WOLF Michael C.， HAUPT Kevin and PHELPS Quinton E.

(1. Department of Natural Resources， Fish and Wildlife Division， 204 Main Street East Baudette， Minnesota 56623， USA; 2. U.S. Fish and Wildlife Service， Carterville Fish and Wildlife Conservation Office 9053 Rt. 148 Suite A Marion， Illinois 62959， USA; 3. West Virginia University， Division of Forestry and Natural Resources， 307C Percival Hall， Morgantown， West Virginia 26505， USA)

Abstract: The establishment of a highly efficient planktivore， Silver carp (Hypophthalmichthys molitrix)， in the Mississippi River Basin has been documented as having negative impacts on native planktivore populations. Silver carp have the potential， to alter the trophic food web through negative interactions with native planktivores in large river systems. Due to increasing Silver carp ranges， interactions between Silver carp and other native planktivores is likely to increase. One abundant native planktivore that is often found in similar habitats as Silver carp is Gizzard shad (Dorosoma cepedianum). Potential competition for resources exists because of overlapping habitat use and forage. Given the similarities between these two planktivores， we sought to evaluate daily growth rates and determine hatch dates of age-0 Silver carp and Gizzard shad in the Middle Mississippi River. Silver carp and Gizzard shad were collected from the Middle Mississippi River using an electrified surface trawl from June through September with sizes ranging from 21—65 mm total length for Gizzard shad and 24—85 mm total length for Silver carp. A sample of age-0 fish (n=697) was collected at the end of the growing season including 292 Gizzard shad and 405 Silver carp. Size structure (total length in mm) of Silver carp (49.05±0.41;± SE) was significantly greater (T= –16.43， df=695， P＜0.001) than Gizzard shad (38.95 ± 0.45). Hatch dates were similar between Silver carp (July 26th to August 18th) and Gizzard shad (July 28th to August 19th). Growth of Silver carp (2.24 mm per day) was significantly greater (P＜0.001，T= –11.1， df=82) than Gizzard shad (1.72 mm per day). Mortality was similar between Silver carp (28.1%)and Gizzard shad (25.1%) at this early life stage. The faster growth rates in Silver carp result in Silver carp entering the winter season larger in size with likely an increased capacity to consume plankton and decreased predation ultimately leading to higher recruitment. Knowing that there exists an overlap between Silver carp and Gizzard shad diets and habitat could alone call cause for concern. Adding that spawning occurs at a similar time and Silver carp have the ability to outgrow Gizzard shad early in life leads to major concerns about food web balance. Knowing that invasive planktivores have already shown to alter food webs in aquatic systems in North America， extra caution should be taken.

The role that invasive species play in the extinction of native species is assumed but not always understood[1]. Research has shown that invasive species introductions can cause increased competition for native species including competition with the invader and native species[2]. While these competitive interactions can be complex， the results are typically negative for the native species[3]. More research is needed to better understand the impacts of invasive species.Specifically， in aquatic systems invasive species can impact native species both negatively and positively through predation， competition and other more complex relationships (i.e.， altering water chemistry)[4，5].One specific system that is highly effected by invasive species is the Laurentian Great Lakes where the introduction of Sea Lamprey (Petromyzon marinus)hindered several sport and commercial fisheries[6]. Increased competition can occur when the invasive species consumes a similar resource (i.e.， plankton).Wells[7]found drastic changes in the plankton community of Lake Michigan following the introduction of the Alewife (Alosa pseudoharengus).

In 1973， Silver carp (Hypophthalmichthys molitrix) were brought to Arkansas by a private aquaculturist[8]. At that time this was an unknown exotic species that was a low trophic-level filter-feeder creating an ideal candidate to improve water quality in aquaculture ponds[8]. By January of 1980， Silver carp were caught by commercial fisherman in a tributary of the White River， a Mississippi River tributary，flowing through two private fish hatcheries with Silver carp[8]. The Mississippi River’s highly connected systems with a diverse range of habitats are susceptible to invasion[9—12]. Rapid increases of Silver carp densities were observed following introduction in the Lower Mississippi River， Middle Mississippi River，Missouri River， Illinois River， and Ohio River some resulting in percent catches near 50% of all fish sampled[13—17].

Since their introduction， Silver carp have expanded their range and established dense populations as a result of their dispersal capabilities， reproductive output and environmental tolerances[12，18—22]. While some artificial barriers may serve to slow down Silver carp dispersal[23]they still appear to be expanding their range and establishing new populations[24]. Current management efforts focus on containing both Silver carp and Bighead Carp (Hypophthalmichthys nobilis)collectively referred to as Asian Carp[12，25，26]but further actions (e.g.， targeted harvest or added barriers)may need to be taken to minimize their negative impacts on native species. The Middle Mississippi River is a region of interest for Silver carp research because of the proximity to the Great Lakes， through the Illinois River shipping canal， and the Upper Mississippi River. Modeling conducted on the Great Lakes suggests Silver carp could establish in certain portions of the Great Lakes[27]. High densities of Silver carp exist in the Middle Mississippi River and the potential for rapid expansion call for focused research efforts to better understand the effects of Silver carp.

Silver carp are a large-bodied， highly effective planktivore that may disproportionately reduce plankton and shift zooplankton communities[28—30]. With their ability to deplete plankton and negatively impact native fishes there exists a distinct concern that Silver carp could degrade the populations of native filter feeders like Gizzard shad (Dorosoma cepedianum)[14，17，31—34]. Specifically Gizzard shad diets were found to have considerable overlap with Silver carp diets[32]. Previous research suggests that Silver carp introductions have decreased fish diversity and commercial catches of preferred fish[28，35]. There could exist a lag time between the introduction of Asian carp and the negative impacts observed in native fishes[17]. Additionally， Irons，et al.[14]found that Gizzard shad had reduced body condition following the introduction of Asian carp. Little is known about the early life history of these two possible competitors.Therefore， this study aimed to compare early life history traits; size structure， hatch dates， daily growth rates and mortality rates of age-0 Silver carp and Gizzard shad in the Middle Mississippi River.

1 Methods

In order to effectively obtain large sample sizes of late season age-0 Silver carp we developed an electrified surface trawl. We floated a 7.62 m treated nylon shrimp trawl (manufactured by Memphis Net &Twine Co.， Inc.) with 38.1 mm body mesh and 3.18 mm bag mesh by attaching a 0.76 m long 0.15 m diameter polyvinyl chloride (PVC) capped pipe to each 0.76 m by 0.38 m metal otter board and attaching a float rope and floats to the headrope. To electrify the trawl， we used methods similar to those described in Freedman，et al.[36]Essentially， we electrified the trawl by adding five 30-cm droppers (cathodes in DC mode and electrodes in AC mode) to one rope and a wire along the headrope (anode in DC mode and electrode in AC mode)[36]. Electrodes were attached to a Wisconsin physics box (made by ETS electrofishing Systems，LLC) connected to a Honda 6500-W generator.

The electrified surface trawl was used to collect both Silver carp and Gizzard shad in the Middle Mississippi River on August 29thof 2014. The trawl was deployed in an open water habitat and deployed for 3 minutes. The Silver carp and Gizzard shad were measured to the nearest mm. Five individuals of each species from every five mm group (20—25 mm，25—30 mm， 30—35 mm， etc.) were selected for age analysis. Sagittal otoliths were removed from Gizzard shad[37]and lapilli otoliths were removed from Silver carp[38]. Otoliths were mounted on microscope slides and the number of daily growth annuli was estimated. Hatch date was calculated by subtracting the annuli count from the date captured.

Size structure was compared between Silver carp and Gizzard shad using aT-test (α=0.01). Growth was estimated by subtracting the size at hatch from the size at capture and divided by the age in days result-ing in mm per day. Size at hatch was found in the literature， Silver carp=6 mm[39]and Gizzard shad=5 mm[37].Growth was compared between species by calculating the mean growth (mm/d) with aT-test(α=0.01). Mortality was estimated using the catch curve method and then compared between species.

2 Results

Age-0 fish (n=697) ranging from 21—85 mm were caught with the electrified surface trawl including 292 Gizzard shad ranging from 21—65 mm and 405 Silver carp ranging from 24—85 mm (Fig. 1).Size structure was significantly larger (T=–16.43，df=695，P＜0.001) for Silver carp (49.05±0.41) mm than Gizzard shad (38.95±0.45) mm

Mean total length (mm) differed (T=–16.43，df=695，P＜0.001) between Gizzard shad (38.95±0.45)and Silver carp (49.05±0.41)

Thirty-nine Gizzard shad were aged with ages from eleven to thirty-four days and forty-five Silver carp were aged with ages from ten to thirty-two days.Some length categories did contain at least five individuals and in these instances all individuals were aged. Subsequently， Gizzard shad hatch dates ranged from July 26thto August 18thand Silver carp hatch dates ranged from July 28thto August 19th. Hatch dates were then grouped into five-day groups to minimize the variability and an age-length key was used to extrapolate the results to the entire sample (Fig. 2).Mean Silver carp growth (2.24 mm per day) was significantly greater (P＜0.001，T=–11.1，df=82) than Gizzard shad growth (1.72 mm per day) (Fig. 3).Mortality was similar between Silver carp (28.1%)and Gizzard shad (25.1%) at this early life stage.

3 Discussion

Given that the size structure is different between Gizzard shad and Silver carp it is expected that either the hatch date or growth also differ. Since the age structure did not appear to differ， Silver carp and Gizzard shad spawning events may be triggered by similar conditions. Differing growth rates between these two planktivores could explain the rapid expansion of Silver carp and decline of Gizzard shad. Silver carp outgrow Gizzard shad and likely exit the window of predation described by the optimal foraging theory[40，41].

Fig. 1 Length frequency histogram with Gizzard shad (top) and Silver carp (bottom) sampled from the Middle Mississippi River in 2014

Fig. 2 Hatch dates for Gizzard shad (top) ranged from 7/27 to 8/16 and Silver carp (bottom) ranged from 8/1 to 8/21

Fig. 3 Mean growth did differ (P＜0.001， T=–11.1， df =82)between Gizzard shad [(1.72±0.03 mm) per day] and Silver carp[(2.24±0.03) mm per day]

If Silver carp do indeed exit this window of predation faster than Gizzard shad and occur in higher densities[14，42]Gizzard shad could decline in the future. If Gizzard shad are declining due to competition with the invasive Silver carp， other planktivorous prey species (e.g.， Emerald shinerNotropis atherinoides) could also be affected[17，42]. Negative impacts on the populations of native prey species could have a detrimental bottom up effect on primary consumers. If native piscivores are not able to compensate for the decline of native baitfish by consuming more Silver carp， growth and condition could suffer.

The research we conducted should be expanded to see if these patterns pertain to other streams and environments. This information will help determine environmental conditions favorable for spawning events， determine growth rates， and provide valuable information relating to the interactions of these fishes during early-life history.

Additionally the electrified surface trawl appears to be an effective method for sampling small(21—85 mm) planktivorous fishes in the Mississippi River basin but the effectiveness was not evaluated.More studies should be conducted similar to Freedman，et al.[36]to better understand the effectiveness of the electrified surface trawl and other surface trawls(e.g.， Mamou trawl manufactured by Innovated Net Systems).

When considering our results and findings， two limitations of this research should be considered.First， the duration of this study was short and our findings should be supported by a longer term study.Second， the electrified surface trawl had not been used before but did prove useful for capturing both species during the early life stage. Even with these perceived limitations， we believe our findings accurately represent the age-0 Silver carp and Gizzard shad populations in our study area. While some researchers have assessed Silver carp growth and others have assessed Gizzard shad growth we know of no study comparing the two species at this early life stage collected in the same habitats. Because of the novelty of the gear used， care should be taken in comparing catch rates generated in this study relative to prior research. We also believe that the electrified surface trawl should be tested in concert with similar gears to determine gear efficiency and investigate which method is best suited for sampling small bodied planktivores in large rivers. Furthermore， future research on early life history of both species should be conducted to assess the entire first year of life entering into the first winter (e.g.， recruitment transition phase). Additional research should also be conducted on how size of either species can affect predation by piscivores or competition with other planktivores.