Introducción de peces en el lago Pyrin durante la época romana

Using sedimentary DNA from the lake for Long-term changes in terrestrial and aquatic biota (fish) have led to rapid evolution in paleo-ecological research. Although widely applied today, gaps in knowledge persist in this area, prompting ongoing research to ensure the reliability of sedimentary DNA signals.

Based on the latest literature and seven original case studies, researchers have Synthesized cutting-edge analytical procedures for efficient DNA sampling, extraction, amplification, quantification, and/or inventory generation.

They employed Ancient sedimentary DNA (Sedadna) through high-throughput sequencing technologies and presented recommendations based on current knowledge and best practices.

Reintroduced Fish

The ancient DNA from the Redon Pyrenean Lake in Catalonia reveals that people may have intentionally added fish to the lake in the 7th century to provide food while grazing sheep. This occurred several hundred years before the first historical documents, and its discovery revolutionizes our understanding of past activities in the Pyrenees.

This conclusion comes from a study Led by researcher Cref Elena Fagín and CSIC researcher Cref Jordi Catalan published today in Communications in Nature. These findings align with archaeological discoveries in the vicinity, strengthening the notion that the region had grassy areas used by late Roman and Visigothic inhabitants for livestock.

To uncover this, the team examined the hidden history preserved in lake sediments spanning 3,300 years within just 30 centimeters. Generally, sediments do not retain fish DNA and can be lost, but the team employed an innovative methodology: analyzing fish parasite DNA, which was well-preserved.

«Fish cannot access high mountain lakes and can only reach them if someone carries them, making their presence an indicator of human intentionality and activity,» explains Elena Fagín, lead researcher and one of the main authors of the study.

Historical documents began recording the presence of fish in European lakes in the 14th and 15th centuries, detailing fishing and trade rights, «but due to the memory stored in sediments, we can go back several years, before documentation existed, and pinpoint it more accurately,» adds the author.

Data suggests that the fish population in the lake has remained constant over the years despite changes in the human population in the surrounding area, although it may have been impacted by climate trends, such as cold or warm periods.

Aside from parasites, the study also analyzed other environmental indicators, such as photosynthetic pigments also present in sediment, providing insights into the amount of photosynthetic organisms that lived in the lake in the past.

It was observed that the presence of fish parasites coincided with an increase in lake productivity: there was more photosynthetic activity. According to the team, this may be due to increased sheep (mainly) grazing in the late Roman and Visigothic era causing higher soil erosion, leading some nutrients from the land to be transported to the lake during rainfall or snowfall.

«The nutrient contribution favors phytoplankton growth and is reflected in the growth of photosynthetic pigments preserved in the sediments,» explains researcher from Cref and the University of Barcelona Marisol Felip, also a co-author of the study.

Parasitic Gill Organisms

To delve into the history of mountain lakes, so-called «sediment cores»—cylindrical samples extracted from the lake bottom through drilling—are typically used. They observe certain parameters, such as phosphorus and nutrients, as well as heavy metals or remnants of fossils and environmental DNA, among others.

This can help determine, for example, how the climate was and if certain types of life were present. «To get an insight into the value objects of these sediments, every 3 centimeters contain about 100 years of history,» explains Elena Fagín.

Taking this study a step further, researchers utilized, for the first time, fragments of ectoparasite DNA—organisms living on the external surface of the fish, in this case, dependent on it for feeding or completing their life cycle.

«Until now, searching for fish DNA in high mountain lakes was like looking for a fish in an Olympic pool, because depending on where it died, you may or may not find it in the sediment core Parasites are spread throughout the lake, so they are a good indicator of FISH presence,» explains Jordi Catalan, a CSIC researcher at Cref and one of the main authors of the study.

The parasites most easily detected belonged to the genus Ichthyobodo and Flagellate protozoa that live on the skin and gills of fish, although others observed were Kinetoplastea and other groups like Oomycota, Ichthyospores, and Ciliophora.

«These findings demonstrate the potential of ancient parasite DNA in understanding historical human activity and document impacts which have been unclear in settlements within high mountain ecosystems so far,» adds Elena Fagín.

Redon Lake: 40 Years of Research

Lake Redon is located in the Pyrenean Catalani and currently holds a population of approximately 60,000 trout, mainly for recreational fishing. A unique aspect of this lake is that, situated at an altitude of 2,240 meters in the Pyrenees, it has been relatively isolated from human activity for centuries. This allows any ecosystem changes to be more easily attributed to environmental or human factors, such as fish introductions or grazing.

Researchers Marisol Felip and Jordi Catalan have been studying this lake for about 40 years, continuing to investigate various processes. «It’s like having a natural historical archive where we can uncover past ecosystems and also an outdoor laboratory where we can observe how climate changes and human activity influence the ecosystem,» conclude these two researchers.

This research was led by Cref and the University of Barcelona, CSIC, the National Institute of Biology in Slovenia, and Edith Cowan University in Australia.