The Mystery of the Blue Flesh
Anglers who have never caught a cabezon, lingcod, or the lingcod’s cousins, kelp and rock greenling, are sometimes startled when they clean and fillet them. Although some have white fillets, a high percentage of the cabezon fillets are blue or bluish-green while upwards of 20% of lingcod fillets are the same. Are the turquoise-tinted fillets safe to eat? Yes, while the fillets are a pretty blue color before cooking, they turn white and are delicious to eat when cooked. In fact, the meat of the cabezon and lingcod, whether blue or white, didn’t seem to bother the voters on Pier Fishing In California who in an informal vote on favorite eating fish, rated lingcod #1 while halibut and cabezon tied for #2.
Why the fillets are blue is always somewhat of a question among anglers, deckhands, seafood workers, and fishery scientists themselves. The short answer is that the culprit causing the color is something called biliverdin, a bile pigment that causes the blue-green color of their flesh. But how does the biliverdin actually color the fish? Why do some fish have the biliverdin while other fish living in the same environment, i.e., rockfish, not have the same condition? And why are only about 20% of the lingcod bright blue (both outside and inside) while most of the lings have different colored skins and white fillets? Don’t they eat the same foods?
I first asked AI — Why do lingcod and cabezon have biliverdin in their system? AI — “Lingcod and cabezon have biliverdin in their systems because it is a natural bile pigment that causes the blue-green color of their flesh, though the exact reason for its accumulation in some individuals is still being researched. While potential factors include genetics, diet, or liver function, the specific mechanism for why some fish accumulate biliverdin while others don't is not fully understood. This blue coloration is completely harmless, is normal for these species, and the flesh turns white when cooked.” It’s an accurate compilation of facts but still leaves the main question open.
Next I consulted the only ichthyology book I own, Fishes An Introduction to Ichthyology by Peter B. Moyle and Joseph J. Cech, Jr. Not a single word about biliverdin and why some fish have the blue color.
Finally I took a look at the books of the guru that I turn to whenever I have a question about fish—Milton Love, head of the Love Lab at the University of California, Santa Barbara. In his first book he commented on cabezon: “The color [of the flesh] may come from copper-based compounds in the shellfish they consume.” — Probably More Than You Want To Know About The Fishes Of The Pacific Coast, Milton Love (1996). In his second, truly encyclopedic book, he said (again in regard to cabezon): “And what’s with that blue-colored skin and muscle? That flashy hue is caused by high levels of bile pigment called biliverdin, which caused the blood serum of the fish to be blue-green (or occasionally purplish) and which also finds its way into the muscles. The source of the pigment (which is also found in a number of other sculpins) is unclear, although the most likely source is the breakdown of heme (as in hemoglobin) or other molecules porphyrins. Why these fish should carry such a high concentration in their blood is unknown. Also possible is that the color comes from the fish’s diet, as some invertebrates store bile pigments from the algae they eat. The level of biliverdin found in cabezon blood serum are higher than those found in humans with jaundice and how these fish cope with nominally toxic levels of this material remains a mystery. Arrgh, me smells a thesis here, matey.” — Certainly More Than You Want To Know About The Fishes of the Pacific Coast.” — Milton S. Love (2011). Until a student out there takes up the question for a Master’s Thesis and provides an answer we are still somewhat clueless.
I finally turned to the Internet as all good researchers do and I found an interesting article in the Times Colonist newspaper of Victoria, British Columbia. Dated August 29, 2021, written by Rochelle Baker, and assisted by marine ecologist Aaron Galloway, it contained some interesting information.
An early interesting comment was one the writer had on anglers, “most fishermen, some who tout the fish [blue lingcod] as lucky, theorize the harmless colour occurs about 20 per cent of the time and is derived from the food the fish consume — speculating it’s the result of a particular type of crab, squid or kelp.”
Those comments mirrored those seen on a thread on Pier Fishing in California discussing the blue fillets of lingcod: (1) I was told it’s the squid and octopus they eat. Like Methaline Blue effect (2) Friend of mine said squid and octopus have copper based blood. (3) Blue because their circulatory system uses copper to transport oxygen (compared to iron in our human hemoglobin). The responses: No! That is true for crustaceans, but not fish. (4) It’s all about what they are eating that makes them white, green or blue. (5) They turn blue because they have been feeding on crab. Cabazons also have blue fillets, same scenario. From what the biologist told me when I was in the commercial seafood industry in a different state, that’s what they told me. The response: Most bottom fish in the rock and kelp environments feed on crabs but few have this blue flesh. (6) They take on the color of the kelp that's why they're blue. (7) I always understood it is from the ink in the squid and octopus they eat. (8) Always thought it was from eating octopus. The ink from the octopus is what made the meat turn blue. (9) Very few know why the meat turns blue. Response: It’s because they eat more octopus and squid and the ink turns their body blue. (10) They’re blue from eating octopus, but blue fillets signal awesome fish tacos. some of the best fresh seafood there is on the planet. (11) Eating Squid... good eating. (12) Blue or green is from them feeding more on crustaceans than fish. Cut the stomach open and notice the food contents, crabs, shrimp & lobster. (13) I’ve heard it’s genetics, and I’ve heard it’s diet. I’m sure Google has the right answer. (14) AI figured it out already. It’s from eating octopus. (15) Nobody 100% knows. The theory that it's the ink in the squid doesn't hold much water with me. Where I fish probably 1 out 10 are blue. I don't believe that one fish ate that much more squid than the rest. Especially when you cut its belly open and find rockfish and smelt. (16) So the $100,000,000 question is would it [the blue lingcod] be considered a different species? Some feel it’s because of octopus ink that they are that color but they all eat octopus. Can anyone really answer? A lot of us have caught both colors and they are identical except for color.
Unfortunately, these answers were mostly conjecture, an answer to the main question is still lacking. However, the article does provide additional information that may be useful down the road. Hints may be gotten from a study of a catch survey done along the U.S. coast that involved more than 2,000 lingcod. Survey scientists and Galloway used various criteria to reach several findings. “The most significant and surprising find was the probability that blue flesh was highest in females, as well as those caught in shallow water, or ones that were smaller. The overall incidence rate of blue fish ranged from four per cent to 25 per cent depending on the region where they were caught, which supports anecdotal estimates by fishermen that one out of every five ling cod has bluish flesh. However, sex was still the biggest predictor, as 80 per cent of all the blue lingcod were female, Galloway said. Yet, blueness isn’t solely a gender trait as a proportion of male ling cod are also blue.”
The article wasn’t done. “Tests from 175 fish showed that blue-fleshed lingcod had a lower concentration of fatty acid said Galloway, noting higher fatty acid counts — derived from food sources — generally indicate good health or fitness in fish. So, a dietary difference in shallow waters might trigger a blue colour, compared with fish that spend most of their time in deeper habitat… our hypothesis, and some of the fatty acids we traced, indicate the blue fish might be starving, or going through some sort of nutritional stress,” Galloway said. “Research on other fish indicates poor diet can trigger an increase in blue bile pigments,” he said. Ideally, experiments in a laboratory setting could explore if nutritional stress would turn a lingcod blue, Galloway said. Or if blue fish with a good diet could turn into a more typical brownish-red. “Or is it once [the fish] get to a state of blueness that they are locked in for the rest of their life,” he said. “I find that a very interesting question. It’s also not clear if blue lingcod are born blue and inherit the trait from parents.”
One final article, actually a scientific abstract: The blue-green blood plasma of marine fish by Igor Solar and William Halmeck, offered up some final information and I quote it in full:
The primary reason that the flesh of fish is a white is that it is not visible to a predator or to prey, as it is positioned beneath the skin. Therefore no adaptive advantage exists to acquire a different color than that of most protoplasm which is colorless or clear to white. However, not all fish flesh is white. Some fish have myoglobin, which produces the predominantly red flesh in most tuna and other open-ocean fish like mako sharks and swordfish. The protein myoglobin stores energy which is required by these fishes to swim at high speeds for longer periods of time.
Some fishes like trout and salmon have colored flesh ranging from orange to pink to almost red because they have been feeding heavily on crustaceans, which contain pigments called carotenoids. Occasionally in the past hatcheries have added paprika to the trout chow they feed to trout to give color to the flesh.
The lingcod (Ophiodon elongatus) is a member of the greenling family. Around 20% of lingcods have blue-green to turquoise flesh. Though usually white-colored like halibut or cod, lingcod occasionally looks as though it's been marinated in Blue No. 2 food dye. A pigment called biliverdin seems to be responsible for turning the blood serum of these fish blue. The rock greenling and the kelp greenling, which are related to the lingcod, are sometimes tinted turquoise, as is an unrelated sculpin called the cabezon, which shares the same habitat. All of these species are found in the Pacific Ocean off the coast of California and utilize bottom habitat. So, there must be something about their diets, which leads to acquiring biliverdin, but it is unclear what that might be.
Along with its presence in avian egg shells, other studies have also shown that biliverdin is present in the blue-green blood of many marine fish, the blood of tobacco hornworm, the wings of moth and butterfly, the serum and eggs of frogs, and the placenta of dogs. With dogs this can lead, in extremely rare cases, to the birth of puppies with green fur; however, the green color fades out soon after birth. In the garfish (Belone belone) and related species, the bones are bright green because of biliverdin. The green coloration of many grasshoppers and lepidopteran larvae is also due to biliverdin.
More data and more questions but still no answer on how exactly the biliverdin works or why it only shows up in some fish while not in others. Fish may live in the same environment and eat the same items in their diet but have totally different colors.