Saturday, 10 September 2011

Aquarium tour: It came from the Amazon

In the centre of the aquarium is our walk-through tank. One of the first in the world, though now outclassed by many newer buildings, it was actually created from an old 19th century bear pit. That in turn was a modification of the original building, one of two lime kilns that were on the site before it was acquired by the zoo back in 1835. A great deal of the buildings in Clifton and the surrounding area were built with cement made on the Bristol Zoo site.

There are various species of Amazonian fish in the walk-through tank, but the largest are our Pacu, Colossoma macropomum. As you might guess from the name, they are very large fish, about 1m long and probably weighing around 30kg. They are actually fairly closely related to their smaller, but more famous, cousins, the various species of predatory piranha, but unlike them are omnivorous, with a strong preference for vegetation and fruits of trees.

The name Pacu is in a native Brazilian language, but incorrectly applied to Colossoma sp, in English. The correct local name for the fish is tambaqui. Pacu is the name given to smaller, but related, species such as Metynnis.

Pacu are one of those species which really should not even be considered for home aquaria. In the UK, fish stores typically do not even stock them, but in the US it is more frequent for them to be sold, usually at a length of 5cm or so. The inevitable result is that they rapidly outgrow their accommodation, and either die young or are dumped. In warmer parts of the US they could probably survive in the wild, so there are stringent laws against this. Even though the trade in this species in the UK has now mainly ceased, occasional youngsters turn up mixed with other species, and there are still some in private hands, so the result is that any public aquarium in the UK is contacted at least once and often several times a month by people wanting to rehome overgrown fish of this or other species. Part of the problem is a persistent myth that fish grow to fit the size of their accommodation – this is simply not true. What does happen is that fish in improper accommodation suffer stunted (but still massive) growth and die young, which is not the same thing at all. I am unaware of any estimates of the potential lifespan of a Pacu, but 20 years+ is probably quite possible, so if you are unprepared to provide a heated pool for this length of time do not buy them.

In the wild, pacu are solitary except when breeding. When the forest floods they disperse widely, feeding on fruit falling from the trees and helping to disperse the seeds. As the seeds take longer to pass through their guts than more well-studied dispersers such as birds and primates, the seeds can travel long distances – probably several kilometres, whereas mammals and birds typically only spread seeds a few tens of metres. In the dry season they are confined to the main river channels, and survive lean periods by relying on their extensive fat reserves.

The pacu’s vegetarian diet and high fat content makes for a highly edible fish, and they are extensively wild-caught and these days also aquacultured. Overfishing is a potential threat, and in heavily fished areas large individuals are becoming rare. As an aquaculture subject they have great potential, as unlike most such fish, which are fed a diet of animal protein (salmon for example are fed fishmeal), the pacu can be raised on an all-vegetable diet, which is much less wasteful. Unfortunately, they are also now being raised outside their native range, such as in India and other parts of South East Asia, and this raises the possibility of escapes establishing themselves in the wild, to the detriment of native species. This has only taken off in the last 5-10 years, so much still remains to be learned about best practice. In the meantime, if anyone reading knows what they taste like, please leave a comment.

(image from wikipedia)

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