Fisheries bycatch mitigation measures as an efficient tool for the conservation of seabird populations

The impact of industrial fisheries on marine biodiversity is conspicuous in large pelagic vertebrate fisheries bycatch. In seabirds, this led to the decline of many populations since the 1980s following the rise of global fishing efforts. Bycatch mitigation measures were implemented in the 2000s, but their effects on the concerned seabird populations remain poorly quantified and understood. We studied the effects of bycatch mitigation measures on the demography of the white-chinned petrel, one of the most bycatch impacted seabirds whose populations suffered dramatic declines before the implementation of mitigation measures. To do so we 1) built multi-event capture-recapture models to estimate the demographic parameters of a population from Possession Island (southern Indian Ocean) over thirty years, 2) assessed the effect of climate and fishery covariates on demographic parameters, 3) built a population matrix model to estimate stochastic growth rate according to the management in fisheries bycatch, and 4) estimated changes in breeding population density using distance sampling data. The population declined from the 1980s to the mid-2000s, while trawl and longline fisheries occurred with no bycatch mitigation measures. The negative effects of fishery bycatch through additive mortality and of rat predation on breeding success were likely the main drivers of this decline. Both modeled population growth rate and observed breeding densities showed an increase since the mid-2000s. We explained this trend by the improvement in survival probability following implementation of fishery bycatch mitigation measures and in breeding success probability with the local control of the rat population and changes in sea ice conditions on foraging grounds. Synthesis and applications: We provide a holistic approach to assess the effects of management measures by analysing datasets from sampling methods commonly employed in seabirds studies. Our conclusions should encourage the eradication of invasive predatory species in seabirds breeding areas and the strengthening of bycatch mitigation measures for the vulnerable seabird species, especially in international waters, but also the development of such measures considering the other marine large pelagic species threatened by fisheries bycatch (sharks, rays, turtles and marine mammals) since it could be crucial to avoid populations’ extinction.