I heard that in the past, but then I heard (somewhere on ddf) that "baby salmon" is actually trout. Which is true?
interesting,
I was sitting next to a guest in shull so I welcomed him and asked him what he does for a living, and we had this discussion about the color of salmon, he is a rav hamachshir in EY and he came for some expo/kashrus event not sure
so he mentioned that there is this rule that pink fish is bchezkas kashrus becuase only salmon has this color,
there was a study that checked why farmed raised salmon have a grayer color and wild salmon have a dark pink color and they found that the wild fish eat some food that makes them red,
so once this study was published fish farms started adding red dye to their feed so the flesh turns pink, which basicly raises concerns about the chazaka of the color
a quick google
Naturally, that salmon and most farm-raised fish would actually be gray-ish in color. Instead, to make the fish look more like their wild caught cousins, many farmers add food dyes to their fish feed.
so the OU went ahead and fed other random fish the same dye that they feed the farmed salmon and found that it only effects pink fish (which is salmon) to turn into a darker pink, but white fish would stay white.
from the OU
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 Hoffman-La Roche’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. [4]
https://oukosher.org/blog/consumer-kosher/salmon-colored-with-astaxanthin/So to answer your question based on the OU article it seems that all pink fish are kosher which includes salmon and trout
he also mentioned that OU believes that there is no Bishul Akum on tuna since it's not olah al shulchen meluchim and its not cooked only steamed, and the use of salmon changed as well since a big percentage is used raw for sushi, but not everyone agrees with them.....