Maritime traffic accounts for more than 90% of world trade. Noise, pollution, and litter are its drawbacks, affecting especially vulnerable migratory fish. Here, a motivated team of citizen scientists analyzed maritime traffic from three estuaries of the south Bay of Biscay and three from the south of the Iberian Peninsula, where the European eel is critically endangered, during the season of the entrance of glass eels. More than 164,000 data points about ship types and positions were collected. The results showed that traffic differences between estuaries would explain, at least partially, the different eel conservation statuses. The participants appreciated learning about ships and nature conservation and acquiring an awareness of the real volume of shipping and its potential impacts. All the citizen scientists, new and experienced, would like to get involved in ocean research again.

Understanding the effects of global change is crucial to aid in the conservation of critically endangered species such as the European eel (Anguilla anguilla). Emerging pollutants (microplastics) and legacy ones such as heavy metals, as well as thermal pollution, are deleterious for this species; however, their molecular impacts on glass eels entering estuaries remain understudied. Investigating the molecular signatures of environmental stress can provide insights into some of the factors contributing to A. anguilla population decline. In this study, expression changes in candidate genes associated with general and thermal stress (hsp70 & hsp90), oxidative stress (sod1, sod2, cat, gpx1), heavy metal detoxification (mt2), and neurotoxicity (ache) were measured in gill tissue of glass eels exposed to polystyrene (PS) microbeads in an aquarium, and also in individuals sampled from six Bay of Biscay estuaries with different levels of microplastics, heavy metals, nitrites, nitrates and thermal pollution. In the aquarium experiment, a significant positive correlation was found between hsp70 expression and PS concentration. On the other hand, in the glass eels sampled from the wild results showed that gene expression patterns tend to cluster based on estuary's pollution levels. Specifically, hsp70 gene expression was upregulated in eels from the most polluted estuaries, suggesting signs of damage. This study highlights the molecular effects and signatures of emerging and legacy pollutants on the critically endangered European eel, under natural and controlled conditions.

Citizen science and environmental DNA (eDNA) are increasingly used to detect and monitor invasive species in freshwater ecosystems, yet their combined application to investigate invasion drivers and spread has rarely been explored in southern Europe. In this study, we integrated both analyses to assess the expansion of the invasive signal crayfish Pacifastacus leniusculus in Asturias rivers (northern Spain), where the species was first recorded in 2013. Citizen scientists collected and filtered 69 water samples from 39 sites for eDNA extraction, and recorded visual observations, physico-chemical parameters and anthropogenic litter. Signal crayfish-specific nested PCR detected presence at 15 of the 39 sites (38.5%), including locations absent from official records, particularly in coastal reaches and headwater streams. Signal crayfish DNA was significantly associated with fishing litter, suggesting intentional introductions by recreational fishers. It occurred more frequently in sites with high chloride concentration, consistent with the species' tolerance to disturbed environments. More frequent detection from coastal reaches than in midstream or headwater sections suggested a downstream colonization gradient, but when corrected by the connectivity between basins it points to human-mediated introductions rather than natural dispersal. Overall, our results show that integrating citizen science, eDNA and litter assessments provides an effective approach for identifying invasion fronts and potential introduction pathways, supporting the management and control of this globally invasive species.

Maritime traffic accounts for more than 90% of world trade. Noise, pollution, and litter are its drawbacks, affecting especially vulnerable migratory fish. Here, a motivated team of citizen scientists analyzed maritime traffic from three estuaries of the south Bay of Biscay and three from the south of the Iberian Peninsula, where the European eel is critically endangered, during the season of the entrance of glass eels. More than 164,000 data about ship types and positions were collected. Results showed that traffic differences between estuaries would explain, at least partially, the different eel conservation status. The participants appreciated learning about ships and nature conservation and acquiring awareness of the real volume of shipping and its potential impacts. All the citizen scientists, new and experienced, would like to get involved in ocean research again.