Taking into account the continuous increase in freshwater introductions, and to support the recent European legislation on invasive alien species, the identification of priority pathways and gateways of introductions is of utmost importance to develop adequate control strategies. The aim of this paper was to analyse the main pathways and gateways of introductions of freshwater alien species in Europe. Based on a thorough review of the scientific and grey literature, information on pathways, country and year of initial introduction of all freshwater alien species in Europe, was retrieved. The spatial and temporal patterns and trends of biological invasions in freshwater ecosystems in Europe, in relation to different pathways, were assessed. Our results pinpoint the major importance of aquaculture, pet/aquarium trade and stocking activities as pathways of introduction of freshwater alien species in Europe. For species native to some European countries, shipping and inland canals were the most important pathways, highly responsible for the entry of many harmful species. Germany, the United Kingdom and Italy were the main entry gateways of freshwater alien species in Europe. We found a geographical pattern related to some pathways of introduction in Europe: introductions through inland canals were concentrated in Central/North-eastern Europe, while introductions through pet/terrarium/aquarium trade were mainly observed in Central/Western Europe. While Chordata species entered Europe mainly through the three major above mentioned pathways, many harmful Arthropoda and Mollusca entered through shipping and inland canals. The information gathered in this study clearly indicates the entry routes that should be prioritised by Member States, for which stronger control and management actions should be implemented and prevention efforts concentrated under the scope of the related new EU Regulation.
The Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis are two species of invasive bigheaded carp currently invading North American rivers and watersheds. Bigheaded carp were accidentally introduced into the lower Mississippi River basin in the early 1970s and have since invaded many water bodies in the Midwestern United States. Evidence of bigheaded carp reproduction and recruitment in the upper Mississippi River upstream of Lock and Dam 19 (LD19) at Keokuk, Iowa, thought to be a critical constriction point to their upstream establishment, has been limited to a few isolated detections of eggs, larvae, and juvenile life stages since 2012. Therefore, a more comprehensive assessment of bigheaded carp reproduction in this critical management zone was needed. We used quadrafoil light traps (n = 1,387) deployed during May–September 2016–2018 in Pools 17–19 of the Mississippi River to monitor for advanced larval bigheaded carp in low‐velocity habitats. Throughout the sampling period, we captured 1,747 larval and 35 postlarval bigheaded carp (N = 1,782). Bigheaded carp were collected on 15 sampling events that spanned from May 31, 2016, to September 13, 2018, with associated hatch dates estimated to represent 10 unique reproductive events from May 2016 to September 2018. The individual captures and backdated hatch estimates revealed a protracted spawning period of up to seven events in 2016, one event in 2017, and two events in 2018. Bigheaded carp were only captured in Pool 19, possibly due to the drifting requirements for egg maturation and the low‐velocity downstream reach of Pool 19. This research provides confirmation that bigheaded carp spawned upstream of LD19 are capable of transitioning past the yolk sac stage upstream of this bottleneck to more advanced larval stages. Knowledge of reproduction and larval retention and the field‐based evidence of protracted spawning fill critical research gaps needed for the management of bigheaded carp.
Invasive species can have varying and unpredictable reproductive potential among native and invaded populations or along invasion fronts, potentially contributing to invasion success. Understanding the reproductive potential (fecundity and egg size) of low‐density, leading‐edge populations is important to inform the management of Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis populations in the upper Mississippi River. Therefore, we compared batch fecundity and eggs/g of ovary subsample (an egg‐size surrogate) of Silver Carp (n = 145) and Bighead Carp (n = 75) along a decreasing invasion gradient from Pool 20 (higher abundance) to Pool 17 (lower abundance) in the Mississippi River. Bighead Carp in Pools 17 and 18 (461 ± 22 eggs/g [mean ± SE]) had significantly fewer eggs/g than those in Pools 19 and 20 (higher abundance; 483 ± 13 eggs/g). Log‐transformed batch fecundity for Bighead Carp was positively correlated to log‐transformed fish length, and batch fecundity was higher in Pools 17, 18, and 19 than in Pool 20. The eggs/g values for Silver Carp, which have been previously shown to be independent of fish length, were similar among all pools (94 ± 11 eggs/g). Batch fecundity for Silver Carp was significantly higher in Pools 17–19 than in Pool 20, where densities are higher. Our results suggest that at lower densities, Bighead Carp produce more, larger eggs and Silver Carp produce more eggs, which collectively could increase their reproductive success along invasion fronts.
Fecundity is a necessary metric to assess reproductive potential (e.g., spawning potential ratio and recruitment overfishing) in invasive fishes, yet the direct measurement of fecundity is often difficult, impractical, and time consuming. Therefore, the ability to estimate fecundity from field measurements (i.e., TL, relative weight [W r ], or ovary weight) can be a valuable tool for fisheries managers to estimate reproductive potential. The goal of this study was to relate the batch fecundity of invasive Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis to metrics that can be quickly measured to eliminate the need for direct enumeration. Silver and Bighead carp were collected from the upper Mississippi River (UMR) in the summer of 2016 and 2017. Subsamples of mature (spawning capable) ovaries were preserved, enumerated, and then related to ovary weight, TL, weight, and W r . An equation for estimating Silver Carp batch fecundity from ovary weight in the UMR, corrected for ovary membrane weight, was developed: {ovary weight -[0.222 × (ovary weight 0.7589 )]} × 794 (SE = 11). Batch fecundity for Silver Carp from Pools 17-20 of the UMR averaged 1,230,930 AE 68,983 eggs (mean AE SE). Ovary weight was the strongest predictor of fecundity, but TL, weight, and W r also showed significant relationships with fecundity for Silver Carp. Batch fecundity for Bighead Carp averaged 1,043,953 AE 82,360 eggs, but the number of eggs per gram showed significant spatial variation; therefore, no fecundity equation was developed for this species. Future work should evaluate the frequency of Silver Carp spawns within a season and how batch fecundity varies among multiple spawning events. Fecundity estimates can be used to determine spawning potential ratios, which can help in evaluating management strategies.
Fishes utilize several different species-specific reproductive strategies adapted to their environment. Invasive species may display reproductive strategies that allow successful reproduction in varying environments, such as batch and protracted spawning. Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix, referred to as bigheaded carp, are invasive species that have been introduced worldwide. In some invaded ranges, they either have a protracted spawning period and/or spawn multiple times in a season, which can be advantageous in novel environments. Reproductive strategies can vary between river systems or with environmental conditions, and understanding differences in strategies can improve management of bigheaded carp. To determine reproductive strategy, we evaluated the histology of spawning capable ovaries from Silver Carp (n = 58) and Bighead Carp (n = 38) collected during May-September 2016 and June 2017 from Pools 17-20 of the upper Mississippi River (UMR). We found that 87.9% of Silver Carp (n = 51) and 86.8% of Bighead Carp (n = 33) had an ovary that contained primary growth, cortical alveolar, and vitellogenic oocytes simultaneously. Multiple stages of oocytes within an ovary indicated that individuals were capable of spawning multiple times during a single year, and the presence of postovulatory follicle complexes demonstrated that individuals had already spawned while oocytes were still present. Based on histological samples, bigheaded carp appear to be indeterminate batch spawners, with oocytes continuing to develop throughout the spawning period. Silver and Bighead carp in the UMR may exhibit protracted and batch spawning and employ flexible spawning strategies among different river systems, which can increase the probability of successful recruitment in a variable environment.Fish species display several types of reproductive strategies, including total or batch spawning, determinate or indeterminate fecundity, and synchronous, groupsynchronous, or asynchronous spawning, which have evolved in response to varying environmental conditions. Certain spawning strategies may be advantageous and contribute to the success of a species. For example, total spawners, which are always determinate spawners with
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