 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 DSM, 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?
To answer this question, it is important for one to understand exactly what carotenes are. Astaxanthin (the primary carotene found in wild salmon) is in the same family as beta carotene, the chief anti-oxidant (and principal pigment provider) found in carrots, apricots, squash and sweet potatoes. But carotenes do much more than provide yellow, orange or red color. Carotenes are essential to the health of both humans and fish as they also eliminate free radicals and oxygen singlets from the blood, enhance immune functions, act as anti-mutagens and anti-carcinogens and are a resource for the body’s manufacture of Vitamin A. Excess carotenes, which the body does not need for maintaining life functions, are stored in different parts of the body in different creatures, depending on their genetic make-up. In salmon and trout (for example), carotenes are stored in the muscle tissue (i.e. flesh), making the flesh pink or red. (The skin on the other hand, in an Atlantic salmon for example, would retain a blue/silver color.)
Only fish such as salmon and trout retain natural carotenes in their reddish-pinkish flesh. Only fish that have the ability to store natural carotenes in their flesh can retain artificial carotenes, such as DSM’s astaxanthin “Carophyll-pink”, in their flesh. Other fish are not red because they do not store carotenes whether natural or artificial in their flesh.
It is interesting to note that in humans, carotenes are stored (amongst other places) in skin (as opposed to flesh). That is the reason why eating excessive amounts of yellow-orange fruits can cause a person to develop a “jaundiced” look, and why astaxanthin is marketed for human consumption as the active ingredient in oral tanning pills (though it does have the undesirable side effect of causing one’s perspiration to be orange). Astaxanthin does not change the skin color of farmed salmon and trout, nor does it change the flesh color of humans or other fish.
In conclusion, the OU Poskim maintain that red flesh is still a siman muvhak for a kosher fish, as only salmon and trout have a red or pink flesh  , and that our policy of accepting reddish-pinkish fillets without skin is justified.
by Rabbi Chaim Goldberg
OU Kosher, Fish
 Shulchan Aruch 83:1
 Though some small tropical fish have red flesh, they are not available for commercial consumption.
 This is not an extension of the Beis Yosef’s mesorah regarding red fish roe (see Shulchan Aruch 83:8 and Shach ad loc. #27) but is based on modern-day independent research about salmon fillets.
 Rabbi Belsky was recently sent a sample of a “white” wild king salmon steak from British Columbia. He confirmed that even this was pink enough to clearly be a salmon, and that no other fish could be substituted even as a “white” salmon.