I know that sharks and things like that get bigger and gain weight, but why don’t my salt water fish ever get any fatter? They eat and eat and eat and eat and eat and eat but never get bigger or die from eating too much or anything. Is it a calorie thing or what?
Well I guess that a really fat rolly polly fish would have dificulty swimming and not be able to catch prey,thus losing “weight”
Really though fish do have visible fat. You can see it when cleaning a oily type fish. Around here carp, but salmon and trout are also oily fish.
Contaminates settle in the fat so most large fish are best thrown back rather than eaten.
Logically thinking only – I’m a very UNscientific person–they eat mostly plant and/or protein matter–few, if any, carbos–and they are almost constantly moving. Something like the principles of the Atkin’s Diet. ?
I can tell you haven’t done much fishing steeljaw. Fish in the wild can get tremendously fat depending on the available feed.
I would suggest that the fish in your aquarium don’t get any bigger because of the limitations of their habitat.
Fish can and do get fat. Some record bass and catfish were quite “fat” relative to other “normal” bass and catfish. As a WAG I would imagine, however, it’s relatively rare for fish (salt or fresh) to put on weight as it would not tend to serve any useful evolutionary purpose vs the risks involved in not being able to move quickly enough to either catch prey or not be prey so the ability to accumulate significant fat deposits never developed.
steeljaw, how do you know they don’t get fat? Did you cut open your fish & look?
Fish in general don’t store and get fat like mammals because their metabolism works differently. In mammals the majority of energy used to “run the body” comes from carbohydrates, while extra fat is mosty packed on for a rainy day. Fish on the other hand get the majority of their energy from burning fat, and carbohydrates are relativly useless. The two main ingredients that make up the bulk of commercial salmon feed are fish meal (for protein) and fish oil (the fat used for energy). Fish also tend to have liquid oily fat as opposed to white crisco-like fat in their bodies. They need to have unsaturated fats in their bodies because they are cold-blooded and saturated fats become solids at the temperatures they normally live in. You can see white fat deposits in some farmed fish, especially salmon - although feed formulas and feeding practices have reduced this condition. The type of fish also tells you how fat they are likely to become. Blue tuna are almost constantly swimming at high speeds and would burn off more calories than a big old catfish hanging out in the swamp. Tropical (pet) fish food has even less fat than salmon feed, so it’s unlikely you’d ever fatten up a pet fish unless it was a lethargic bottom species and you fed it constantly… even then it wouldn’t get rolly-polly like an overweight cat; just well-rounded and hefty looking.
No, really, I’ve cleaned plenty of catfish that I would definately call “fatty,” usually when they 4 lbs or above.
Just my $.02.
Blubber, thats fish fat. Whales are big fat fish.
Uh, handy, whales are big fat mammals.
How come fish never get fat?
…cuz they eat fish?
<<ducks, runs>>
You are right chriszarate, they are Cetaceans 
Others have pointed out that some fishes can get quite fatty, so I’ll just second what’s already been said on that score.
Carbohydrates are not useless to fish, although they may be relatively rare in the natural diet of some fishes. Most carnivorous fish do use fats more efficiently than mammals, but fats make up a small fraction of their diet. In artificial situations, some fish can adapt to high fractions of carbohydrate in their diets:
From Bond, C.E. 1996. Biology of Fishes, Saunders College Publishing, p. 441-442.
I’m also under the impression (but not positive) that salmon feed contains an unusual amount of lipids for cultured fish (up to about 30%, I think), while feed for catfish measures in at less than 10% lipids with about 25% carbohydrate. According to Bond again…
Ibid, p. 442.
Yes! Fats can actually be dangerous to fishes if fed in excess, causing damage to the liver. I suspect pet fish food has a low fat content (especially in fats with high melting points, which are very difficult for fish to digest). This is the most likely reason for your fish not becoming fat.
It also occurs to me that fish may want to control the fraction of fats deposited in their bodies within a certain range for bouyancy control, however I’m not sure if this provides a more ultimate evolutionary reason for your fish not getting fat, just something to think about.