Paddlefish (Polyodon spathula) COSEWIC assessment and status report: chapter 3

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Species Information

Name and classification

Common Name

The order Acipenseriformes contains two extant families, and one fossil family of primitive fishes. The extinct family Chondrosteidae is known only from fossil evidence from the Lower Jurassic to the Lower Cretaceous periods (Scott and Crossman 1973). The sturgeon family (Acipenseridae) consists of 23 species in four genera of freshwater and anadromous fishes of the northern hemisphere (Scott and Crossman 1998). Five of the 16 species in the genus Acipenser (which dates back to the Upper Cretaceous Period) are known from Canadian waters: white sturgeon (A. transmontanus), Atlantic sturgeon (A. oxyrhynchus), green sturgeon (A. medirostris), lake sturgeon (A. fulvescens), and shortnose sturgeon (A. brevirostrum). The paddlefish family (Polyodontidae) consists of two extant monotypic genera. The paddlefish, Polyodon spathula is native to North America, and the Chinese paddlefish, Psephurus gladius, to China (Dillard et al. 1986). Paddlefishes are one of the oldest known fishes, having first appeared some 300 to 400 million years ago. At least one fossil genus (Crossophilis) is known from the Eocene Green River Shale deposit of Wyoming.

Morphological description 

Paddlefish are primitive Chondrostian (fishes having a persistent notochord and largely cartilaginous skeleton), ray-finned fishes distinguished by their large mouth and elongated, flattened, paddle-shaped snout or rostrum (hence the common name), which may comprise up to half the length of the body. They have skeletons composed mainly of cartilage, skins with a few reduced rhomboid scales on the up-turned (heterocercal) tail fin, with the upper lobe larger than the lower (Figure 1). There is also a long, pointed opercular flap that reaches nearly to the pelvic fin. Males can be distinguished from females by the raised urogenital papilla (Trautman 1981; Becker 1983; Scarnecchia and Schmitz 2003).

The paddlefish is a uniform dark blue-grey to black on the back and side, with a lighter underside. The average size range is 0.5 to 1.2 m in length and the average weight ranges from 0.9 to 9 kg. Larger fish may measure up to 2 m in total length and weigh more than 80 kg (Becker 1983, Trautman 1981; Dillard et al. 1986; Parker 1988).

Genetic description

The genetic population structure of the paddlefish in Canada is unknown. Three genetic strains are currently recognized in the United States (U.S.): the upper Missouri river basin in Montana and North Dakota, the Missouri and Mississippi River basins in the south central and central U.S., and the Alabama River system (NatureServe 2007). However, there is a low level of genetic variability among populations, and the potential for polytypy is probably low as populations in the different river systems have only been relatively recently isolated (Carlson et al. 1982; Starnes 1995).

Designatable units

All Canadian specimens occurred within the Great Lakes-Upper St. Lawrence Biogeographic Zone of the Freshwater Biogeographic Zone classification adopted by COSEWIC. There is no evidence to support the identification of designatable units below the species level.

Eligibility

The paddlefish is recognized as a species native to North America (Nelson et al. 2004) and Canada. It appears that they were once more widely distributed (Hubbs and Lager 1958; Eddy and Underhill 1974; Burr 1980; Trautman 1981; Cooper 1983; Hubbs et al. 2004), but never common in the Great Lakes. Canada was never a significant part of the distribution, and the species has disappeared from the Great lakes Basin, as well as other highly peripheral areas such as Maryland, Michigan, New York, North Carolina and Pennsylvania (Graham 1997), which probably never represented a significant part of the range (Cooper 1983).

Very few specimens were ever collected in Canada, and the last was in 1917. Therefore it is difficult to determine if these were part of a once larger population (Reid et al. 2007). Aboriginal Traditional Knowledge might prove useful in answering the question, since this unique fish would not have gone unnoticed. However, at the time of writing such information had not been made available.

The onus is not to prove that it was native to Canadian waters, but that it was not. At this point in time, since there is no firm evidence to the contrary, one cannot, without negating the precautionary principle, definitively state that it was not native to Canadian waters.

The issue was best summarized by Becker (1983) who considered the species to be native to the fauna of the Great lakes, but encountered on their way to natural extirpation.