Paddlefish (Polyodon spathula)
|Also known as:||American paddlefish, duckbill cat, Mississippi paddlefish, spadefish, spoonbill cat|
|Size||Length: up to 221 cm (2)|
|Weight||up to 90.7 kg (2)|
Classified as Vulnerable (VU A3de) on the IUCN Red List 2004 (1) and listed on Appendix II of CITES (3).
The paddlefish has a highly distinctive appearance, so named for its peculiar, elongate, paddle-shaped snout, which may act as a sensory organ or help to channel plankton into the mouth (2) (4). The paddlefish’s genus name, Polydon, derives from a Greek word meaning ‘many tooth’, referring to their hundreds of gill rakers, specially adapted to their method of filter-feeding plankton (5). This large fish is grey to blue-black, paler underneath, lacks scales, and has a deeply forked, shark-like tail (5) (6). Males are generally larger than females (7).
Currently found in 22 U.S. states that are part of the Mississippi River basin (7), including the Missouri River into Montana, the Ohio River, and their major tributaries (2).
A freshwater fish (although capable of surviving in brackish water) that generally inhabits slow-flowing water of large rivers, usually at depths greater than 1.3 m (2) (7). Access to areas with sand or gravel bars is required during migratory breeding events (7).
The paddlefish is one of the few freshwater fish to feed by straining plankton from the water, which it does by sweeping through the water with its lower jaw dropped and the sides of the head inflated (4), allowing it to filter feed and ventilate its gills simultaneously (7). The large snout is covered with electroreceptors used to gather information about the surrounding environment, including locating prey (7).
The peak breeding season occurs in spring, during which large shoals of paddlefish migrate upstream and congregate in specific breeding areas to spawn. Spawning appears to require very specific environmental requirements and therefore generally only occurs every two to three years based on environmental stimuli (7). Single females can lay a huge number of eggs, from 300,000 to 600,000 (8), after which no parental care is invested and many will die (7). Males attain sexual maturity in around seven years, females in nine to ten (9). These relatively long-lived fish may live up to 55 years, although the average lifespan seems to be around 20 to 30 years (7).
The paddlefish has suffered heavily from legal and illegal harvesting in the past, due to its valuable meat and eggs, which are sold as caviar. However, the species is now threatened more by habitat destruction and river modification as a result of dams throughout the Mississippi River basin (7). Dams have helped eliminate traditional spawning sites, interrupt natural spawning migrations (9), and separate paddlefish populations, which limits gene flow and therefore genetic variability (7). Additionally, agricultural development along much of the Missouri, Ohio, and Mississippi rivers and their tributaries has increased soil erosion, and the fertiliser and pesticides used run off into these rivers. These problems are combined with severe industrial pollution and municipal waste in a number of areas across the fish’s range. Although much of the caviar and meat produced now comes from farmed paddlefish, commercial harvest was still permitted in six states as of 1997, and sport harvest in 14 states (9).
The historical decline of paddlefish has led to greater regulations on paddlefish harvesting (7). The Mississippi Interstate Cooperative Resource Association (MICRA) was set up in 1991 to address the fishery management issues in the Mississippi River Basin, with several states actively participating in the five year MICRA paddlefish project by stocking rivers with this species (1). Indeed, over a million hatchery-reared juveniles have been tagged and released since 1994, helping to bolster current numbers and to provide valuable data (9). The results of tag monitoring and harvesting reports indicate that the total population of paddlefish exceeds 10,000 individuals and can sustain current harvesting levels (1).
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- Electroreception: a biological ability to sense electrical impulses and fields through a series of electroreceptor sensory organs, often found in sharks, skates and rays.
- Gill rakers: a series of bony, comb-like projections located along the front edge of the gill arch.
IUCN Red List (May, 2012)