Why Franklin's Gulls have pinkish cast on belly

As noted in prior post, many of the Franlin's Gulls I saw had a pinkish cast to their belly area, a feature seen every year in spring and that wears off as the feathers are worn. So where do they get the pinkish cast seen on the gull in this pic (double-click to see it best)?

Not only do Franklin's Gulls get this but so do some Ring-billed Gulls as well as some Elegant Terns, though they had some other similar substances. It has been found to be due to a carotinoid substance called astaxanthin. And it is within their feathers not in the preen oil they produce.

An article in the Journal of Field Ornithology by McGraw and Hardy hypothesized that some of these species get the pinkish coloration because they are getting a lot of this astaxanthin in their diets when they are growing their feathers. This article, "Astaxanthin is responsible for the pink plumage flush in Franklin's and Ring-billed gulls" states, "It is tempting to link the increase in sightings of pink Ring-billed Gulls since the late 1990s with the introduction of pure, synthetic astaxanthin to the diets of hatchery-raised salmon."

It seems that natural astaxanthin is in the food that wild salmon eat and that is responsible for the pink color of their flesh. Since these farmed salmon (which are not nearly as healthy for people to eat due to reduced omega 3 content and they are harmful to other fish and the ocean environment) don't eat healthy, they supplement with synthetic astaxanthin (which has been approved by the FDA as a dye for this and some other foods). And astaxanthin is derived from petroleum products (so when you eat farmed salmon you are getting them with dye made from petroleum-ick!).

As much as I have found this pinkish cast to be interesting, I would really rather see my gulls (and my fish) with their natural color.
SeEtta