Disease or Predation
Fish pathogens have the potential to produce severe epizootics of clinical disease but also are known to exist in a carrier state. They include viral, bacterial, and parasitic agents. In some instances, disease outbreaks can severely deplete local populations, but these extreme events have not yet been documented in wild pallid sturgeon populations. Some pathogens of notable importance for pallid sturgeon recovery are a rhabdovirus, Viral Hemorrhagic Septicemia Virus (VHSv), and the Missouri River sturgeon iridovirus (MRSIV).
Viral Hemorrhagic Septicemia Virus is a fish disease that has caused large-scale mortalities in numerous species (Kim and Faisal 2010) and has been described as an “extremely serious pathogen of fresh and saltwater fish” (APHIS 2006). While it has not been documented to affect pallid sturgeon, it also has not been found within the range of the species. However,VHSv has been documented in the Great Lakes (APHIS 2006). Various shipping canals have created a connection between the Great Lakes and the Mississippi River so it is possible that through time, this virus could reach areas occupied by pallid sturgeon. Because this pathogen can cause large- scale mortalities in fish populations, and it has a wide range of potential carriers, we believe it is important to monitor for VHSv within the range of pallid sturgeon.
Missouri River sturgeon iridovirus is a concern in the context of pallid sturgeon recovery because it causes mortality in hatchery-reared pallid sturgeon (Kurobe et al. 2011) and its effect on free-ranging sturgeon populations is unknown. The MRSIV was originally documented during artificial propagation efforts of shovelnose sturgeon at the Gavins Point National Fish Hatchery in 1999. However, this iridovirus also can infect pallid sturgeon (Kurobe et al. 2011). This disease is known to cause substantial mortality in hatchery-rearing environments (Kurobe et al. 2011). Study fish surviving initial viral outbreaks still harbor the virus even though they may appear healthy (Hedrick et al. 2009; Kurobe et al. 2011). While initially identified in a hatchery environment, additional testing has documented that this virus is found in the wild; of 179 Scaphirhynchus tested from the Atchafalaya River between November 2003 and May 2004, 8 (4%) were identified as positive for the virus and 5 (2.8%) were suspected of carrying the virus. Subsequent testing with more sensitive methods also confirmed the presence of the virus in the wild, suggesting that it may be endemic in portions of the species’ range (Hedrick et al. 2009). The effect of the virus on wild populations is not known.
Little information is available implicating piscivory as a threat affecting the pallid sturgeon. Predation on larval and juvenile fishes of all species occurs naturally. However, habitat modifications that increase water clarity and artificially high densities of both nonnative and native predatory fishes could result in increased rates of predation. Pallid sturgeon larvae and fry passively drift post-hatch (Kynard et al. 2007; Braaten et al. 2008). This behavior likely exposes naturally-spawned pallid sturgeon to predation but was moderated historically by high fecundity and turbid waters. However, anthropogenic changes that affect habitats could result in increased vulnerability to predation. In the impounded areas of the upper Missouri River, larvae may be transported into the clear headwaters of reservoirs like Fort Peck and Lake Sakakawea. These reservoirs are or have been artificially supplemented with predatory species like walleye (Sander
Maintaining artificially elevated populations of certain species in these reservoirs has been hypothesized as a contributing factor in poor survival of larval and juvenile pallid sturgeon.
Walleye and sauger (S. canadensis) are capable of eating wild paddlefish up to 167 mm (6.6 inch (in.) body length (305 mm/12 in. total length) and, thus, likely could consume naturally-produced pallid sturgeon larvae, fry and fingerlings (Parken and Scarnecchia 2002). When looking at data for sample locations closest to reservoir headwaters, it appears that no age-0 paddlefish were found in walleye, but were present in sauger, a native species closely related to walleye. Yet, Braaten and Fuller (2002, 2003) examined 759 stomachs from
7 piscivore (fish eating) fishes in Montana, they found no evidence of predation on sturgeon. Predation vulnerability of pallid sturgeon (> 40 mm) by channel catfish (Ictalurus punctatus), smallmouth bass (Micropterus dolomieu), and walleye appears to be low, provided other prey species are available (French 2010; French et al. 2010). More data are needed to adequately assess predation effects on eggs and larval pallid sturgeon in order to evaluate implications on recruitment success (see also Invasive Species/Aquatic Nuisance Species under Factor E Other Natural or Manmade Factors Affecting its Continued Existence).
Factor C Summary
At the time of listing, neither disease nor predation was discussed as a threat, primarily
due to limited information. New data have highlighted both disease and predation as issues of potential concern and should be considered as likely threats. At this writing, data are inadequate to quantify the magnitude of the threat either may impose.