New population estimates of a critically endangered species, the Balearic Shearwater Puffinus mauretanicus, based on coastal migration counts

Abstract

SummaryThe Balearic Shearwater Puffinus mauretanicus is considered one of the most threatened seabirds in the world, with the breeding population thought to be in the range of 2,000–3,200 breeding pairs, from which global population has been inferred as 10,000 to 15,000 birds. To test whether the actual population of Balearic Shearwaters is larger than presently thought, we analysed the data from four land-based census campaigns of Balearic Shearwater post-breeding migration through the Strait of Gibraltar (mid-May to mid-July 2007–2010). The raw results of the counts, covering from 37% to 67% of the daylight time throughout the migratory period, all revealed figures in excess of 12,000 birds, and went up to almost 18,000 in two years. Generalised Additive Models were used to estimate the numbers of birds passing during the time periods in which counts were not undertaken (count gaps), and their associated error. The addition of both counted and estimated birds reveals figures of between 23,780 and 26,535 Balearic Shearwaters migrating along the north coast of the Strait of Gibraltar in each of the four years of our study. The effects of several sources of bias suggest a slight potential underestimation in our results. These figures reveal the urgent need to reformulate the population viability analysis for the species, and then if necessary reconsider its conservation status.

Demography of the critically endangered Balearic shearwater: the impact of fisheries and time to extinction

“…; Arroyo et al . ). In addition to population growth rates, the deterministic model was used to estimate other important information, such as the stable age distribution, generation time, reproductive value and sensitivities and elasticities.…”

“…To calculate N min , we took the 20th percentile of the distribution of population size following the equation (Dillingham & Fletcher ):where equals 23 780 individuals, and CV N equals 0·03, using the mean and its 95% CI of that estimate provided by Arroyo et al . (). We set f at a conservative value of 0·1, typical for endangered species.…”

“…All the vital rates used in the model were derived from this study, except the probability of skipping breeding, which we obtained from Oro et al (2004). To initialize the models, we used the highest available estimate of current population size, obtained from at-sea censuses (Arcos et al 2012;Arroyo et al 2014). In addition to population growth rates, the deterministic model was used to estimate other important information, such as the stable age distribution, generation time, reproductive value and sensitivities and elasticities.…”

“…whereN equals 23 780 individuals, and CV N equals 0Á03, using the mean and its 95% CI of that estimate provided by Arroyo et al (2014). We set f at a conservative value of 0Á1, typical for endangered species.…”

A comprehensive large‐scale assessment of fisheries bycatch risk to threatened seabird populations

“…However, many species are long‐lived with age‐structured populations and major gaps exist in knowledge of young age classes (juveniles, immatures) and adult non‐breeders, which are challenging to track because of their smaller size or long periods spent entirely at sea (Hazen et al, ; Phillips, Lewis, González‐Solís, & Daunt, ). As non‐breeders can represent a large component of the total population (Arroyo et al, ; Saether & Bakke, ), neglecting them leads to biased estimates of bycatch risk. Also, due to their generally more dispersed distributions and, for juveniles, naïve foraging behaviours, these life‐history stages are often at greater risk than breeding adults (Gianuca, Phillips, Townley, & Votier, ).…”