Blind shots: non-natural mortality counteracts conservation efforts of a threatened waterbird

Waterbirds are particularly affected by the high hunting pressure they face in many regions, which in some cases is compromising conservation actions for threatened species. The marbled teal Marmaronetta angustirostris is one of the most endangered waterbirds in Europe. In order to restore its population, several conservation actions have recently been undertaken, including a population reinforcement programme in Spain using captive-bred birds. With the aim of assessing the success of the reinforcement programme to establish a long-term self-sustaining population, we identified mortality causes of marbled teal, evaluated the survival of individual birds of the reinforcement programme and estimated the viability of the population under different management scenarios. We used data from wild and captive-bred individuals tracked by GPS since 2018 (n = 42) and from a mark-recapture programme initiated in 2015 (n = 297). We recovered 15 dead birds or transmitters: 20% died of natural causes, 60% of non-natural causes (including all anthropic causes) and 20% of unknown causes. Furthermore, the GPS tags of 24 birds unexpectedly stopped transmitting without any indication of malfunction, and for 66.7% of these disappeared birds, the cessation was suspected to be caused by illegal shooting. Survival during the hunting season was higher for males (31.3%) than for females (12.5%), and for the wild (50%) than for the captive-bred birds (9.4%), probably due to differences in migration patterns to North Africa. Population viability models revealed that maintaining the breeding population at the current mortality rates is only possible with a permanent release programme of captive-bred individuals, and that in order to establish a self-sustaining population, non-natural mortality would have to be reduced by at least 40%. We recommend management measures to reduce marbled teal mortality, such as limiting legal hunting to hours with clear visibility, prosecuting illegal shootings, controlling exotic predators and improving water management to reduce disease outbreaks. Some improvements can be implemented in captive-breeding programmes, such as earlier release times and incorporating anti-predator training. Las aves acuaticas se ven especialmente afectadas por la elevada presion de la caza que sufren en muchas regiones, comprometiendo en algunos casos las acciones de conservacion de las especies amenazadas. La cerceta pardilla Marmaronetta angustirostris es una de las aves acuaticas mas amenazadas de Europa. Recientemente se han emprendido varias acciones de conservacion, entre ellas un programa de refuerzo con ejemplares criados en cautividad en Espana. Con el objetivo de evaluar el exito del programa de refuerzo para establecer una poblacion auto sostenible a largo plazo, identificamos las causas de mortalidad, evaluamos la supervivencia de los individuos del programa de refuerzo y estimamos la viabilidad de la poblacion bajo diferentes escenarios de gestion. Utilizamos datos de individuos silvestres y criados en cautividad rastreados por GPS desde 2018 (n = 42) y de un programa de marcaje y recaptura iniciado en 2015 (n = 297). Recuperamos 15 aves o transmisores muertos: el 20% murio por causas naturales, el 60% por causas no naturales y el 20% por causas desconocidas. Ademas, 24 aves dejaron de transmitir inesperadamente sin ninguna indicacion de mal funcionamiento, para el 66,7% de las desaparecidas se sospecho que el cese fue causado por disparos ilegales. La supervivencia durante la temporada de caza fue mayor para las silvestres (50%) que para las criadas en cautividad (9,4%), probablemente debido a las diferencias en los patrones de migracion al norte de africa. Los modelos de viabilidad poblacional revelaron que el mantenimiento de la poblacion reproductora con las tasas de mortalidad actuales solo es posible con la liberacion permanente de individuos criados en cautividad y que para establecer una poblacion autosostenible, la mortalidad no natural tendria que reducirse al menos un 40%. Para ello, recomendamos que se establezcan medidas de gestion como limitacion de la caza legal a las horas de clara visibilidad, persecucion de la caza ilegal, control de depredadores exoticos, y la mejora de la gestion del agua para reducir los brotes de enfermedades. Pueden introducirse algunas mejoras en los programas de cria en cautividad, como adelantar el momento de la suelta e incorporar entrenamientos de comportamiento antipredador. The assessment of the marbled teal population reinforcement programme in Spain carrying out releases of captive-bred birds showed that 60% of the mortality is due to non-natural causes, mostly illegal shooting. To establish a self-sustaining population, non-natural mortality should be reduced by 40% and management measures such as prosecution of illegal shooting, control of exotic species and improved water management are recommended.image