Monitoring the olm population in Krka NP

Monitoring the olm population in Krka NP
The olm is the only true stygobiont vertebrate in Europe, and the species is specific because it is the only living representative of the genus Proteus.

Its nearest relative is the genus Necturus from North America, indicating that the olm has been present in this area for more than 200 million years. This incredible species witnessed both the rise and fall of the dinosaurs.

Monitoring the population of the olm, Proteus anguinus, is conducted using the linear transect method during periods of low water levels, when flow is weak and visibility is optimal. The researcher, a cave diver, dives along a previously defined transect line of known length, and counts all the individuals they see underneath and to the left and right of the line. These data are then analysed in the program DISTANCE 8.0 to give an absolute assessment of the population, i.e., the number of adult individuals of olm per square metre. The final result is calculated as the relative size of the adult population and sexually mature individuals for the entire locality (known part of the cave). This method is sufficient for this type of monitoring, in which the primary aim is to track population trends over longer periods of time.

The olm is a target species of the NATURA 2000 ecological network, Krka National Park area (HR2000918). In the part area, this species is present in caves in the Miljacka slap waterfall area: Miljacka II, Miljacka III, Miljacka IV and the system Miljacka I-V, and in the Nasuprot Torku pit (pit opposite of Torak) at the mouth of the Čikola into the Krka. Systematic monitoring of the olm population has been ongoing in the Miljacka II cave and cave system Miljacka I-V since 2017 (previously from 2012 through the Proteus project), since 2019 in the cave Miljacka III, and started for the first time this year in the cave Miljacka IV. Monitoring along the defined transect in the caves in the Miljacka slap waterfall areas was conducted during August and November 2022. Specimens of olm, both adult and subadult, were recorded in all caves. The estimated population in the cave system Miljacka I-V is 12 individuals/3600 m², followed by the populations in the caves Miljacka II and Miljacka III, each with 5 individuals/3600 m². The estimated population size of 18 individuals/3600m² in the cave Miljacka IV is the result of the small number of recorded individuals and the small number of repeated transects, indicating the assessment limits (and low precision), and therefore for a more precise assessment of this locality, greater efforts are required with a minimum of two monitoring sessions each year.

An analysis of the population trend over a five-year period in the cave system Miljacka I-V and the cave Miljacka II indicated a negative trend, even though no direct causes of threat were identified. Since the olm population is present within all parts of the cave, these oscillations are also possible due to the movement of individuals within the cave system due to water levels and food accessibility. For the caves Miljacka III and Miljacka IV, the number of individuals and number of monitoring sessions was not sufficient to assess trends. Based on the initial data, it appears that the population in the cave Miljacka III is similar to that in the cave system Miljacka I-V. The area of Krka National Park provides a safe haven for this species, however, the water flowing through these underground systems can potentially carry harmful compounds that could have a negative impact on the olm and other cave fauna.

With the aim of finding the potential cause for the observed negative trend, further research and monitoring will be conducted in order to ensure a timely response, and to preserve the habitat conditions and population stability of the olm.

Photo: Petra Kovač Konrad, Dušan Jelić