Fish
Back to the Monitoring Hub
Increase diversity
Data collection in progress
Iberian Highlands
Spain
Problem: Various fish, such as cyprinids (carp), need to be able to move to different stretches of the river in different seasons, especially in Mediterranean areas where summer droughts affect their reproductive and survival strategies. The presence of artificial barriers prevents these species from completing their biological cycles normally, leading to local extinctions with the consequent loss of biodiversity and ecological processes.
Action: The project will increase connectivity for fish species by working with the regional government to remove dams and weirs and install fish ladders.
Indicator: Changes in the number of fish species detected before and after the removal of barriers will indicate any changes in river connectivity.
Methods
The number of fish species present were assessed using DNA metabarcoding. Baseline samples for environmental DNA analysis were taken upstream and downstream of two weirs prior to their removal. At each sampling site, one litre of water was collected and passed through a filter. The sample was then stabilized with a preserving liquid and sent to NatureMetrics for processing. DNA from each filtered sample was then extracted, purified, amplified and screened for the presence of fish species.
The protocol and spatial design are being reviewed, and the river barriers across the landscape are being mapped, to identify those that should be prioritised for removal in 2025. The final sampling strategy for fish monitoring will be decided based on the outcomes of this mapping. At a minimum, the baseline eDNA analysis will be revisited in the last year of the project, but a more targeted approach may be designed, depending on the barriers prioritised for removal.
Before (left) and after (right) removal of the Molino Bajo weir. Photo: AEMS – Ríos con Vida.
Results
DNA was successfully amplified from all four samples taken prior to barrier removal (Fig. 1). The only native freshwater fish species detected at all four sites was trout (Salmo trutto). European hake (Merluccius merluccius), a marine fish, was also detected at one of the four sites, but this likely came from contamination via the movements of a bird or mammal. DNA from some bird and mammal species were also found.
Figure 1: Presence of fish (purple), bird (yellow) and mammal (red) species found in the four water samples. As depicted by the large purple circles, trout (Salmo trutto) was found at all four sites, regardless if the samples were taken up- or downstream of the barrier.
Interpretation
Because the same single native fish species (trout) was detected both upstream and downstream of the barriers to be removed, it is unlikely that removing these barriers will have an immediate effect on the fish communities in these parts of the Cabrillas river. However, other native fish, particularly from the family Ciprinidae, are known to be present in the wider river basin and so as more barriers are removed in the future continued monitoring will reveal whether new species return to upstream parts of the river.
Lessons learned
Metabarcoding approaches only reveal the presence/absence of species in an area of river. In the future this methodology may be accompanied by electrofishing to provide more detailed information on the number and age/size of different fish species present, in addition to mapping additional barriers across the river system and identify priorities for barrier removal.
Partners
