TRIPP Sara and PHELPS Quinton

(1. Big Rivers and Wetlands Field Station， Missouri Department of Conservation， 3815 East Jackson Boulevard， Jackson，Missouri 63755， USA; 2. West Virginia University， Division of Forestry and Natural Resources，307C Percival Hdall， Morgantown， West Virginia 26505， USA)

Abstract: Asian carp are expanding their range throughout the Mississippi River; however， abundance is thought to be highest in reaches within close proximity to the Illinois River. In the Mississippi River， Lock and Dam 19 (L&D 19) at Keokuk， Iowa is the primary barrier to slow the expansion upstream. As Asian carp abundance increases below L&D 19， it is important to investigate potential means of control (i.e.， reduction through harvest and barriers) that will prevent complete invasion of the Mississippi River above L&D 19. Silver Carp and Bighead Carp were collected below L&D 19， a subsample were implanted with ultrasonic transmitters to evaluate passage through the lock chamber and the remainder were used to determine population dynamics at the leading edge of invasion. Although the dam portion of the structure poses a complete barrier to upstream expansion， we documented lock chamber passage demonstrating the lock chamber has the capability to provide passage upstream. Based on the results of the population assessment， in order to induce recruitment overfishing at this leading edge of invasion， Asian carp will need to be intensively harvested at 300 mm and larger. The combination of commercial fishing efforts and research investigating ways to prevent passage upstream must be employed.

Invasive species are one of the leading threats to native species and biodiversity， second only to habitat loss[1]. Introductions of these non-native species can have harmful， often even catastrophic environmental and economic impacts[2—4]. Despite the consequences in terrestrial， freshwater， and marine ecosystems， biological invasions continue to accelerate[5].In response to mounting concerns regarding the scale of invasive species issues， many agencies， policymakers， and stakeholders are beginning to develop management strategies for preventing new introductions and reducing the impacts of current invasive species[5]. Management or eradication plans have been developed for exotics such as feral swine (Sus scrofa)[6]， Round Goby (Neogobius melanostomus)[7]，zebra mussels (Dreissena polymorpha)[8]， and countless others as populations have increased and their distributions have expanded across many states and countries. The invasive species currently receiving the most attention and publicity is the group of fishes known as the Asian carp， which includes Bighead Carp (Hypophthalmichthys nobilis)， Black Carp(Mylopharyngodon piceus)， Grass Carp (Ctenopharyngodon idella)， and Silver Carp (Hypophthalmichthys molitrix)， with emphasis on Bighead Carp and Silver Carp.

Bighead Carp and Silver Carp were brought to the United States approximately 40 years ago[9]. Since that time， the range of Bighead Carp and Silver Carp has rapidly expanded throughout the Mississippi River Basin[10，11]. This rapid invasion can be attributed to life history traits such as a broad environmen tal tolerance， feeding efficiency， rapid growth rates，high fecundity， lack of predators， and high dispersal rates that are generally exhibited by successful invasive species[12，13]. Silver Carp and Bighead Carp continue to expand their range being found as far north as the state of Minnesota in the Mississippi River and east in the Ohio River past West Virginia and Ohio[14].Fortunately these recent expansions have only been documented adult occurrences with the potential for spawning adults， but no establishment (Spawning verified by collection of confirmed eggs or larvae) has been documented[14]. While spawning has been verified in Pool 18 of the Upper Mississippi River， no recruitment has been documented， so there is still a chance to prevent the establishment of Asian carp in the Upper Mississippi River.

While the immediate focus has been to prevent Asian carp from spreading from the Illinois River into the Great Lakes， other areas where reproduction and recruitment has not yet been documented (i.e. the Upper Ohio River and the Upper Mississippi River)are beginning to gain more attention. When state and federal agencies within the Mississippi and Ohio River Basins were surveyed about the amount of money spent for Asian carp prevention and control between June 2012 and June 2014， the response yielded a total cost of approximately ＄94.5 million， but only ＄12.2 million was spent outside of the Chicago Area Waterway Systems (CAWS)[14]. In these other areas where Asian carp have not yet been established it will be imperative to build upon the tools that have proven successful in the Illinois River Waterway and CAWS. Some of the research conducted on the prevention and control of Asian carp invasion includes population monitoring and assessment， commercial harvest to reduce abundance， assessment of the effectiveness of dispersal or exclusion barriers， exploration of market development， outreach and awareness campaigns， and new technology development[9，14]. Due to the similarities (i.e.， natural pinch points or restricted movement areas， established populations downstream) among the Illinois River Waterway/CAWS and certain reaches of the Upper Ohio and Upper Mississippi Rivers some of these techniques may be most efficiently implemented at specific sites. Because of this， the objective of this study was to determine if the area below Lock and Dam 19 (L&D 19)on the Upper Mississippi River is one area where previously tested prevention or control techniques (i.e.exclusion barriers and harvest) could be implemented successfully. Specifically， we used telemetry to determine whether potential barriers could be placed at the entrance of lock chamber to prevent Asian carp passage upstream into Pool 19. We also evaluated population demographics through local population assessment to develop the input parameters needed to model if harvest could cause recruitment overfishing and be used to reduce the Asian carp population below L&D 19.

1 Study area

There are a series of 29 locks and dams within the Mississippi River that create a stairway of travel for commercial and recreational traffic from Minnesota down to the Gulf of Mexico. Approximately halfway between the head of navigation in Minnesota and the confluence of the Ohio River， sits L&D 19 at Keokuk， Iowa. Lock and Dam 19 is the most downstream hydropower dam on the Mississippi River with the second highest lift on the Mississippi River at 12 meters. The dam and original lock were completed in 1913， with 119 sliding gates with 10 meter spillways below creating an insurmountable barrier for fish coming upriver[15]. Due to the construction differences(i.e.， extremely high lift， turbines， insurmountable sills below the gates) compared to the other locks and dams within the Mississippi River， the dam portion of L&D 19 acts as an effective barrier to upstream movement， with the lock chamber being the only option for upstream movement past Keokuk， Iowa[15].Unfortunately， Coker[15]suggested that the location of the lock in regards to the water flow and the manner of operations， impede the lock chamber from functioning as an effective means of passage for native fishes. Tripp， et al.[16]also found that while the majority of lock and dam complexes in the Upper Mississippi River are not complete barriers to movement，L&D 19 did pose a complete barrier with neither native nor invasive fish passing through.

2 Methods

In order to monitor the movement and potential passage of Asian carp around L&D 19， we employed a monitoring array that was a combination of stationary receivers (Vemco VR2W) placed on navigation buoys， bottom stands， and a lock chamber attachment.Two stationary receivers were strategically placed below the lock chamber entrance (one approximately 1.5 kilometers downstream and another directly below the entrance)， one receiver was placed within the lock chamber， and two other stationary receivers were deployed covering the channel 1.5 kilometers upstream of the dam (Fig. 1). The local array surrounding L&D 19 is just a small portion of the entire stationary receiver array that extends from above L&D 19 in the Upper Mississippi River at Keokuk， IA(River Kilometer [RKM] 591) down to the Lower Mississippi River at Caruthersville， MO (RKM 1350)and also includes most major tributaries (Fig. 1). To focus on the potential passage at L&D 19， Asian carp were sampled within 5 kilometers downstream of L&D 19. Asian carp were collected using day time electrofishing to ensure capture of a wide range of sizes and ages[17]. Twenty-five Asian carp (15 Silver Carp and 10 Bighead Carp) were surgically implanted with ultrasonic transmitters (Vemco V16-5H;69 kHz; 2 year battery life) using methods described in Tripp，et al.[16]. A wide range of sizes were implanted so that we could investigate whether passage was affected by size. Transmitter detections from the stationary receivers were summarized to describe Asian carp movement through L&D 19.

Fig. 1 The Upper Mississippi River VR2W array from Keokuk， IA to Caruthersville， MO with the local array surrounding Lock and Dam 19 enlarged in the callout boxStationary receivers (VR2W) locations represented with the grey circles

The remaining Asian carp collected were weighed， measured， sexed， eggs were removed to estimate fecundity， and the lapilli otoliths were removed for age estimation. Lapilli otoliths were processed and aged according to methods in Seibert and Phelps[18]. Fish age was estimated by two experienced readers. If age estimates differed between readers， the otolith was reevaluated in concert so that a consensus was reached. Growth was estimated using the von Bertalanffy growth equation in Fisheries Analysis and Modeling Simulator (FAMS):

whereLt= length at timet，L∞= theoretical maximum length in the population，K= growth coefficient，andt0= time when length theoretically equals 0 mm[19].For von Bertalanffy growth equation，L∞was held constant at the largest length collected during the sampling. Conditional natural mortality was estimated in FAMS using the Hoenig method[20]. This baseline information was used to develop the input parameters(i.e.， maximum age， growth， mortality， length-weight relationship， fecundity， and maturation schedule)needed to model the population (Tab. 1). This model will allow us to determine if a commercial fishery has the potential to be a long-term solution for control of

Tab. 1 Population parameter estimates for Pool 20 Silver Carp used to develop Spawning Potential Ratio models in Fisheries Analysis and Modeling Simulator (FAMS)

Asian carp and quantify the level of exploitation required to prevent their continued expansion. The spawning potential ratio (SPR) was simulated using FAMS software[19]for various exploitation rates (i.e.，5%—85%) and target harvest lengths (e.g.， 300， 400，500， 600， and 700 mm)[21]. Goodyear[21]defined spawning potential ratio as an index that can be used to define the number of adults needed to maintain recruitment in the population. A SPR level of 0.2 was used as the critical number of adults needed to prevent or induce recruitment overfishing[21，22].

3 Results

During the summer of 2013， ten Bighead Carp that ranged from 625 to 1130 mm were implanted with transmitters as well as 15 Silver Carp ranging from 600 to 960 mm. For the first year， both Asian carp species tagged directly below L&D 19 and those tagged further downstream in the Upper Mississippi River were detected on the receiver within 1.5 kilometers downstream of the lock chamber entrance， but there were no detections on the receiver closest to the lock chamber， within the lock chamber or above the lock chamber. The following year in the fall of 2014，the stationary receivers again detected Asian carp from multiple locations on the receiver within 1.5 kilometers downstream of the lock chamber， but detections from three Silver carp (tagged within 1.5 kilometers of L&D 19) were detected on the receiver closest to the lock chamber. On October 15， 2014，one of those three Silver carp entered the lock chamber and was detected by the stationary receiver within the lock chamber for the next 50 minutes， after this short time this Silver carp was also detected on both the stationary receivers above L&D 19， representing passage had occurred by this Silver carp through L&D 19. Since only one of the 25 Asian carp has passed through the lock chamber， we were unable to perform analysis to determine if there was differential passage success based on size.

Because passage is occurring at some level， it is important to investigate potential means of control that may prevent or delay the complete invasion of the Mississippi River above L&D 19， using the local population demographics. To do this， 346 Asian carp(294 Silver carp and 52 Bighead carp) were collected and sacrificed for population demographic information. The Silver carp size structure was truncated with the majority of fish ranging from 550 to 750 mm，while the Bighead carp length-frequency distribution was skewed toward larger fish with the majority of the fish ranging from 900 to 1000 mm (Fig. 2). Both populations lack small (＜500 mm) fish. The Silver carp age structure was also truncated with only 6 year classes present， while the Bighead carp age frequency covered a broader range of ages ranging from 6 to 16 years of age (Fig. 3).

Due to the low sample size of Bighead carp population modeling was only conducted on Silver carp.Based on the local population demographics (Tab. 1)，the spawning potential ratio model reveals that the most efficient method to decrease the reproductive potential of the population is to include all Silver carp ≥300 mm in exploitation efforts. Therefore if all Silver carp ≥ 300 mm are included in the harvest， only 30%exploitation would be needed to begin to see the effects of recruitment overfishing， but if only larger fish are harvested (＞500 mm) an exploitation rate of over 85% would be needed (Fig. 4).

Fig. 2 Length-frequency of Silver and Bighead carp collected within 5 kilometers downstream of Lock and Dam 19

Fig. 3 Age-frequency of Silver and Bighead carp collected within 5 kilometers downstream of Lock and Dam 19

4 Discussion

Because lock chamber passage was the only means of expansion beyond L&D 19， many thought some level of passage had to be occurring， but we now have evidence of passage at L&D 19. While only one of the 25 transmitter fish has been documented passing through the lock chamber， this study does provide the needed evidence that this is in fact a pathway for expansion. Although the dam portion of the structure does pose a complete barrier to upstream passage， we now have evidence that the lock cham-ber has the capability to provide passage upstream.Therefore in order to aid in controlling the Asian carp population at this leading edge of invasion， Asian carp will need to be intensively harvested (＞30% exploitation) at sizes 300 mm and larger to achieve recruitment overfishing. The combination of focused commercial fishing efforts and research investigating ways to prevent passage upstream must be employed.

Fig. 4 Spawning potential ratio model for the Silver carp population below Lock and Dam 19 simulated for various length limits and exploitation ratesHorizontal dashed line represents the 20% SPR threshold required to induce recruitment overfishing in the Silver carp population

As biological invasions continue to increase， the use of large-scale removals of invasive species are being considered for many ecosystems[23]. Aquatic ecosystems have already provided some key examples regarding the effectiveness of removal programs in controlling invasive species. Some of these examples include， the Lake Trout (Salvelinus namaycush) suppression program in Yellowstone Lake[24]， Lionfish(Pterois volitans) removals in the Western Atlantic Ocean， Caribbean Sea， and Gulf of Mexico[25]， Sea Lamprey (Petromyzon marinus) in the Great Lakes[26]，and Asian carp in the Illinois River[27，28]. However there is no “one size fits all” approach for the control of invasive species， so understanding the local population dynamics and life cycle of the target species is imperative for successful invasive species management[29]. As noted in this study as well as many of the others mentioned above， the success of these removal or control efforts will hinge on the use of local population demographics in developing the target exploitation goals and also the willingness to change the target size of fish harvested.

While we documented the first Asian carp passage through L&D 19， we also realize that due to the limited passage through the lock chamber this makes L&D 19 a natural pinch point restricting fish movement. Because this area is a natural pinch point， the population demographics from this specific area were used to develop models similar to studies conducted in other areas that may benefit from reduction efforts through harvest. In these cases， multiple control strategies (i.e.， harvest， barriers) have been employed to control invasive species such as Asian carp[28，29]. In 2011， the Minnesota Ad Hoc Asian carp Task Force suggested that an integrated control strategy (i.e.， barrier construction， monitoring， education， research， and removal) will be necessary to slow the spread of Asian carp. Barriers to prevent movement and the spread of Asian carp have been investigated as an integral part of the control strategies[30]. Further studies will need to determine the feasibility of implementing one of the many types of deterrence barriers; however telemetry studies such as this one can provide information about when and where Asian carp are entering the lock chamber， leading to the most effective placement of the barriers. At L&D 19 and many other areas where we have the potential to prevent the complete invasion of Asian carp， these multi-tiered approaches will be a crucial step in Asian carp prevention and control.