The true size of Arapaima gigas

Hallo everyone! This is my first post here is about a special topic with which I have dealt since many years. I was always obsessed by the outsized freak animals, especially big fish and big reptiles. You can often read fantastic sizes which are allegedly reported for some species. But after a long research for several species, it turned out, that many of the often-cited sizes are not true in fact. In some cases old big-fish-stories which had never any reputation actually found their ways into scientific literature, because nobody ever doubted it. I have still many older books which quote the size of the great white shark for example with 11 or even 12m. But there was never such a big great white shark, and it turned out later, that the alleged specimen of this size from which it was said that it was caught at the coast of Australia, acutally never existed. There are many similar cases, and some of them are very popular and widely cited as facts, including some fish species which are often discussed in this forum. One of the most extreme cases is the arapaima, which is normally cited to grow up to 4,5m or even 5m (in newer literature you can find more often sizes of 3m as maximum). But have you ever seen an arapaima only close to this size?
I once wrote a longer article about this issue for a site about freshwater fishing in Thailand ( can be seen here: http://www.megafishingthailand.com/content/view/168/53/ ), which I show here again:



[FONT=Times New Roman Standard, serif][FONT=Times New Roman Standard, serif]How big can Arapaimas grow?[/FONT]


[FONT=Times New Roman Standard, serif]The arapaima is a big fish. Indeed a very big fish, especially for a freshwater-species. But is the arapaima, or pirarucu how it is called in South-America, really the largest freshwater-fish of the world, and how big can it actually grow? In popular and even scientific literature, as well as in the web, you can often read that this species can reach a length of 4,5m and a weight of 200kg. But is this really true? If you look for pictures of the arapaima, you will find a whole lot of photos which show very big fish, some of them well over 2m, but you will never find a picture of one which comes only close to the alleged size. Furthermore, this dates alone are highly dubious, because the relation of weight and length is absolutely unrealistic. It is really strange why never one of the authors and zoologists who wrote about the arapaima became critial at this. As there are many actual recorded specimens with known weights and lengths, it is easy to compare it with the figure of a 4,5m arapaima which would weigh 200kg. One example for comparison would be the record-arapaima which was caught only a short time ago in a fishing-pond in Thailand. It was 2,63m in length and had an enormous weight of 185kg. This are actual dates, and show very well that the arapaima is a very stocky fish. So if a 2,65m long arapaima weighs already 185kg, is it really realistic that one of 4,5m which is nearly two times the length, would weigh only some kilogramms more? No, it is far away from realitiy, because weight increases with the cube and such a fish would have the proportions of an eel and would be only skin and bone. A hypothetical arapaima of 4,5m would weigh at least 970kg, or even more, as many fish show a tendency to become stockier with increasing size. This is really strange, because there are never any mentions of a weight more than 200kg. [/FONT]
[FONT=Times New Roman Standard, serif]The question is now: How big grow arapaimas really?[/FONT]
[FONT=Times New Roman Standard, serif]A zoologist named Karl Heinz L[FONT=Times New Roman Standard CE, serif]ü[/FONT]ling, who reseacherd this question, found out, that the actually largest ever recorded arapaima from South-America was “only” 2,32m in length and weighed 133kg. The length-weight-relations are nearly identical to those of the 2,63m-specimen.. The arapaima has a very wide range, and even keeping in mind that this species is in many regions overfished, so that old and big specimens are becoming scarce there, it would be really strange if there was never an authenticated fish which was actually close to the biological maximum size this species can reach.[/FONT]
[FONT=Times New Roman Standard, serif]During the last decades, several million arapaimas were caught, even in very isolated regions, where there was never something as commercial fishery. This fish is very fast-growing and can reach large sizes in a comparably short time, and it seems that it also don[FONT=Times New Roman Standard CE, serif]́[/FONT]t live unusually long, so there should be always at least some individuals which managed to reach their full size during their lives.[/FONT]
[FONT=Times New Roman Standard, serif]So from where are the wide-spread dates of 4,5m long arapaimas? They came from a german naturalist named Schomburgk (not to confuse with the famous german zoologist Hans Schomburgk which lived from 1880-1967) who wrote in 1936 about his voyage to Guyana that the arapaima is said to reach a length of 4,5m and a weight of 200kg. He has never seen himself such a fish, and there was also never a recorded specimen of this length, how you can sometimes read. Schomburgk was only told by members of native tribes that the arapaima could grow that big. Thousands of authors used this dates since this time without any critical research. That means that there was never an actually recorded 4,5m long arapaima, and this dates are nothing than hearsay. It is even possible that this was nothing than a translation-error between Schomburgk and the native-tribes, who spoke very probably not the same language as Schomburgk.Given the fact that there is actually a recorded arapaima from Thailand, which weighed 185kg, it is possible that the largest ever living arapaimas in South-America actually reached a weight of roughly 200kg, but even at this weight, they would be only somewhere between 2,70m and 2,80m. [/FONT]
[FONT=Times New Roman Standard, serif]It is interesting that there are some arapaimas from Thailand which are much larger than the actual record specimens from South-America. But those fish came all from fishing-ponds, where they could grow with plenty of food and without any species-specific parasites or diseases which would stunt their growth in their original habitat. Many fish can reach comparably easy record-sizes in man-made ponds withouth strong competition or predators, and especially with alltimes-available food. [/FONT]
[FONT=Times New Roman Standard, serif]As summary you can see that the stories of 4,5m arapaimas are nothing more than tell-tales, but this should not make you forget that this is still a really immense fish-species. But it is neither the longest nor the heaviest freshwater fish of the world. The european catfish or wels Silurus glanis has a similar background of highly tell-taled exagerated sizes, but the actual authenticated record specimen was with a length of 2,78m at least longer than the largert known arapaima, although it was with 144kg not as heavy. The world-record Mekong giant catfish Pangasianodon gigas was at a length of 2,7m nearly of the same length as the world-record arapaima, but with 293kg much heavier. There are also some other freshwater species which can surpass the arapaima in length and/or weight, so the arapaima is not more actually entitled “largest freshwater fish of the world”.[/FONT]


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The largest arapaima about which I could find valid information was this monster with a length of 2,65 m and 185kg which you can see here: http://www.megafishingthailand.com/content/view/158/58/
I don´t doubt that they have the potential to grow even bigger, but there is surely a limit. I would be very interested in your source about the speciemens over 3m. Were they only spied or really caught? Big fish are very often highly overestimated in size if they are still in the water, and very often even on land. And we still have the sad phenomen of exageration.
It is true, the relation between length and weight differs always among different members of a species, but only to a distinct degree. It is for example just impossible, that a 4,5m arapaima would weigh only a little bit more than an authenticated specimen of "only" 2,65m. The weight of specimens with the same length can perhaps differn +/- abourt 50%, but surely not for several hundred percent. And keep in mind that a specimen which shows excessive growth will hardly ever look like it would be starving. A 4,5m arapaima with 200kg would be as impossible as a 4,5m human with only 200kg.
As the breeding of arapaimas in Asia is a comparably recent development, we have surely still not seen the maximum of the size they can reach. But what I wanted to show was mainly that those old dates of 4,5m and 200 kg are not just impossible, but also have no base in actual facts, and that you will never find an arapaima in south america which is over 3m.

I would really like to know where you have seen an arapaima of 4,2m, or even better see a photo of it. Even if you search for a long time, you can´t find a single photo of an arapaima which exceeds at least 3m. And even at asian fishing ponds specimens like the one with 185kg are true sensations, and very far away from being normal (even if this is not the maximum biological size they can possibly reach). So if 5m are not really a stretch, why are there no photos of arapaima of at least 3m? And keep in mind how huge such a fish would be, it would have a weight of more than 1000kg.
Species like the giant freshwater stingrays and also several species of freshwater sturgeons grow doubtless longer than the arapaima, and its weight is also surpased by species like the giant Mekong catfish.

The giant arapaimas in Thailand were released in the 90ies, that means they had already a very long time to grow. If they would have really the potential to outgrow 3m with ease, they had reached this size since many years. But even the 185 kg monster was only a bit over 2,5m.
So why are there nowhere photos and actual records of bigger ones? Why aren´t there masses of monsters around 3m. I can not only find photos of arapaimas over 3m but also none with at least 3m? A size of 2m seems to be comparably common, but surely not 3m. Why couldn´t they find during the research a single specimen over 2,32m among many thousands?
And keep in mind that the conditions in aquariums are not the same conditions as those in nature. Many species with a good growth potential can reach sizes much bigger than the average sizes in nature if they are kept in aquariums or ponds (many sturgeons for example which can grow extremely fast with enough food).
And there are extremely large freshwater sturgeon, for example the giant chinese paddlefish Psephurus gladius. Even if the often-cited length of 7m as maximum is extremely doubtfull, they can still grow very large. Just last year a 3,6m long specimen with a weight of 250kg was caught and sadly killed, and there are (or were) bigger ones. And some of the giant freshwater stingrays can reach more than 4m, even if most of their length comes from the tail, but they are still very heavy.

The arapaima is very big indeed, I don´t doubt that they can reach sizes about 2,5 and perhaps even 3m. But this doesn´t mean they can reach every size.
And you are not correct, most fish don´t grow for their whole lifes, this is a widespread myth. Many, especially smaller fish nearly stop their growth after some time, and even in the few species which show visible growth for many years, the growth will slow down more and more.