Conservation of the Giant Freshwater Pearl Mussel (Pseudunio auricularius = Margaritifera auricularia Spengler, 1793) in Spain

Resumo

Freshwater mussels are one of the most threatened faunal groups in the planet. Among them, the Giant Freshwater Pearl mussel (Pseudunio auricularius, Spengler 1793) is the most endangered unionid bivalve in Europe, and the population of the Ebro River basin in Spain is in a critical situation. The aim of this thesis is to increase the scientific knowledge of the current conservation status of the species in Spain in order to apply meaningful management actions to reduce its risk of extinction. The decline of freshwater mussel (Order Unionida) populations in the Ebro basin is widespread, but the situation in the associated canals is critical, especially since 2013 when an episode of mass mortality of P. auricularius in the Canal Imperial de Aragón (CIA) was recorded. In the framework of a long-term study aiming to assess mussel densities in the canals, the obtained results showed a continued and generalized decrease of all native mussel species, ending with the local extinction of Anodonta anatina (Linnaeus, 1758), Potomida littoralis (Cuvier, 1798) and Unio mancus Lamarck, 1819, together with thousands of P. auricularius' dead specimens. One possible explanation for these declines is aging and possible death due to senescence. To find out the relationship between shell length and age in P. auricularius in the Ebro basin, its growth pattern was analysed by counting the shell rings of adult and young specimens together with growth data of captive-bred juveniles. The generalized von Bertalanffy and Richards sigmoidal models presented the best fit, showing that the growth rate of P. auricularius is relatively slow after 30 years, when maximum growth is reached. The maximum age estimated for a specimen in the Ebro basin was 68 years. Another possible explanation of the population decline was environmental pollution derived from anthropogenic activities. Considering the lack of previous knowledge on the species' sensitivity, acute toxicity tests against heavy metals and ammonium were carried out for the first time using juveniles from a captive breeding program. In this way, the lethal concentration values (LC50 and LC10) were determined for cadmium, copper, nickel, zinc, chromium, lead and ammonium. In addition, non-observed-effect concentration (NOEC) values, lowest-observed-effect concentration (LOEC) values and maximum acceptable toxicant concentrations (MATC) were assessed for each contaminant tested. When comparing the results of P. auricularius with available data from other mussel species, it seems to be more sensitive to cadmium and copper, and less sensitive to nickel, lead, and total ammonia. As one of the main actions planned for the recovery of the species, laboratory cultures were established to develop a captive breeding protocol for juveniles of P. auricularius, by testing the effects of several conditions on survival and growth. The best results corresponded to a treatment in glass containers at a density of 0.2 ind/L, using river water, substrate, detritus and phytoplankton, and without extra aeration. The highest survival and growth rates achieved were 60% at 100 days and 2.56 mm shell length at 30-32 weeks of life. Captive breeding is considered one of the most important management tools to improve the conservation status of P. auricularius, and these valuable results allow establishing, for the first time, the best conditions for maximizing juvenile survival under laboratory conditions. Finally, and with the objective of increasing the survival probabilities of the living specimens left in the CIA, a translocation plan was initiated in 2017. Adult specimens were translocated from the CIA to the Ebro River using plots previously characterized as suitable for the survival of P. auricularius. A total of 638 specimens (291 specimens in 2017, 291 in 2018, and 56 in 2019) were translocated. A monitoring survey, one year after, allowed estimating a survival rate between 40 and 70%. On the other hand, the control group left in the CIA had a lower survival rate of just 19.7% after one year. Based on these first results, it was confirmed that the specimens of P. auricularius translocated to the main river channel showed higher survival than those that remained in the canal. Overall, the autecological information collected will provide support for the application of the best restoration measures in order to bend the curve of decline of this iconic riverine species in Spain.