Saturday, 6 August 2011

A Tour through the Aquarium: American Paddlefish

The first large tank in the Aquarium is dedicated to a variety of ‘primitive’ fishes, whose ancestors split from the ancestors of more modern fish such as perch and carp in the distant past. Still surviving today, the Chondrostean fish are characterised by a great reduction in bone and a skeleton that is mostly cartilage. They also often have a shark-like heterocercal tail, and were at one time thought to be close to the sharks. It is now plain that they are modified ray-finned fish, and the surviving forms are not necessarily even closely related to each other. We have two species at Bristol, the Sterlet (a small European species of sturgeon) and the subject of this article, the American Paddlefish Polyodon spathula.

Until 2003 there were two species of paddlefish, the American and the Chinese. No Chinese paddlefish (which grew much larger than the American species) have been seen for several years now and the species is sadly almost certainly extinct. The cause of its demise is probably water pollution and the dams on the Yangtze, the only place it lived.

The American paddlefish is in slightly better shape, as re-stocking of some areas has been started. It is extinct in Lake Erie however, and overfishing of this very slow-growing fish has severely restricted the population in many areas

The natural home of the paddlefish is the Mississippi river drainage system, where it lives in slow moving, deeper waters. They breed towards the heads of rivers, often migrating hundreds of miles. They lay their eggs in midwater over rock or shingle, and when the adhesive eggs hatch they young are swept down river to mature. They probably take at least eight years to reach adult size of over 1.5m, and may live as long as 50 years – possibly more.

The most distinctive feature of the paddlefish is its enlarged rostrum. This has several functions, mostly associated with feeding. Paddlefish are primarily filter feeders, although they will also feed on larger food items at times, and swim with their mouths open using their gill rakers to trap zooplankton. It is thought the rostrum acts as a stabilizer while they do this, helping to keep the head in the right position for most efficient feeding. The rostrum also contains electoreceptors, which may be used for finding large concentrations of plankton, or navigation in murky water.

Commercially, paddlefish caviar, similar to that from sturgeon, has been a prized delicacy. As a result, the paddlefish is now CITES Appendix 2 listed, which means that a licence is required for trade to occur. They can grow quite fast in the appropriate circumstances, and an arrangement has been reached in the US where a single licensed producer commercially raises paddlefish to a suitable size then leases them to owners of reservoirs and similar water bodies to be grown on. When the fish reach a suitable size for harvest, either for meat or caviar, the owner of the reservoir receives some commercial benefit. As the paddlefish feed on the natural zooplankton in the reservoir, they need no additional feeding. Paddlefish caviar retails at around $67 for a 1.5oz jar, so producing it is a profitable business.

Unfortunately, people in other parts of the world have also experimented with paddlefish farming. In recent years, many have been caught in the lower Danube in Europe, and reportedly from rivers in Russia. It is not yet clear whether these are recent escapes from fish farms, or if they are a self-sustaining introduced population. As they do not take bait, removing them if it becomes necessary could prove extremely difficult, depending on whether they are spawning in a single site or not.

(images from wikipedia, USGS)

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