Eating Fish... warm-bloodied, cold-bloodied, squirrels, homeothermic, exothermic, endothermic, point-endothermic, fungi, mold, etc.

Ken Jones

Staff member
Date: June 11, 2003
To: Pier Fishing In California Message Board
From: halibut hunter

Subject: Rumor?

I heard that if you catch a fish, that it’s not good to eat it right after it dies because its’ still warm because of its body temperature. What do you guys think? Besides I usually prepare my catch and placed it in the fridge and eat it the next day.

Posted by chuam

I don't see a problem with eating it as soon as possible. The only issue you would have to deal with would be parasites but those will still be there the next day after being in the fridge over night.
Actually, the best sashimi I ever had was when I was tuna fishing. We caught the tuna, dispatched it and then cut it up. The tuna was dead for less than 10 minutes from deck to stomach. Marcus

Posted by gyozadude

Basic science... Fish are cold-blooded creatures. Except for large whale sharks, most fish have metabolisms that follow closely ambient water temperatures. They regulate temperature by migrating to appropriate waters that have more favourable temperatures.
I certainly hope you're not over 14 years old. Otherwise, I'd have to contact your parents and teachers and have any diplomas recalled for this science transgression alone! Gyozadude, “Yes - I can roll my own potsticker skins”

Posted by BigUnInDaBoat

I believe some tuna are also “warm blooded”... but yea I have kept fresh fish in a fridge for a long as it was cleaned properly and rinsed it will keep. BUIDB

Posted by gyozadude

Warm or semi-warm or actually cold? Fish ARE cold blooded, and if marine biologists were required to take graduate level heat transfer classes, they'd probably conclude that most large fish (over say 50lbs) will be able to maintain some measurable core body temperature above the ambient water temperature. This is just a fact of heat transfer and metabolism. A by-product of producing power by burning fuel and oxygen is heat. Any large fish, sharks included, that metabolize food for power, will generate heat, and by virtue of size, will establish a significant temperature gradient inside their bodies. Tuna may be slightly better designed by their round shape to retain heat.
The question I don't read being answered by marine biologists on these gov’t research grants is whether the fish are warm blooded by nature, or if it’s a beneficial quirk of evolution that's primarily a heat transfer glitch in big fish. The answer should be pretty straightforward to measure and test.
Firstly, my kids whom are still children will still produce massive amounts of heat on a cold winter’s night when they kick off their blankets. Secondly, the complete enzyme and amino acid profile of most warm-blooded creatures usually demonstrates inability to sustain our high level cognitive abilities at temperatures below, say, 30 deg. C. In fact, if human core temperatures drop below 32 C, we go into coma and sustained periods may lead to death. But fish have a different physiology that has a different set of enzymes and amino acid profiles that allow the creatures to function despite large core body temperature swings.
The test for tuna warm-bloodedness should be simple: A) when they are babies, do they exhibit the same and therefore steeper temperature gradient in body temperature of a true warm-blooded creature? and b) do they retain most of their higher level cognitive (well, in a fish’s case, their predatory behaviours) when the temperature drops?
The answer should be fairly simple. You don't find young tuna in San Francisco waters often (except in El Nino years) You have to head out on long range trips that hit warm currents. And very similar related species like bonito are not considered warm blooded. Only the largest of fish are likely to be warm blooded.
In any event, fresh tuna is far better than older tuna and the original post questioning whether aging fish tastes better, has a clear answer. Fresh is better.
Even amongst beef aficionados who think aging improves the flavour of beef... they need to head to Japan and have some Kobe veal raw in a steak Tare-tare. The stuff is hands down the best beef in the world and extremely flavourful and very fresh, the best stuff being delivered by Shinkansen (bullet train) to cities all over Japan for several hundred dollars per kilo. Even the prized (aged) Scottish beef I've had in Glasgow takes a far second to Kobe stuff. It just goes to show folks that there are myths and rumours about aging meat or fish that should be clearly false.
Gyozadude, “Yes - I can roll my own potsticker skins”

Posted by John Mykkanen

Cold and warm blood are bad terms that Biologist don't use.
Thermal regulation is more appropriate. In tunas it has been shown that the blood circulatory system actually heats the blood. That is, blood warmed by muscle (the outer deep red tissue does most of the work) activity is used to heat up cooler blood coming from the gills. Instead dumping all the metabolically generated heat in conduction through the muscle and then convection into the water. Or in forced convection where warm blood leaves the warmer center of the fish and enters the cooler gills, the veins, which carry the warm blood “comingle” with the cooler arteries forming a “heat exchanger and carry the heat (thermal energy) with the arterial blood back into the center of the fish. There some other similar tricks they also use.

Posted by nopal

Great Whites, Tunas, Makos, and Squirrels. Just some thoughts: Tunas, Makos and Great Whites do, for the most part, maintain a constant core temperature. Great Whites, for example maintain a body temperature of up to 27 degrees higher than the surrounding water. Regardless of how they regulate that temperature, I’d like to see a study, which demonstrates that the cognitive abilities of a Great White DON’T diminish if its core temperature drops (and Great Whites are quite intelligent too boot). As any angler worth its salt knows, water temperature clearly affects the behavior of fish, so perhaps an argument can be made that body temperature does affect a fish's cognitive abilities regardless of whether they can regulate their body temperature internally or not.
It would be pointless to define whether an animal is warm-blooded by nature or evolution, since both choices pretty much mean the same thing: If the animal can regulate its body temperature internally and this is characteristic of the species, then how can't it be part of it's nature? Didn't all so-called warm-blooded animals eventually evolve to be this way? Similarly, a fish’s fry, for the most part, goes through pretty radical changes as it grows, especially if its from an oviparous species (think of the early stages of development taking place in a relatively cold environment versus a warm womb), so comparing their body temperature at their early stages of life is like comparing apples to Buicks.
On the flip side, ground squirrels have core temperatures that fall from about 86 degrees to about 34 when hibernating. That is almost the freezing point of water, and it's probably completely irrelevant, but I still think it's cool (pun intended).
In its strictest terms, the definition of "warm-blooded" simply refers to an animal’s temperature, and it is clear that though rare, there are fish that have relatively warm temperatures in the sense that it is higher than that of the environment in which they live. This appears to be the definition that is used by the original poster, Halibut Hunter.
Gyoza’s orginal post refers to the more traditional definition of temperature self-regulation, but even by this definition some fish do regulate their temperature internally.
It is when Gyoza tries to define “warm-blooded” in more specific terms that we run into problems. Could it be that the term is just meant to be vague?

Posted by gyozadude

When folks talk about core body temperatures and how some species insulate themselves versus the cold relative to others, it comes again down to basic heat transfer through thermal mass. Sure, I read those same articles on blood circulation and red tissue versus white tissue insulation in tuna. Sounds sophisticated. But do a calculation on thermal dissipation on the creature through diffusive layers based on simple metabolism and we'd find quickly that this isn't much more than large animals having heat dissipation issues.
Again, show me babies of the species and their heat transfer profiles and if higher gradients exist relative to their parents, AND then show me their amino acid/enzyme profiles that have a narrow optimum temperature band of operation, then I can tell you that the species is “warm” blooded. But otherwise the species is “cold” blooded. Pretty cut and dry.
Gyozadude. “Yes - I can roll my own potsticker skins”

Posted by nopal

I'm sorry Gyoza, but that’s just too funny. I just have this picture in my head of a tuna at a shrink’s office, trying to work out its heat transfer issues.
John has pointed out that there is a very specific mechanism that tunas use to recycle heat in order to keep their temperature at a certain level. Why would such a mechanism even evolve in the first place if it was useless? If you dig a little deeper, I'm sure you'll find out that tunas actually recycle that heat for a purpose.
But go ahead and do a calculation about human thermal dissipation. I bet you'll discover that humans just have "heat transfer issues" as well, we just happen to have much bigger issues than tunas do. After all, it could essentially be argued that our body temperature is constant because all of that heat we generate simply does not diffuse into the environment efficiently. But that oversimplifies the issue to the point of absurdity, doesn't it? (Post Hoc, ergo Propter Hoc)
Enzyme/amino acid profiles by definition have narrow temperature ranges of optimum operation. That's what an enzyme does, it makes reactions at specific temperatures possible. And you were questioning Halibut Hunter's education? For most animals it all boils down to reaching the optimum operating temperature. That's why lizards sun themselves, dogs pant, and we sweat.
But I have taken the liberty of doing something radical and actually looked stuff up. According to, this is the definition of warm-blooded: “Zoology.- Maintaining a relatively constant and warm body temperature independent of environmental temperature; homeothermic.”

Let's see:

Relatively constant, check!
Warm body temp, well in relation to the surrounding water temp., Umm, I'd say OK, why not? Check!
Independent of environmental temperature, check! (especially for Great Whites!).
So that's 3 out of 3.
Lets look at homeothermic: Does the heat that the tuna recycles come from within the tuna, or outside of the tuna? I'll leave that one for you to answer.
Again Gyoza, my original point was that simply that “warm-blooded” is the kind of term that could potentially be applied to some fish. “Warm-blooded” is a term that has become, as you would say, simplistic and unscientific, and frankly needs a good revision. I just wouldn't use it in a “scientific” discussion, period.

Posted by gyozadude

Heat transfer issues? No, I think you folks misunderstand what I'm saying. The articles tend to make one think that Tuna have special mechanisms for recycling heat against cold diffusing in. In mammals, we use insulative layers and fat to do that, and tuna have some white tissue. But the researchers that publish the papers make it seem that the tunas have some type of heat pump and heat exchanger vascular system. Sounds impressive, until you ask the question: heat exchange with what? What's the source, what's the sink? It's a single vascular system. The fluid pumps through the system. There's no secondary heat exchanger.
In other words, it's fancy nonsense for insulation. The marine biologists should do a calculation on the increase in required metabolic rate to supply pumping pressure to overcome vascular pressure loss due to viscous dissipation, and I'll bet that it will establish the measure temperature gradient between the outside ambient water/skin temp and the tuna's core temperature.
In other words, in any large animal, warm or cold blooded, we would expect the core temperature to be higher simply because living animals aspirate and this generates heat. The heat is a source. The ambient environment is a sink. Hence, we will always have a temperature gradient. I'm in violent agreement with everyone that, yes, if we caught a tuna and cut it up quickly, we might discover that that core temp was significantly warmer than the skin temp.
I think everyone understands this. However, I think we're splitting hairs over how we define warm-blooded and cold-bloodedness, and a lot of the papers on the web by so called fish biologists propose a lot of complicated vascular reasons why this is so. Yet, have any of them presented a simple analysis of core temperature based on total life force thermal dissipation alone? If they could show that this alone didn't account for the majority of internal heat, generated, then I'd be happier with the so called claims of vascular reasons for warmer core temperature.
What I propose is a simple way to judge whether a species is warm or cold blooded that makes it more definite. Heck, it may not be even -necessary- and no one really cares. But I'll present it anyway, just because it's fun! :)
Again, let's go back to my simple and yet, unassailed definition of warm-blooded-ness: A) The young have a more acute temperature gradient because of their smaller size, yet the need to maintain similar core body temperature to function like the parents, and B) the enzyme/amino acid profile of the living fish has evolved around acute efficiency in a narrow temperature band (because the species can maintain that temperature). If these two conditions can be met, then we have a "warm" blooded creature.
As for the rumour that started this whole discussion: Are fish better hot or cold... the fish are whatever temperature they are at and fresher is usually better, since bacterial decay usually follows an Arrehnius equation with temperature but is always positive so the longer you wait, the less fresh the fish.
The only exception would be noted by Ken, who's pointed out that in some species, we need to marinate in some neutralizing solution. But again, in general, better to marinate sooner, rather than later. Gyozadude, “Yes - I can roll my own potsticker skins”

Posted pkjoe

Language for the layperson? Hmmm... all this techno-speak is giving me a headache. Let me see if I understand.
Cold blooded (fishies): Warmer than stuff around them because living things make heat so the temperature difference is incidental.
Warm-Blooded (Us): We HAVE to be at a certain temperature or else we get dumb. So our body makes heat when we need it or gets rid of it when we don't.
Is that a fair summary? PK

Posted by nopal

Language for the layperson? Pretty much, except I'd add that there are some fishes that use and indeed may need that incidental heat in order to function at their optimum, hence the existence of endothermic (self-warming) fishes.
For what I've been able to gather, there is a clear distinction that biologists make between regular (exothermic) fishes and heat-recycling endothermic fishes. Run a search on Google on “endothermic fishes.” There's a lot of fascinating stuff about the subject.
The disagreement between Gyoza and I seems to be about the semantics of “warm-blooded.” He appears to say that it's a term that does not apply to fishes at all since they are not warm-blooded in the same way that we are, and to which I respond that it's a term so vague, that it can be applied to fishes just as easily as humans. Petty, actually.

Posted by nopal

Use: Exothermic, endothermic, point-endothermic.

Posted by Songslinger

This Is Why I love This Board! The entropy of discourse...

Posted by baitfish

Yeah and they talk about a lot of different stuff too... :)

Adam, Fish have to eat, all you need to do is figure out the ingredients.

Posted by Ken Jones

Original subject: not true. When young, I used to fish on the half-day boats out of Mission Bay and quite frequently we would have fresh fish cooked up almost as soon as we caught them. One favorite was bonito (a member of the tuna family). We would clean them and cut thin slices of meat from behind the eyes on the head. Cooked on the galley grill with just a small amount of butter (and sometimes a slight amount of soy sauce)—for just a few seconds—it was delicious. Could not have been more fresh. And many of us have gone camping and cooked fresh fish— right?
But, in the case of sharks and rays you do tend to be right. They are often much better if the fillets are soaked overnight in the refrigerator in a little lemon water or small amount of milk. That’s because sharks, rays and skates should always be cleaned, or at least gutted and bled, soon after capture. Each of these species contains urea in their blood, flesh, and skin to help them maintain the proper salt balance in their bodies. It's good for them, but bad for us. Unless bled quickly the urea will cause the carcass to have an ammonia smell to it and cause the flesh to have an off taste. This urea-induced taste can be neutralized by soaking the fillets in acidulated water (mild vinegar and water or lemon juice and water) for a few hours... If not immediately cleaned, make sure you soak them for a few hours or overnight in the acidulated water. In addition, I have found that the flesh of these fish is improved if kept chilled one or two days (no more) in the refrigerator—even if they were cleaned immediately.”

Posted by hallihunter

I agree as well, sashmi albacore after it is bleed is mighty tasty.
But on another note, to add my two cents, fish especially tuna when fighting for their lives can increase their internal body temperature several degree’s, compared to the ocean temp. The “rumor” that some may hear, is somewhat true if the tuna is not bleed and put on ice right away. That increase in temp is coupled with the release of enzymes giving a more favorable environment for bacteria to grow and eventually contaminate the flesh. HH

Posted by pkjoe

Hmmm, who's to say a little bacteria isn't a good thing? Fresh isn't necessarily better. After all, without good ol’ bacteria we wouldn't have cheese. PK

Posted by gyozadude

Without good fungi and mold either!

Posted by pkjoe

Are you talking about that Blue Cheese stuff? the one w/ mold running through cause that's just plain not right. I've looked at it but can't bring myself to eat it. Echhh!! :) PK

Posted by gyozadude

Roquefort, Bleu, etc. Man... a good pot of mussels in a Roquefort sauce/broth with white wine. I must have had that at least three times a week when I worked in Paris at the Leon de Bruxelles Restaurant on the Champs-Elysees. It was only like 60FF at the time. It’s a Belgian recipe, but just outstanding with a side of Frite (fries). I make that up north when we get a minus tide that exposes the mussels on the rocks. I always wondered what a poached halibut filet would taste like in a Roquefort reduction with some cream, rue, and some cayenne pepper, with that same side of awesome fries... yum!
Gyozadude, “Yes - I can roll my own potsticker skins”

Posted by pkjoe

Heh! Gyozadude, you make sound almost edible! Maybe I've been a bit hasty in my reluctance to partake. That halibut recipe sounds darn good! PK