Plight of the Paddlefish, by Don Orth

If we were able to travel back in time to the Cretaceous period, the world would appear very different to us.  We would not recognize any of the first flowering plants that appeared in the early Cretaceous period.  We would not recognize our continental boundaries -- the supercontinent of Pangea was breaking apart.  A broad shallow sea covered central North America at the time.   We might recognize some of the dinosaurs, pterosaurs, and ichthyosaurs of the period, but only from museums or the Jurassic Park movie.   However, we would recognize the Paddlefish.   Paddlefish fossils appeared in Cretaceous and Paleocene formations and are clearly Paddlefish.  The American Paddlefish Polyodon spathula is as distinctive as its fossilized ancestors. It can reach 2.1 m (7 feet) and weigh as much as 74 kg (163 pounds).  Only one species of Paddlefish exists in North America.  The only other paddlefish in the world, the Chinese Paddlefish Psephurus gladius, is most likely extinct; no Chinese Paddlefish has been captured since 2003 (Dudgeon 2011).  Therefore, the plight of this unique large freshwater fish must be dealt with effectively. 

 Historical drawing of the Paddlefish from the U.S. National Museum.

Johann Julius Walbaum, a German physician and naturalist, described the Paddlefish in 1792.  Because of the cartilaginous skeleton, the paddlefish was first classified as a shark.  Walbaum named it Squalus spathula, thinking it was closely related to dogfish sharks.  Later the French naturalist Bernard Germain de Lacépède would name the genus, Polyodon.  Polyodon, is derived from the Greek root words meaning “many teeth.”  The basic body form is common to ancestral acipenseriform fishes since the Jurrasic.    The body is fusiform and tapers in the tail region to an asymmetric heterocercal tail.  The skeleton is mostly cartilaginous with minimal calicification.  Only small modified scales appear on the caudal fin base.

The Paddlefish has a very large mouth and a long, paddle-shaped snout that is one-third of its body length.   The paddle, or rostrum, is filled with electrosensory receptors that can detect the weak electrical fields generated by a swarm of zooplankton. The small eyes, numerous slender gill rakers (="many teeth"), and long tapering operculum flap are adaptations for a filter-feeding life in large, often turbid, rivers and oxbow lakes. The electrosensory pores on the rostrum help the Paddlefish orient to abundant zooplankton.  The numerous fine gill rakers help it feed efficiently on numerous small zooplankton, such as cladocerans and copepods. 

Paddlefish rostrum and close-up of electrosensory pores (Helfman et al. 2007)

The Paddlefish swim with its mouth open to filter feed on abundant zooplankton. Their first five years are all about growing to a large body size and they mature late and can live a long life, if not interrupted by fishing.  After five years, Paddlefish increase in body weight rapidly.   Many populations of Paddlefish have been studied to examine individual growth patterns and breeding patterns.  The dentary bone (i.e., lower jaw) can be sectioned to expose annual rings.  The oldest Paddlefish specimen was aged at 56 years old!  

 

Gill rakers of Paddlefish (left), photo by John Lyons.

Transverse sections of dentary bones (right). Photo from Adams (1942).

The Paddlefish is a migratory species that occurs in all large rivers of the Mississippi River as well as large Gulf Coast tributaries.   These are working rivers that have been highly modified for navigation and flood control.  The dams are barriers to migration and modify flow levels that serve as migration cues.  Navigable river channels have been dredged, deepened, and straightened, modifying habitats for Paddlefish.  Pollutants, contaminants, and waterway development continue to constrain populations of Paddlefishes.  The dcline in Paddlefish began in the early 1900s due to unregulated harvest.  Consequently, many states have research and management efforts to regulate harvest of populations. . 

Catch of Paddlefish from Illinois River, circa 1900 ( Forbes and Richardson 1920)

Paddlefish display strong spawning site fidelity and larges shoals of adults gather over clean gravel-cobble stream substrates for spawning.  Females delay spawning until they are between seven and ten years old and do not breed each year. Small eggs (2-3 mm) are produced by a single female and the total fecundity ranges from 9,000 to 26,000 per kg of female body weight.  That is why large female are so important to sustaining productivity of a population. A 20-kg female can produce 520,000 eggs and a 40-kg female produces over one million eggs.   However, these big old fecund female fish (BOFFF) are very rare in exploited populations.   

Fertilized eggs can hatch in 6-10 days while drifting downstream in the river currents; therefore, flow levels determine where larval Paddlefish will end up when they are ready to begin feeding.      Larval Paddlefish hatch at about 8-9 mm and have no paddle.  For a inside look at a Paddlefish larva, view this micro-CT scan.  The paddle begins to appear after larval metamorphosis and juveniles begin to resemble adult Paddlefish at 2 to 3 inches in length.   Recruitment of Paddlefish is usually better during years of high spring flows and recruitment failure is common during drought years.  

Larval Paddlefish SEM (Top)  Photo by William Bemis.  Underside and side views of juvenile Paddlefish, blue stained for cartilage.  Photo by M.C. Davis.

Management of Paddlefish populations is complicated because it requires coordination of efforts from many states that are responsible for this mobile species that does not stay in one state for its entire life cycle. Paddlefish freely move between political jurisdictions subject to differing management strategies and harvest regulations (Prachiel et al. 2012).   The Mississippi Interstate Cooperative Resource Association was organized in 1991 to improve interjurisdictional management.  Paddlefish is a major priority and increased knowledge is needed to manage this highly migratory and valuable planktivore.

  

When Paddlefish caviar sells for $24 per ounce (caviarlover.com), the demand for harvest will remain high even has catches are historical lows. Poaching remains difficult to stop and fraudulent sales of Paddlefish caviar as more expensive black caviar (Sturgeon) will no doubt continue.  Several different management strategies are in play.  One is stocking of juvenile Paddlefish since much is known about raising Paddlefish in captivity (Mims and Shelton 2015).   Some believe that Sturgeon and Paddlefish aquaculture will almost certainly be the major source of caviar in the future. For now, however, regulating harvest to protect females is needed throughout the range.  Some commercial fisheries have been banned to protect small fragmented populations.  Recently Hupfeld et al. (2016) advocated adoption of a basin-wide minimum length limit of at least 810 mm (or 32 inches) on Paddlefish fisheries in the Mississippi River.  Other proposals include a harvest slot and maximum size limits. Many dams serve to concentrate Paddlefish during the spring migration; here they are highly vulnerable to popular recreational snag fisheries.  The Oklahoma Department of Wildlife Conservation (ODWC) piloted a recreational harvest permit for Paddlefish in 2006.  Permit holders can bring their catch for the ODWC staff to clean and fillet.  In exchange for fillets the ODWC retains eggs from the female paddlefish for caviar.    This program has earned approximately $14 million since its inception, while processing 30,000 Paddlefish caught by anglers. 

Commercial harvest of Paddlefish (Pikitch et al. 2005).

McIntire et al. (2016) used “triple jeopardy” to describe the precarious situation faced by migratory fishes, such as the Paddlefish.  Paddlefish need suitable habitats in the feeding and breeding habitats, as well as along the migratory corridors connecting these habitats. Lack of spring high flows means that Paddlefish do not have a strong cue for spawning and recruitment is reduced (Prachiel et al. 2012).   Alterations of large rivers for navigation change the migratory corridors increasing the energy demands for upstream migration.  Construction of dams without fish passage restricts movement to historical breeding grounds.   Loss of oxbows means fewer productive feeding habitats exist.  In addition to the “triple jeopardy,” the Paddlefish now co-occur with large Asian Carp populations.What influence will this abundant non-native planktivore have on the planktivorous Paddlefish?  Without wise management actions, we risk keeping fisheries for Paddlefish trivial compared to their historic potential and we risk the loss of the last Paddlefish species on the planet.  

References

Dudgeon, D. 2011.  Asian river fishes in the Anthropocene: threats and conservation challenges in an era of rapid environmental change.  Journal of Fish Biology 79:1487-1524.

Firehammer, J. A., and D. L. Scarnecchia. 2007. The influence of discharge on duration, ascent distance, and fidelity of the spawning migration for Paddlefish of the Yellowstone- Sakakawea stock, Montana and North Dakota, USA. Environmental Biology of Fishes 78:23–36.

Forbes, S.A., and R. E. Richardson. 1920. The fishes of Illinois.  Illinois Natural History Survey Division.  357 pp.  

Hupfeld, R.N., Q. E. Phelps, S.J. Tripp, and D.P. Herzog. 2016.  Mississippi River basin paddlefish population dynamics: implications for the management of a highly migratory species.  Fisheries 41(10): in press.

McIntyre P.B., C. Reidy Liermann, E. Childress, E.J. Hamann, J.D. Hogan, S.R. Januchowski-Hartley, A.A. Koning, T.M. Neeson, D.L. Oele, and B.M. Pracheil. 2016. Conservation of migratory fishes in freshwater ecosystems. In Closs G, Krkosek M, & Olden JD: Conservation of Freshwater Fishes.

Mims, S.D., and W. L. Shelton.  2015.  Paddlefish aquaculture.  Wiley Blackwell,  Hoboken, New Jersey.  298 pp.

Neely, B.C., B.M. Pracheil, and S.T. Lynott. 2014. Hydrologic variables predict harvest in a recreational paddlefish fishery. Fisheries Management and Ecology 32: 259-263.

Paukert, C. and G. Scholten editors. 2009.  Paddlefish management, propagation, and conservation in the 21st century: building from 20 years of research and management. American Fisheries Society, Symposium 66, Bethesda, Maryland.

Pikitch, E.K. P. Koukakis, L. Lauck, P. Chakrabarty, and D.L. Erickson. 2005.  Status, trends and management of sturgeon and paddlefish fisheries.  Fish and Fisheries 6:233-265.

Pracheil, B.M., L.A. Powell, M.A. Pegg, and G.E. Mestl. 2012. Swimways: protecting paddlefish through movement-centered management. Fisheries 37: 449-457.