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The importance of fish in monitoring biodiversity in river ecosystems

Improving water ecological status and key species populations


by Marija Smederevac-Lalić



Marija is a Research Associate Professor at the University of Belgrade - Institute for Multidisciplinary Research, Serbia. She leads a programme of work and a group of project partners tasked with the collection of data on the current status of water ecological quality and aquatic biodiversity, with particular attention on three project restoration sites along the Danube River Basin.


The group's work is focused on the creation of a framework for a trans-boundary monitoring scheme that will improve the outcomes for assessing water ecological status i.e.,water quality and functioning, and the status of fish community, abundance and structure along the Danube.


We caught up with Marija to learn more about this fascinating scientific strand of the project.





Tell us about your work as part of DANUBE4all?


The main aims of the team of project partners I work with are to understand the current status of water ecological quality and biodiversity, particularly as it relates to aquatic biodiversity, and to deliver new insights on how changes in river connectivity impact it.


As a first of its kind project, DANUBE4all will provide integrated knowledge of the basin-wide distribution of endangered and invasive species in the Danube based on a combined assessment of rivers, floodplains, the Delta, and coastal systems. While we will work on the collection of monitoring data for the whole Danube, our restoration actions will focus on the three DANUBE4all demonstration sites only, where work has already begun on the monitoring and assessment of biota and water (ecological) status. As river restoration measures are planned at each of these sites, these ‘before and after’ assessments of biota and fish status will demonstrate how effective these restoration implementations are and what impact improving river connectivity can have on biodiversity status.


At a wider level, our work will conceptualize, and importantly demonstrate, how we can more accurately monitor biota and the water ecological status at a transboundary level across the Danube countries. Fish are a key element in river ecosystems and therefore it is vital to understand their status in order to get a comprehensive picture of wider aquatic biodiversity.


Left image: Barbels (Barbus barbus) from the Danube in Serbia (Photo: Smederevac-Lalić, 2023)

Right image: Riparian zone of the Danube River in Serbia (Photo: Smederevac-Lalić, 2023)



Fish provide many ecosystem functions and services – they are a food source and support local economies but crucially, they are of great value in terms of aquatic diversity.


The position of fish in the food web and their sensitivity to a wide range of influences make them suitable indicators of the biological integrity and overall health of a water ecosystem. Healthy fish populations indicate a healthier aquatic environment. In addition, as fish are relatively long-living organisms compared to other aquatic groups, cumulative environmental changes over the longer term can be detected by monitoring them (Alonso et al., 2011). As fish integrate the full spectrum of chemical, physical and biological influences, they make for excellent indicators of the history of a particular habitat across time.


The fish community, especially in large rivers, is characterized by a great diversity, which reflects the structural richness of the habitat and the structure of a fish community - particularly the abundance of certain species – is indicative of the comprehensive conditions of that habitat.


A wide range of abiotic, spatio-temporal variables relates to the habitat requirements of certain species and their life stages (Schiemer, 2000), to species composition, population numbers and age structure.


To explain, abiotic means the non-living physical and chemical factors that shape an ecosystem. In a freshwater ecosystem such as a river, these could be water temperature, sunlight levels, oxygen concentration, nutrients, substrate composition, and water flow. Spatial variables relate to variations according to geographical location, while temporal variables relate to variations across different time scales, such as seasons or years.


When we look at the policy framework for defining ecological status, we can see that fish are central to the process. The EU Water Framework Directive (WFD) considers the composition of the fish community, abundance, and structure as relevant elements to assess the ecological status. National and regional river basin management plans base their management decisions on the response of aquatic organisms to their environmental conditions.


Conserving fish helps to create unique and vibrant ecosystem. It’s imperative that we focus on improving ways to assess the health of fish communities so we can gain a comprehensive view of biodiversity in our aquatic ecosystems as a whole.


Sterlets (Acipenser ruthenus) in the Danube River in Serbia (Photo: Smederevac-Lalić, 2023)





What do you hope to achieve as part of DANUBE4all?


So as to understand what restoration actions to prioritize and where, and also how to upscale and replicate these actions in other areas, we will need to model the availability of suitable habitats for both protected species and critical invasive species. This will involve creating basin wide maps of distribution for at least 15 of these species. For these models, we will draw on information from published studies, open available data sets, databases from our DANUBE4all project partners, and from consultation processes with experts.


We plan to cover approximately 800 km of river stretches in our transboundary monitoring of fish status in the Danube. Our aim at the first research site is to increase the levels of rheophilic juvenile fish along the shorelines of restored river banks by 50% after the implementation of restoration measures. At the second site we aim to increase rheophilic fish biomass as a result of planned groyne-optimization there, and at the third site, to increase fish biodiversity (higher species number) compared to the situation before in the reconnected lagoon systems in the Delta research site.


It will be a busy 2024!






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