The American bullfrog is a tough and noisy guest in ponds and pools

It is best to tackle invasive exotics as soon as they set foot on the ground, otherwise it will soon be mopping up with the tap open. Sometimes it succeeds, such as with the Indian house crow in Hoek van Holland, which got off the boat in the port of Rotterdam sometime in the 1990s. Another success story is the American bullfrog.

In 2010, this bullfrog had settled in two ponds in Limburg, where they were all quickly and successfully scavenged. Those frogs most likely had crossed the border on their own from Belgium, where the American bullfrog is still abundant in the Grote Nete valley east of the city of Antwerp (part of the Scheldt river basin).

There are also two isolated populations in Hoogstraten and Arendonk in Belgium. Both places are almost on the border with the Netherlands, just below Breda and Tilburg. The American bullfrog is officially no longer found in the Netherlands. But he is still knocking at the door.

Two men who are doing everything they can to prevent the bullfrogs from crossing the border again are Teun Everts and Rein Brys, both of whom are employed as biologists by the INBO – the Institute for Nature and Forest Research in Belgium. Brys coordinates the e-DNA research (more about this later) that is used to map the distribution of rare fish and amphibians, but also to detect and combat exotic species. As a doctoral researcher, Everts focuses specifically on the bullfrog.

The INBO biologists assume that there were several hotbeds of introduction in Flanders in the 1980s and 1990s. “We know from at least one fish farmer where the bullfrog larvae must have accidentally come along with a fish transport. And fatheads have been sold at the famous animal market in Mol for at least ten years. People came from the surrounding area, but also from the Netherlands and Germany,” says Everts. It was always said that it would be too cold here for this heat-loving frog species to reproduce. Not a completely illogical assumption, according to Everts: “In America, that frog occurs along the entire East Coast, up to the Rocky Mountains and the deserts of Central America, but only in the southernmost regions of Canada.” Toronto is on the 43rd parallel, Brussels on the 51st.

That is not true, we now know. It just takes longer. Normally it takes them a year to go into metamorphosis (from tadpole to frog). Here it can take up to two years. In comparison, most native frogs take several weeks or months.

The bullfrog primarily causes ecological damage in Europe. Because of two reasons. First of all, it is quite a large animal – with a head-trunk length (so without stretched hind legs!) of 15 to 20 centimeters – and a not very picky eater: it eats everything that fits in its mouth. From fish and amphibians to small waterfowl and everything in between, eggs and larvae and even wasps and mole crickets (the ‘earth cricket’, one of the largest insects in Western Europe at five to seven centimetres).

When the pond is really completely empty, they switch to their own larvae

Rein Brys biologist

It is the vulnerable native amphibians that suffer the most. Not only are they on the menu of the bullfrog, this American exotic also carries diseases that are serious for the native fauna. Such as the deadly ranavirus and a skin fungus that causes thickening and disrupts gas exchange (amphibians partly breathe through the skin). The bullfrog itself is virtually immune to both, explains Everts.

Another inconvenience is the enormous noise the males make. In America they have it bullfrog named because the sound it produces resembles a bull’s bellow – it’s worth googling that.

The frog does so well here, despite the cold, because it hardly experiences predation pressure – in America it is eaten by snakes and crocodilians, here only a heron ventures sporadically. In addition, it has a particularly high reproduction rate: bullfrogs can lay up to 20,000 eggs (for comparison: most native frogs and salamanders lay 20 to 200 eggs at a time) and in long summers they can produce two clutches per year.

With this method we recently intercepted two wandering males quite quickly

Teun Everts biologist

It sounds contradictory, but a third advantage is that the bullfrog is cannibalistic. Brys explains: “The adult frogs are already serving at multiple trophic levels. And when the pond is completely empty, when animal food is no longer available, they switch to their own larvae, which live on algae. Thus, in a sense, they are self-sufficient as a species. It also means that the fatheads are naturally afraid of their adult counterparts. They have to make sure they get away as soon as they get legs. That is a driving factor behind the spread.”

The frogs and tadpoles are actively fought with traps – the tadpoles are easier to catch, because they don’t hide on land. In some places, the adults are also shot by hunters – those beasts are incredibly tough.

It is generally known where the frogs are, but putting traps in the right ponds remains a bit of guessing and hoping that they are there and swimming in them. And it’s extremely labour-intensive, because you have to go to the bitter end – if you leave even two, it’s all pointless. With the e-dna research that Brys coordinates, this can become much more of a precision job: “We take water samples, filter them and analyze all the DNA present. From this we can determine which species are or were present in the relevant water feature, how many there are and whether they reproduce there. It is actually a kind of forensic investigation.” Everts: “With this method we were recently able to intercept two roaming males quite quickly who had crossed the border into Belgian Limburg from Antwerp.”

Read an interview with biologist Anneke ter Schure: ‘E-dna tells which organisms have lived in a certain area, even if they have long since left there’

In 2022, 91,939 bullfrogs were caught in Belgium (almost ten times as many as in 2019). They were simply destroyed for years. Recently they are going to some zoos as food for the ibises. Those zoo birds have nothing to complain about, at least about their food. The American bullfrog is a delicacy.

Why specifically the French – especially in English speaking countries – are known as ‘frog eaters’ is not clear. Yes, frogs are sometimes on the menu in France. But that also applies to Italy and Spain. In Eastern Europe they were traditionally eaten during Lent (frogs, like snails, do not count as meat for the Catholic Church). It is common throughout Southeast Asia and China to eat frogs. As well as in Uruguay, for example (the Aztecs already did that) and of course in the South of the US. Enough recipes available.

Frog is super clean, tender, very lean and refined white meat

Everts and Brys were more than willing to participate in this article on one strict condition: that they could participate in the tasting. So in Brys’s well-equipped kitchen we prepared a dozen firm buttocks, in three ways: French style, Sechzuan style and dirty South style. (Disclaimer: The aforementioned virus and skin fungus are not dangerous to humans – we have not tried the frozen and thawed frogs raw, however).

Does it look like chicken? Yes, in the sense that it is pale in color and little pronounced in taste. Frog, like chicken fillet, consists almost entirely of white muscle cells: they are made to make short-term intense efforts (compared to, for example, the red muscle cells in a cow’s cheek or duck breast, which are made to perform long-term work and therefore have a different biochemical composition). responsible for a redder color and deeper flavor).

But a frog has to hurl its whole body into the air in one jump with such a momentary effort. So those white muscle cells on his back legs are neatly and tightly bundled in juicy packets. Such a calf or a tender thigh is juicy and really a bit more refined in taste than the average chicken. It is by no means muddy or ditch-like or anything like that, should you imagine such a thing. Frog is super clean, tender, very lean and refined white meat.

The beautiful autumnal, faded, red-green colored tail fillets of the larvae can also be deep-fried

Lean white meat should certainly not be overcooked, as that will eventually make a frog dry. But the large buttocks of such a bull frog can take quite a bit. The meat is refined enough on its own to serve as the star of the show, seared briefly on both sides in plenty of butter, slaked with a little pastis and plenty of garlic and parsley. But it can also carry the Chinese spice violence of the sechzuan peppercorns, star anise and cassia just fine, with a soy sauce and ginger.

Perhaps the most satisfying preparation is – the wise of the land, the honor of the land – to make it KFBF (Kentucky Fried Bull Frog). Marinate in buttermilk with hot sauce and then bread with a mixture of flour and cornstarch for an extra crisp finish. Better than chicken. Mark my words.

PS: When it comes to saving the native amphibians, it makes more sense to eat those fatheads en masse (which also easily grow to be as long as your hand). The beautiful autumnal, faded, red-green colored tail fillets of the larvae can also be deep-fried. But as a frog they are still tastier. So we could also raise them in an aquarium at the back of the restaurant, so that we can point out our own frogs just like our own lobster. That doesn’t make anyone worse.