Traditional control measures for the American bullfrog consist of a combination of setting fykes to trap frogs and tadpoles, shooting juvenile and adult frogs, and draining ponds to eliminate their habitat. Single fykes anchored on the bank are often used to catch frogs, while double fykes are used to trap the larvae. A single fyke consists of a long net called a “leader” that opens into a series of funnels that lead to a final trapping compartment. By anchoring the leader on the bank, all frogs that swim by are led to the funnels, where they are then caught. A double trap has the same structure, only there are funnels on each side of the leader. These double traps are usually set up away from the banks in the ponds to mainly catch tadpoles. Although adult frogs can be caught with traps, the catching efficiency tends to be relatively low. Of course draining a pond is the most effective way to eliminate frogs from a pond, but this is not always feasible or desirable and also only works as a control if the migrating frogs are also caught to ensure that they do not occupy neighboring ponds.
This video illustrates how these fykes are usually set up and used.
Unfortunately, these existing techniques such as placing traps and emptying ponds, are not sufficient anymore to halt the spread of bullfrog populations in Flanders. The LIFE 3n-Bullfrog project aims to combine the aforementioned traditional control measures with a new method to stop the spread of the American bull frog in Flanders.
For the isolated populations in Kasterlee, Hoogstraten, Arendonk, Sint-Agatha Rode and Huldenberg, the traditional control measures with fykes will be continued. In these populations, these methods are sufficient to control bullfrog numbers. In Hoogstraten, for example, the population has been virtually wiped out. In subsequent years, aftercare is essential to prevent new reproduction from occuring and to remove individuals that may have been left behind.
Left: double fykes are set up unanchored in ponds to catch tadpoles. - Right: an adult bullfrog, caught using fykes.
In the valley of the Grote Nete, where the smaller populations of American bullfrogs are linked by a network of ponds, traditional control techniques are no longer sufficient. Here, the Life project will experiment with releasing sterile male frogs in enclosed ponds. Release of sterile males is a technique (appropriately called sterile male release technique or SMRT) that is currently already used for the control of insects such as mosquitoes and parasitic flies. However, this project is the first test to apply this technique to invasive amphibians. After several years of research, scientists at PXL University have succeeded in breeding sterile specimens with fertile frogs, which were caught using fykes. You can find out how this is done on our Facebook page. To create sterile frogs, fertilized eggs of the captured fertile frogs are briefly put under high pressure. This causes the developing embryos to become triploid. A triploid frog has three sets of chromosomes in each cell instead of the normal two. Because these frogs have an abnormal number of chromosomes, they cannot reproduce and are thus effectively sterile. These sterile embryos then develop into bullfrog tadpoles, which are released into enclosed breeding ponds in Balen. Here the tadpoles can safely develop into young frogs, after which they are transferred to other enclosed ponds in a second site in the Scheps nature reserve. In this second set of ponds the young frogs grow into adult frogs which are then able to compete with the present feral fertile frogs. This technique is currently considered the most sustainable and animal-friendly option we have to control American bullfrog populations in Flanders. Triploid sterile males will attempt to fertilize clutches from feral bullfrogs, which of course fails. If there are enough sterile animals present, most of the clutches will be fertilized by sterile animals. As a result, fewer tadpoles hatch each year and fewer frogs grow into adults. If enough clutches fail in this way for several years, it is even possible that no new frogs make it to adulthood. This way the population growth will stop and the bullfrog populations will stop expanding in Flanders.