Small pelagic fish, trawling discards and breeding performance of the critically endangered Balearic shearwater: improving conservation diagnosis

Conservation diagnosis should identify which demographic parameters (or vital rates) are having a negative effect on the population growth rate of a threatened organism. Diagnosis can analyse how the environment is influencing the vital rates of this organism. In the present study, we tried to improve the diagnosis of a critically endangered seabird, the Balearic shearwater Puffinus mauretanicus, through analysis of the spatio-temporal variability of breeding performance, using long-term data sets covering most of its breeding range. We also analysed the potential influence of 2 food resources, namely small pelagic fish and trawling discards, as measured by purse-seine and trawling landings, respectively. We found inter-annual variability in breeding performance, which could be related to the changes in the availability of both small pelagic fish and trawling discards. Results showed that these variations affected all study colonies in a similar way, thus supporting previous investigations that identified a major foraging area for the whole breeding population. A fitness landscape suggested that current values of breeding success are not responsible for the sharp decline of the species, although incoming fisheries policies could affect its breeding performance in the long term. Factors affecting adult survival, both at sea and in colonies, remain the most important threat, and conservation efforts should concentrate on these issues to prevent the extinction of this endemic species of the Mediterranean region.