A hidden killer: What happened to the amphibians of Gerês?

Since the 1990s, the amphibian population in the Peneda-Gerês National Park has suffered a worrying decline. However, it was only recently that the culprit was discovered: a ranavirus with the complicity of an exotic fish.

For the last thirty years or so, the lagoon in the Peneda-Gerês National Park has been the scene of a strange phenomenon: the mass disappearance of amphibians. According to Armando Loureiro, a biologist from the Institute for Nature Conservation and Forests (ICNF), it was only “in 1998-99 that, after a series of field missions carried out in Gerês, specifically in a lagoon in the Carris mines that exists high up in the Gerês mountains, the existence of a problem with the amphibians of Peneda-Gerês was noted”. The biologist also revealed that these first field studies showed “that there were some newts, more specifically the marbled newt (Triturus Marmoratus), that presented skin lesions”, thus indicating the possibility of “a sanitary problem”. This situation initiated a study on the cause of mortality and lesions of certain animal communities.

The study of the lagoon’s biodiversity, reveals the INCF biologist, involved “motorisation with annual counts” until “the progressive reduction [of amphibians] was noted, triggering the need to investigate the causes”. In an initial phase, there was no consensus as to the causes of these outbreaks of amphibian mortality that were seriously affecting the balance of the lagoon’s ecosystem, until, “in the middle of this story, we found fish species that were not expected to be found there, exotic American fish that someone introduced”, the biologist reveals. Supported by this systematic monitoring, the group of biologists observed the appearance and growth of the sunfish population (Lepomis gibbosus), and with this crucial data, the biologists found a new direction for research, discovering that this species, besides being invasive, was also a carrier of the ranavirus, a virus that had a devastating effect on amphibians. 

With this new data, the researchers found a possible culprit for this problem, however, using samples preserved in the museum of natural history, they confirmed that the ranavirus was already present in the middle of the Carris Lagoon, before 1990, apparently before the arrival of the fish that also carries the virus. 

In this sense, the investigation returned to the starting point, not discarding the sun perch from the equation. The research continued, until, according to the ICNF biologist, the conclusion was reached that “it was a set of factors, which created this situation, on the one hand, a virus [already present], and, on the other, the problem of the introduction of an invasive exotic species”. Only recently, in December 2022, a study entitled Invasive fish disrupt host-pathogen dynamics leading to amphibian declines, published in the journal Biological Conservation, clarified this issue, thus answering the question initially posed by the group of biologists, “the additional predation pressure would have increased stress levels and led to a disruption of the newts’ immune system, consequently causing an imbalance in their relationship with the ranavirus and increased susceptibility to the disease”.

Demography of the adult marbled newt (Triturus marmoratus) population of Lagoa dos Carris, Peneda-Gerês.

The biologist sees the mission of trying to eliminate this fish and recover the lost biodiversity as a difficult objective, explaining that “any measure we have to take there is not easy, because we are talking about a natural space with a high conservation status and […] the most remote space in Portugal”. However, the group of researchers is analysing the alternative of emptying the reservoir and “capturing and eliminating the sun-perch”, or else emptying the reservoir until the “amphibians from the surrounding area disperse and occupy that space to re-establish some balance”.

Currently, according to the biologist, in the rail lagoon, “there are practically no amphibians, and in this case, the barramundi species, an American species that has no interest in fishing, is one of the major responsible for this environmental catastrophe”, since, even though it is not the carrier of the “correct” strain of the virus, it has created the conditions for its propagation among amphibian populations.

Luís Martins, Freelancer

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