An integrated approach to control invasive faunistic aquatic alien species: a case study on Lithobates catesbeianus

Abstract

It is widely accepted that invasive alien species (IAS) form a serious threat to native biodiversity. The importance of IAS is recognized by Europe in the publication of the EU regulation on invasive alien species (EU 1143/2014). This obliges member states to prevent new introductions and control established invasive populations of the species mentioned on the consolidated Union list of the regulation. The American bullfrog is one of the listed species. In Belgium it is widespread in the valley of the river Grote Nete between Lier and Balen. It is also present in some isolated populations in 3 municipalities in Flanders. In this thesis a management framework based on applied research was set up for the American bullfrog to enhance the effectiveness of the control or eradication of the species. Invasive alien species are known for their behavioural plasticity which make them highly adaptable to varying environmental circumstance. Therefore, a behavioural study of the American bullfrog with the use of radiotelemetry was carried out in the valley of the river Grote Nete. Nine adult American bullfrogs (Lithobates catesbeianus) were tagged with an internal radio transmitter and tracked for one year. The mean ± SD core range area (KDE50) was 15.00 ± 22.41m2. The home range area (KDE95) had a mean ± SD of 429.78 ± 510.97m2. Shores of larger eutrophic ponds and small temporary pools in alluvial forest were chosen as habitats. The latter was used primarily outside the breeding season by half of the individuals, whereas the shores of ponds were the favourable place during the breeding season for all individuals. The study revealed a mean action radius of 270.78 ± 199.17m and individuals were able to move up to 742m in a single displacement. These results show that the dispersion of the American bullfrog in a valley system as the Grote Nete can proceed very rapidly. A positive correlation between weight and distance covered within one movement was found, which could suggest that dominant individuals can cover larger distances in search of an optimal habitat for reproduction, foraging or hibernation. Since bullfrogs can spend a substantial time of their activity season in inaccessible habitat as marshes and brooks, active control methods can only be efficiently used during the reproduction period when adults come to the ponds to breed. Direct control methods for the American bullfrog are limited to the removal of larvae and (sub)adults using fyke netting traps, shooting, or draining the ponds they inhabit. These measures have proven to be successful in the early stage of invasions but are insufficient to control widespread populations, especially in inaccessible habitats. To tackle this problem an investigation in the use of passive population control techniques was set up. The sterile-male-release approach where the introduction of sterile males in a population offers an indirect technique to lower their reproduction success and subsequently control the expansion of the population was explored in this matter. Sterilization can be executed with chemicals so a study on the effect of the injection of bisazir in male bullfrogs was carried out. Individuals were caught and injected with a dose of 0, 12.5, 25, 50 and 100 mg/kg bisazir to induce DNA fragmentation in sperm cells and subsequent cause sterility. The results indicate that injecting 50 mg/kg bisazir causes significant fragmentation in the sperm of Lithobates catesbeianus. Before using chemicals in situ their potential risk for the environment must be documented. As a first step the inherent ecotoxic properties of bisazir were evaluated in both acute and chronic aquatic tests: microtox (Aliivibrio fischeri), microalga (Raphidocelis subcapitata), duckweed (Lemna minor), and water flea (Daphnia magna). The no observed effect concentration (NOEC) was 1 mg/l for Daphnia reproduction. Based on these results the predicted no effect concentration (PNEC) value was 20 µg/l. These results show the inherent ecotoxic properties of the compound and raise questions on the safe applicability in aquatic habitats. Therefore, other safer environmental methods to induce sterility were investigated. A technique adopted in aquaculture to produce sterile triploid fishes with a higher biomass was adjusted and refined to be used on the American bullfrog. To produce sterile individuals a temperature and pressure shock protocol was used on fertilized eggs to create triploid individuals. The cold shock did not result in triploid individuals while 54% of triploids were obtained from pressure shock, the remaining individuals being aneuploids. The triploid and control larvae were reared for more than one year and the control larvae had a greater length after 7 and 9 months, their weight did not differ. At metamorphosis no difference in length and weight was found between the two groups. The subadult bullfrogs from both groups showed 9 months post metamorphosis a similar scaled body mass index but lower than individuals living in the wild. This method was named the Sterile Triploid Method (STM) and the individuals produced could be used to passive control widespread populations of American bullfrog. Control or eradication programs are often fragmented leading to cost inefficient and inadequate results. The scientific findings of this thesis were used to set out a more efficient management protocol to control the American bullfrog in Flanders. The boundaries of this integrated management strategy are defined and carried out in the LIFE 3n-Bullfrog project currently active until 2026. The integration of raising awareness, an updated Early-Warning-Rapid-Response protocol, applied research and (trans)national dissemination and training provide a state-of-the-art approach to manage invasive alien species based on the case study on the control of the American bullfrog in the Flemish region of Belgium. This approach is based on the close cooperation of scientists, policymakers, field managers and conservationists to create a science-based policy for the management of invasive alien species. Innovative applied research poses an important foundation in the steering process to establish efficient control measures. The several stages of which this framework is built, are crucial in the strategy to tackle the invasion in the most integrated way and can be transposed to control other invasive alien species.