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The cavilat (Cottus hispaniolensis) is an endemic fish of the upper basin of the Garonne River (southwest of France and north of Catalonia) with a very small global distribution area, being present in the Iberian Peninsula only in the Aran Valley, where it is home to about 15% of its populations globally. Historically, their populations were distributed continuously along the main course of the river and in the lower parts of some tributaries, but today they are fragmented, distributed in a mosaic form and with low densities.

Cavilat populations have experienced a sharp decline in recent decades. In the case of the Spanish population, it has been considerably reduced over the last decades, reaching historic lows after the strong floods of 2013. Despite the actions to improve habitat and captive breeding carried out since then, the population of cavilat is still very scarce without knowing in detail what the causes are. Therefore, it is now a priority conservation species in our country, listed in the “Endangered Category” according to the latest edition of the Red Book of Vertebrates of Spain.

On the other hand, according to the entity, the scarcity of basic scientific research on cavilat has created a knowledge gap about its biological and ecological requirements. In addition, this would make it difficult to understand how their populations are regulated and would prevent the development of a correct conservation plan for the species.

The general objective pursued by the project has been to determine the basic biological and ecological requirements that can serve as a basis for the development of the National Conservation Strategy (ENC) of cavilat (Cottus hispaniolensis).

The specific objectives were as follows:

• Precise delimitation of the current distribution area of cavilat in Spain.
• Evaluation of river connectivity in the species’ distribution area and analysis of the current fragmentation of its populations.
• Evaluation and update of the conservation status of Cottus hispaniolensis.
• Enhancement of the cavilat population in a selected river section through the implementation of physical habitat management measures, focused on promoting reproductive success.

• A detailed mapping of the current distribution of cavilat was carried out, based on a study of the riverbed of about 40 hectares.
• Preparation of three curves of habitat preference for the cavilat, whose importance has been greatly highlighted by the entity since they are the first to be carried out for this species in Spain.
• Delimitation of the potential distribution area of the cavilat, in terms of suitability of physical habitat for the species, and analysis of the influence of three different climate change scenarios: one with a warming of the waters of 0.5 ºC, another of 2 ºC and a last of 5 ºC.
• Carrying out the inventory of barriers to fluvial continuity in the area of the species, drawing up a map with all the information and 22 descriptive sheets of each of the hydraulic works that may hinder the passage of specimens upstream or downstream.
• Analysis of the genetic fragmentation of the Spanish population of cavilat, carried out on 135 new samples from 8 groups in addition to another 151 samples previously analysed within the framework of the LIFE+ LimnoPyrenees project, which has allowed the genetic analysis of 286 specimens in total, captured in 6 sections of the river. The analysis has revealed that there are between 5 and 6 major genetic groups, which are distributed over a large part of the Garonne River.
• Quantitative censuses of the population of Cottus hispaniolensis were carried out, based on the analysis of 49 specimens captured in 13 sections of rivers sampled. Thanks to this analysis, it was possible to verify that there are at least three age groups, which would indicate that the species has reproduced in 2020: fry (less than one year of age), juveniles and adults, the latter being the most abundant. The data obtained in this study have been communicated to the Iberian Society of Ichthyology (SIBIC) for inclusion in the Spanish Fish Farming Chart.
• Design of a map to identify potential areas for reintroductions and restocking of cavilat and preparation of a protocol that establishes the basic criteria to be taken into account in translocation, captive breeding and repopulation actions. According to the entity, the knowledge of the stretches of river that can act as “donors” and those that can do so as “recipients” of cavilat specimens, constitutes one of the most relevant milestones of the study and represents the culmination of all the previous actions, being essential information for the recovery of the species. In addition, the protocol generated from this information can be an essential tool for the management and conservation of their populations.
• Improvement of the cavilat breeding habitat , by placing 8 artificial structures of 13 units each (i.e. 104 nests in total) along two river sections of the river, to improve the quality of the nests and, with this, improve reproductive success.
• Carrying out maintenance campaigns and monitoring the use of breeding structures.
• Dissemination and communication of the project through different channels, including the creation of a website, the publication of a press release and several press releases and the publication on paper of a leaflet with all the information about the project. In addition, an informative day was held on the main results of the project and presentations were made in two scientific forums: at the Iberaqua-net Congress and at the “Workshop on trout mobility (Salmo trutta)” in Moulis (France).

This project has contributed to reducing uncertainties about the requirements of cavilat (Cottus hispaniolensis) in Spain and to providing information for practical and applied use for the conservation and improvement of its population. In this line, the study carried out by the University of Lleida has achieved a series of relevant results to advance in the knowledge of its requirements, which lays the foundations for being able to develop a conservation strategy for the species.

Firstly, the sampling of about 30.5 km of riverbeds in the headwaters of the Garonne, which is home to an estimated population of about 640 specimens of cavilat, has made it possible to determine the current and potential distribution of the species in the middle-lower section of the Spanish part of the river basin. Likewise, it has been found that the optimal physical habitat is found in somewhat deep areas and with a medium-low water velocity, thanks to the elaboration of habitat preference curves, highlighted by the entity as a relevant step for the recognition of the ecology of the species and the conservation management options of its population and habitat. As for the three climate change scenarios analysed, the results obtained allow us to conclude that the populations are not significantly affected, at least in terms of habitat alteration.

On the other hand, the analysis of the fluvial continuity and the possible “barrier effect” of existing hydraulic works on the populations has made it possible to identify 22 works that can hinder the passage of specimens. In addition, it has been observed that this effect causes the axis of the Garonne River and the lower sections of its main tributaries to be segmented into 6 sections, as there are 11 impassable infrastructures for movements of the cavilat upstream.

In addition, a map has been drawn up to identify the potential areas for reintroductions and restocking of cavilat along the river, identifying 3 sections as potential “donors” and 2 sections as potential “recipients” for the translocation of individuals. According to the entity, the knowledge of the stretches of river that can act as “donors” and those that can do so as “receivers” of specimens constitutes one of the most relevant milestones of the study, since it is essential information for the recovery of the species. In addition, the protocol for translocation, captive breeding and repopulation actions, generated from this information, can constitute an essential tool for the management and conservation of their populations.

Finally, an improvement of the cavilat breeding habitat has been carried out, through the placement of 8 artificial shelter and breeding structures composed of 104 nests in total, with the aim of improving reproductive success in the future. In this sense, the entity highlights that the management of the population of Cottus hispaniolensis in order to recover it can be based on three population strategies, which are not necessarily exclusive: a strategy of introducing specimens in ecologically suitable areas where the target species is not present; a reintroduction strategy in sections where it has disappeared; or a supplementation of individuals in existing populations.