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Fish Story

To the Editors:

In "The Imperiled Giants of the Mekong" (May–June), Zeb S. Hogan and his coauthors state that according to The Guinness Book of World Records the Mekong giant catfish is the world's largest freshwater fish. In the early 1970s I spent a month fishing on an island on the Volga River some 60 kilometers upstream from Astrakhan, and I clearly remember a report about a beluga fish caught earlier that summer that was some 4 meters long and weighed 1,500 kilograms. Clearly, these numbers dwarf those you give for the Mekong giant catfish (up to 3 meters and 300 kilograms).

An Internet search indicates that beluga can reach 5 meters in length, 2,072 kilograms in weight and 118 years in age. Another member of the sturgeon family, called kaluga, can reach 5.6 meters in length, 1,000 kilograms in weight and 118 years in age. On the other hand, giant catfish can reach only 3 meters in length and 350 kilograms in weight.

I am not saying that the beluga is the largest freshwater fish in the world, but it is definitely larger than the Mekong giant catfish. I would appreciate feedback from the authors regarding this question.

Dmitri E. Kourennyi
Case Western Reserve University

Dr. Hogan replies:

I appreciate the opportunity to answer this frequently asked question. Surprisingly, it is not 100 percent clear which species of freshwater fish is the largest. This uncertainty arises in part from our lack of detailed knowledge of the diversity of freshwater fish. (Many poorly studied fish live in exceptionally large, deep or remotely located water bodies.) We also need a clear definition of just what is a large freshwater fish. For our purposes, we defined "largest freshwater fish" as the largest fish in terms of biomass that spends its entire life cycle in fresh water and for which there are reliable records. The largest fish found in fresh water are indeed the sturgeons, but they obtain a majority of their growth in marine and estuarine environments, migrating into fresh water mainly to spawn. Even the hold that the Mekong giant catfish has on the largest–freshwater–fish title is tenuous, because weights of large fish are prone to exaggeration and error. Here are some other contenders for the title, with alleged top weights: the Mekong giant carp (300 kilograms), the Mekong giant stingray (500 kilograms), the arapaima and goliath catfishes of the Amazon (300 kilograms), the Gnooch of South Asia (300 kilograms) and the Wels catfish of Europe (once 5 meters and 300 kilograms, now smaller). Almost all of these species are now rare, so it is difficult to determine their maximum size. I am currently trying to determine which species is in fact the biggest, and I suspect the giant catfish will still come out as one of the top three! Interestingly, in terms of biomass and length, the largest freshwater fish may be the giant stingray of the Mekong, which may attain a weight of over 500 kilograms and measure more than 5 meters from snout to tip of tail.

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