Growth, Fecundity, and Diets of Newly Established Silver Carp in the Middle Mississippi River

—The silver carp Hypophthalmichthys molitrix has spread throughout the Mississippi River drainage. During 2003, we determined its population status and potential impact in the middle Mississippi River (MMR), the conduit between the lower Mississippi River and the upper Mississippi, Missouri, and Illinois rivers. We quantified growth, age structure, fecundity, and diets of silver carp sampled with trammel nets and AC electrofishing in main-channel areas. Mean length at age in the MMR exceeded that of populations in Asia by as much as 26%. Individuals were typically more than 1 year old and 230 mm total length, suggesting that small, young fish were absent. Individuals in this population matured earlier (age 2) than in the species’ native range. Regardless of phytoplankton variation (using chlorophyll a as a surrogate) and zooplankton concentration at MMR sites, phytoplankton was consistently most abundant in diets. Silver carp are finding suitable resources within the MMR, allowing individuals to grow rapidly during early life, persist as adults, and successfully disperse upstream. Successful introductions of nonnative fishes in North America are increasing exponentially (Fuller et al. 1999). One notorious, well-established group is Asian carp (e.g., common carp Cyprinus carpio, bighead carp Hypophthalmichthys nobilis, and grass carp Ctenopharyngodon idella), which are now widely distributed across the United States (Rahel 2000). In the Mississippi and Missouri rivers, common carp contributed more tonnage to the commercial catch than any other fish species for 37 of 47 years of records (Pflieger 1997). While the impacts of other Asian carp remain to be seen, the likelihood of benefits to the native species is remote (Laird and Page 1996). A member of this group, silver carp H. molitrix, is large bodied and omnivorous (Spataru 1977; Bitterlich 1985a, 1985b). Its native range includes several major Pacific drainages of eastern Russia south to northern Vietnam (Fuller et al. 1999). However, it has been widely introduced across the world for food. In 1973, it was introduced in Ar* Corresponding author: [email protected] Received June 17, 2004; accepted June 8, 2005 Published online October 11, 2005 kansas ponds (Henderson 1976) and then propagated and distributed across the state by several state and private fish hatcheries. By 1980, silver carp occurred in the public waters of Arkansas, Louisiana, and Kentucky, apparently all escapees from fish hatcheries (Freeze and Henderson 1982). This species was discovered in the Missouri River in central Missouri in 1982 (Pflieger 1997) and in the Mississippi River drainage in southern Illinois in 1983 (Burr et al. 1996). By 1999, this species occurred in at least eight states within the Mississippi River drainage (Fuller et al. 1999). With spawning requirements similar to those of the previously established bighead carp and grass carp, it was unlikely that natural reproduction could be avoided (Burr et al. 1996). Age-0 silver carp occurred in a ditch in the Cache River, Illinois, drainage (Burr et al. 1996), providing the first evidence of successful spawning in the United States. Larval silver carp have also been captured in backwaters of Pool 25 of the upper Mississippi River (Garvey et al. 2003; Adams 2004). How silver carp may affect native fish species in the United States is a concern (Etnier and Starnes 1993; Laird and Page 1996; Pflieger 1997; Fuller et al. 1999). Its ability to consume detritus, phytoplankton, and zooplankton, combined with its high-consumptive capacity, may negatively affect other aquatic organisms in the Mississippi River drainage. Early life stages of many fishes rely heavily on zooplankton, rendering them susceptible to competition. Further, gizzard shad Dorosoma cepedianum, an important forage species for waterbirds and predaceous fishes (DeVries and Stein 1990), has similar omnivorous behavior as silver carp and may be particularly vulnerable (Pflieger 1997). To determine the potential impact of silver carp, basic information about its life history and