Here’s an update on a topic that has been making headlines lately… fish! While another species may be attracting most of the attention this summer, MedINA is turning the spotlight on the trout that inhabits the Aoos River.
But first, let’s start with the basics:
Freshwater fish account for around 50% of all known fish species worldwide, despite freshwater ecosystems covering just 1% of the Earth’s surface. This highlights the remarkable biodiversity these habitats support. Yet today, one in three freshwater fish species is threatened with extinction.
Fish play a vital role in the functioning of freshwater ecosystems. They contribute to the transfer of energy and nutrients, regulate populations of other organisms through food-web interactions, and help maintain suitable habitats for a wide range of species. They also represent an important cultural and economic resource for people.
In a river, the presence of fish and their populations status are key indicators of ecosystem health. Their survival depends directly on water quality, the river’s natural flow, which allows them to move freely, and the availability of food. By conserving fish and maintaining healthy populations, we contribute to the long-term health and resilience of the river ecosystem.
Voidomatis River © Yorgos Gounezos / MedINA
The fish fauna of the Aoos River
The Aoos River and its tributaries are among Europe’s last wild rivers and are home to a truly exceptional ecosystem. This near-pristine river system supports 11 recorded native fish species, nine of which are endemic to the Balkans, meaning they occur nowhere else in the world except in this corner of Europe!
Among these 11 species are the Ionian trout (Salmo farioides), the European eel (Anguilla anguilla), and other important species such as the Prespa barbel (Barbus prespensis), the Pindus stone loach (Oxynoemacheilus pindus) and the Albanian roach (Pachychilon pictum). Perhaps the most remarkable feature of the Aoos is that it and its tributaries mark the southernmost distribution of many of these species in the Balkans and the only location in Greece where they can be found.
In particular, the Ionian trout survives in only a handful of river catchments in Greece, inhabiting the cold, well-oxygenated waters of mountain rivers. The disappearance or decline of this wild trout from a river signifies more than the loss of a single species; it is a warning that the entire ecosystem is at risk. Conserving and protecting its populations is therefore of critical importance, as reflected in its legal protection status: In Greece, the Ionian trout is a protected species. Current legislation permits recreational angling only under specific conditions, including designated fishing seasons, authorised fishing methods, minimum catch sizes and catch limits.*
The Ionian trout is not the only trout species found in the area. In recent years, the non-native rainbow trout (Oncorhynchus mykiss) has also been recorded in the Aoos and Voidomatis rivers. Native to North America, it was introduced to Greece through aquaculture.
Unlike the native Ionian trout- often referred to as the “wild trout”, the rainbow trout has the potential to disrupt the ecological balance of the river, as it is not a natural component of the ecosystem and competes with native species for food. The extent of its distribution in the Aoos River, as well as the size of its populations, remains the subject of ongoing scientific investigation.
Threats and pressures to the fish populations of the Aoos
The fish populations of the Aoos face more than just the challenge posed by this non-native species. They are under pressure from a range of threats and pressures, including habitat degradation caused by the lack of the required ecological flow, water abstraction and river diversions; artificial barriers that restrict the free movement of fish; illegal fishing and overfishing; and the impacts of the climate crisis. Together, these pressures place increasing strain on the resilience of the river’s native fish populations.
The combination of exceptional biodiversity, ecosystem vulnerability and multiple pressures makes the study of the Aoos fish populations more important than ever. By understanding the interactions between native species and the introduced rainbow trout, and by examining how human activities and climate change are shaping the river’s future, we can establish the foundations for protecting this unique ecosystem.
Protecting the wild trout at the heart of the Ri-Connect II Project
This is where the Ri-Connect II project: The Aoos River as a Transboundary Ecological Corridor comes into play. Working in the Aoos River basin, members of the MedINA team, together with researchers from the Institute of Marine Biological Resources and Inland Waters (IMBRIW) of the Hellenic Centre for Marine Research (HCMR), recently carried out extensive fieldwork. The mission aimed to monitor the ecological condition of the river, investigate the distribution of the native Ionian trout (Salmo farioides) and the non-native rainbow trout (Oncorhynchus mykiss), and strengthen cooperation with local communities, particularly recreational anglers.
Water quality sampling was conducted in the Aoos and Voidomatis rivers. In addition, a preliminary assessment was undertaken to evaluate the feasibility of conservation measures aimed at reinforcing wild trout populations. Sampling and field measurements were also carried out at the Konitsa irrigation dam to assess the feasibility of installing a fish passage that would restore longitudinal connectivity regarding fish migration.
© Yorgos Gounezos / MedINA
To conclude the field activities and coordinate the next steps, a knowledge exchange meeting was organised with the participation of recreational anglers and representatives of the Panhellenic Fly Fishing and Wild Trout Conservation Association “Astraios”. During the meeting, MedINA and IMBRIW presented their joint work in the area, followed by a discussion on the pressures and threats facing the wild trout and the wider river ecosystem. The meeting highlighted a shared vision: to monitor the spread of rainbow trout and track water quality as key steps towards safeguarding the Aoos River.
The trout of the Aoos are far more than just fish. They are indicators of the health of this unique ecosystem. By combining scientific knowledge with close collaboration between researchers and local communities, we can help ensure that the Aoos remains a living, wild river!
Highlights from fieldwork © Yorgos Gounezos / MedINA
These activities were carried out as part of the Ri-Connect II project, funded by the Prespa Ohrid Nature Trust – (PONT).
*The Ionian trout is listed in Annex II of the Habitats Directive (92/43/EEC) and is classified as Vulnerable in the Greek Red Data Book of Threatened Animals, indicating that it faces a high risk of extinction in the wild. It is also included in the National Action Plan for the Five Native Trout Species of Greece, established by Ministerial Decision ΥΠΕΝ/ΔΔΦΠΒ/68085/2148/2021 and published in the Government Gazette (FEK B’ 3722/12.08.2021).