Seasonal changes in the diet of a critically endangered seabird and the importance of trawling discards

Abstract

Pelagic seabirds obtain food from oceans where the availability of their prey changes rapidly both seasonally and spatially. Here, we investigated changes in the trophic habits of the critically endangered Balearic shearwater (PuYnus mauretanicus) through the breeding season and tested for dietary diVerences between sexes and age classes. We analysed & 15 N and & 13 C values in blood of adults during the pre-incubation, incubation and chick-rearing periods and of their chicks. Using a two-isotope mixing model, we estimated dietary contributions based on isotope values from potential prey species which included small pelagic species available naturally and demersal Wsh species available only from trawling discards. Balearic shearwaters showed clear isotopic and dietary variation through the breeding season. During pre-incubation, breeding adults appeared to exploit demersal Wsh, whereas during the incubation and chick-rearing period, they fed mainly on pelagic anchovies (Engraulis encrasicolus) and pilchards (Sardina pilchardus). Similarly, chicks were fed mainly with anchovies, a resource with a high energetic value. This variation in the dietary habits of adult shearwaters during the breeding season was probably related to both natural and Wsheryinduced seasonal changes in the availability of potential prey species within their main feeding grounds. However, changes in the nutritional requirements of the shearwaters could also play an important role. Indeed, diet diVered between sexes during pre-incubation: females fed less on trawling discards and more on small pelagic Wsh than males. This sexual segregation in diet could be the consequence of higher nutritional requirements of females during this period. Our study reveals the diVerential importance of both trawling discards and small pelagic Wsh species for a pelagic seabird depending on the breeding period and illustrates the importance of considering the entire breeding season when making inferences about the importance of speciWc prey in seabird dietary studies.

Exploiting the closest productive area: geographical segregation of foraging grounds in a critically endangered seabird

“…First, the main 2 trophic resources of Balearic shearwaters (e.g. small pelagic fish and trawling discards; see Navarro et al 2009, Käkelä et al 2010) are widely distributed throughout the western Mediterranean basin (Carbonell et al 1998, Bellido et al 2008. Even if the patterns of movement of Balearic shearwaters are influenced by trawling activity (Bartumeus et al 2010), the spatial location of their foraging grounds might not change due to the overlap of marine productivity hotspots (including small pelagic fish) and main fishing grounds along the Iberian continental shelf (Demestre et al 1988, Estrada 1996.…”

Analysis of fatty acids and fatty alcohols reveals seasonal and sex-specific changes in the diets of seabirds

“…In many species the roles of the two 84 sexes become more similar after laying, with both parents sharing incubation and chick-85 rearing. Whilst these different constraints on foraging behavior could result in sex specific 86 variation in diet over the season, this has rarely been investigated (Navarro et al 2009). 87…”

Research priorities for seabirds: improving conservation and management in the 21st century

“…Although discards may serve as an important resource for some seabird populations, they have also been documented to have a negative influence on other seabird populations (Kitaysky et al 2006, Pichegru et al 2007, Österblom et al 2008, Navarro et al 2009). The long-term impacts of discards on seabirds, both positive and negative are still emerging (Arcos et al 2008, Bartumeus et al 2010).…”