Paddlefish

The paddlefish (Polyodon spathula) is a large freshwater fish native to the Mississippi River basin of North America, and one of only two living species of paddlefish worldwide (the other, the Chinese paddlefish, is now believed extinct). Despite a superficial resemblance to sharks, paddlefish belong to Acipenseriformes, the same ancient order as sturgeons, and are true bony fish with a mostly cartilaginous skeleton retained from an early evolutionary lineage. They are best known for their long, paddle-shaped snout, or rostrum, packed with electroreceptors used to detect plankton swarms. Paddlefish populations have declined across much of their range due to dam construction and habitat loss, and the species is now considered vulnerable in parts of its range.

Paddlefish are readily identified by their extraordinarily long, flat, paddle-shaped snout, which can measure roughly a third of total body length.

• Smooth, scaleless, grey to bluish-grey skin, paler underneath
• Long, shark-like tail fin (heterocercal, with the upper lobe larger than the lower)
• Very large gill covers extending back to a pointed flap
• Small eyes positioned well back near a wide, toothless mouth
• Adults typically 1.5-2 m long, with a maximum size exceeding 2 m and over 45 kg

Its paddle-shaped rostrum and lack of scales distinguish it immediately from sturgeons, which have bony scutes and a shorter, conical snout.

Paddlefish live in large, slow-moving rivers, reservoirs, and connected backwaters throughout the Mississippi River basin, from the Great Lakes region and Missouri River south to the Gulf of Mexico. They favor deep, open water with moderate current, often congregating near river confluences, oxbow lakes, and areas with strong plankton production. Water temperatures in occupied habitat typically range from 15-25C. Paddlefish require long stretches of free-flowing river with suitable gravel bars for spawning, making them sensitive to dam construction, which has fragmented and eliminated populations across much of their historic range in North America.

Paddlefish are filter feeders that swim with their mouths open, using their paddle-shaped rostrum, covered in thousands of electroreceptors, to detect the faint electric fields produced by swarms of zooplankton, which they filter through fine gill rakers. They are generally solitary or loosely aggregated, undertaking long seasonal migrations upstream to gravel bars for spawning in spring when rising, warming water triggers reproduction. Females release eggs that are fertilized externally and left unguarded, drifting downstream before hatching. Paddlefish can live over 30 years and grow slowly, reaching sexual maturity only after 7-10 years, making populations slow to recover from disturbance. Their filter-feeding role links them closely to plankton productivity in large river ecosystems.