Saturday, 13 August 2011

Aquarium Tour: Here be Dragons

Gold form S.formosus
Next door to the paddlefish aquarium is another large tank containing large fish from Asia. One of the most important species in the tank is some young Asian Dragon Fish, Scleropages formosus. They are especially significant because they are the results of what we believe to be the first breeding of this species in a public aquarium anywhere. Previous captive breeding has been in outside ponds, mostly in Asia.

Dragon fish belong to a very ancient group of fish, commonly referred to as the ‘bonytongues’, as their tongues are ossified and bear teeth. The group as a whole has representatives or close relatives in fresh waters on every continent, but the closes relatives of Scleropages are all in the southern hemisphere, suggesting that they originated in the supercontinent of Gondwana. How they got north of the equator into south east Asia is unclear – it is suggested they were carried on the Indian subcontinent as it moved north after the break up of Gondwana, but there are no Scleropages in India today, although there are several species of the similar knife fish Notopterus. Outside Asia, there are two species of Scleropages in Australia, but there has always been a sea barrier between Australia and Asia which these obligate freshwater fish cannot cross, so the phylogeography of Scleropages is still obscure. Given that the Australian species are almost identical to the Asian forms, the genus itself probably dates back to the late Cretaceous or even earlier.

The taxonomy of the Dragon fish is somewhat debatable. They certainly vary in colour over their vast range, with some forms being deep red, but mostly silver, sometimes with red fins. It is quite possible that these represent different species, or at least subspecies, which raises problems for their conservation.

Dragon fish and their relatives have numerous peculiarities. As with many fish that live in warm, oxygen-poor waters they can breath atmospheric air, using a vascularised swim bladder as a lung. They are mouth brooders, the males retaining the large eggs in their throats, and providing shelter to the young for some time after hatching.
S.formosus fry December 2008
Dragon fish and their relatives are all surface-feeding predators, taking insects from the surface, small fish, and anything flying over the surface. Mostly these would be dragonflies and similar, but given their size it would not surprise me for them to take swallows and martins as they fly over the surface as well. The South American Arowanas can leap 2 metres out of the water to take insects off overhanging branches, and Dragon fish can probably do the same.

Unfortunately, the Dragon fish is listed by the IUCN as Endangered. In the past they were greatly over-collected for the aquarium trade (the red form being especially prized). As a slow-growing fish with a low reproductive rate, it is susceptible to loss from fishing and habitat destruction. Although now listed on CITES Appendix 1, captive bred fish are available from licensed fish farms in Singapore and a few other countries. Each individual fish is microchipped and accompanied by appropriate liceneces and paperwork before being sold.

Given their large size (over 1m) and somewhat aggressive nature, at least with fish small enough to eat, most Dragon fish are kept singly. Our original parent fish actually originated as a customs confiscation, and lived for many years with us before any breeding behaviour was observed. After several occasions when eggs were laid but either not picked up by the male or did not hatch, improvements in the water in the tank and raising the temperature slightly resulted in successful breeding. Some of the eggs were removed from the male and hatched artificially, and some were left for the male to raise. Unfortunately, the parent-rearing did not succeed, but the artificial rearing resulted in 15 young in December 2008.

Now over 2 years old, nine youngsters are on show. They are still only around half grown, and will probably take at least another 3 or 4 years before they are adult size.

Large fish like these can be a problem for public aquaria. There are far to many juveniles of large fish sold in aquarium shops, and when they grow out they become a problem for their owners. There is a persistent belief that fish grow to the size of their aquarium – this is just not true. What does happen is that they suffer poor health, stunted growth, and die young, which is not the same thing at all. If anyone wants to keep one of these large fish, please think again unless you have a lot of spare cash – proper care for most of these would involve something the size of a Koi pond, with an added heating bill.

Next week – fish of the mangroves

(images from Wikipedia, Bristol Zoo)

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