Summer diet of European Shags Phalacrocorax aristotelis desmarestii in southern Mallorca

Samer Al-Ismail1, Miguel McMinn2, Víctor Manuel Tuset3 , Antoni Lombarte3 and Josep Antoni Alcover1*

1 Institut Mediterrani d'Estudis Avançats (CSIC-UIB), c/o Miquel Marquès, 21, 07190 Esporles, Balearic Islands, Spain;

2 SKUA Gabinet d'Estudis Ambientals SLP, Cr Arxiduc Lluis Salvador 5, entresol, esq., 07004 Palma, Balearic Islands, Spain;

3 Institut de Ciències del Mar (CSIC), Passeig Marítim de la Barceloneta, 37-49, 08003 Barcelona, Catalonia, Spain.

Full paper

Abstract

Analysis of pellets of European Shags Phalacrocorax aristotelis desmarestii collected at a non-breeding roost site in southern Mallorca identified 36 species of fish prey, belonging to 27 genera and to 16 families. This diversity is higher than in the diet of P. a. aristotelis in the Atlantic, and higher than in the previous literature for P. a. desmarestii in the Mediterranean. European Shags in southern Mallorca foraged mainly on fishes with a mean estimated length of 11.6 cm (84.1% ranging from 6.1-15.0 cm in estimated length), most being pelagic species (59.6%). The most important fish in numerical frequency (43.9%) and estimated biomass (37.2%) was the Bogue Boops boops (Sparidae). This species has not been reported in European Shag diet in the Atlantic, and its importance was low in other Ph. a. desmarestii populations studied. The second most frequent prey was sand smelt Atherina (15%), but its contribution to biomass was low (1.4% of estimated biomass) because of its small size, as has been reported from other Mediterranean locations. The occurence of Scorpaenidae (10.7% by frequency, 17.4% of estimated biomass) was higher than in previous studies of Ph. a. desmarestii. Scorpaenids have not been found in the diet of Ph. a. aristotelis. Sandeels (Ammodytidae), a key prey for Ph. a. aristotelis in the Atlantic, were very scarce in this study, as in other recent Mediterranean studies. The relative abundance of species anatomically well protected against predation, such as scorpaenids and trachinids, and the diversity of prey probably reflects the scarcity or absence of other preferred prey. This study reflects the opportunistic behaviour of European Shags in the Mediterranean Sea, foraging on fish with very different ecological requirements, in an environment that is poor but diverse.

Introduction

The European Shag Phalacrocorax aristotelis is distributed along the coasts of the Western Palaearctic with three subspecies recognised: Ph. a. aristotelis, with
66-73,000 pairs breeding along Atlantic coasts from the Kola Peninsula in Russia to southern Portugal (Wanless & Harris 2004); Ph. a. riggenbachi in south-western Morocco; and Ph. a. desmarestii, which is endemic to the Mediterranean, from the Iberian Peninsula to the Black Sea, with an overall breeding population estimated at between 3,000 (Velando & Munilla 2008) and 10,000 (Muntaner & Mayol 2007) pairs. Ph. a. riggenbachi and Ph .a. desmarestii are morphologically smaller than the nominal subspecies, having a smaller crest, and a brighter yellow coloration at the basis of the bill. However, differences between the subspecies are slight, and their taxonomic separation has not been evaluated genetically.

The European Shag is a coastal feeding seabird, showing a strong preference for rocky coasts and small islands with clear, shallow waters over sandy or rocky seabeds (del Hoyo et al. 1992). Birds mainly feed on fish, with a few species dominating the diet, and within a foraging range of up to 20 km around their breeding and roosting sites (Wanless et al. 1991; Velando 1997). Although almost exclusively piscivorous, small numbers of polychaetes, cephalopods, other molluscs and small benthic crustaceans have been reported in the diet (e.g. Barrett et al. 1990; Velando & Freire 1999; Hillersøy 2011). Prey-items taken can differ spatially, even between neighbouring colonies (Velando & Freire 1999), or seasonally, between the breeding season and the rest of the year (Lilliendahl & Solmundsson 2006), depending on availability, suggesting opportunistic foraging behaviour (Barrett 1991).

There is abundant literature on the diet of Atlantic/North European Shags. With exception of the northernmost latitudes where gadoids are often their main prey (Barrett et al. 1990; Hillersøy 2011), different species of sandeels (Ammodytidae) are usually, but not always (Fortin et al. 2013), the main food source for Ph. a. aristotelis (Steven 1933; Lack 1945; Lumsden & Haddow 1946; Snow 1960; Pearson 1968; Harris & Wanless 1991, 1993; Velando & Freire 1999; Furness & Tasker 2000; Pennington et al. 2004; Lilliendahl & Solmundsson 2006). However, fewer dietary studies have been performed in the Mediterranean Sea, where the diet seems to be much more varied, especially during the breeding season, with some fish families such as Atherinidae, Labridae, Sparidae or Gobiidae playing an important role as prey items (Araujo et al. 1977; Guyot 1985; Morat 2007; Cosolo et al. 2011; Morat et al. 2011).

The Balearic Islands hold one of the most important populations of Ph. a. desmarestii (hereafter 'Mediterranean Shag'). Censuses carried out on Menorca, Eivissa and Formentera in 2005, and on Mallorca and Cabrera in 2006, estimated their populations at 1,800 breeding pairs, 95% of the Spanish population for this subspecies, and 18% of its world population (after Muntaner & Mayol 2007); a further census in 2006 and 2007 estimated 2,017 breeding pairs (Álvarez & Velando 2007). It has been suggested that the Balearic Islands could be a source region for individuals dispersing to other areas (García et al. 2011). Despite the importance of this area, the only information on their diet in the Balearic Islands comes from stomach contents analysis of eight specimens, with 16 individual fish identified, and several prawns (Araujo et al. 1977). Shags regurgitate pellets containing fish bones, otoliths and scales, small invertebrates, marine vegetation and even small stones. Although several authors (Ainley et al. 1981; Duffy & Laurenson 1983; Johnstone et al. 1990) estimated that pellets were regurgitated at least once a day, Russell et al. (1995) obtained a mean production of one pellet every four days for birds in the wild. Here, we present diet data inferred from an analysis of pellets from Mediterranean Shags collected on an islet off southern Mallorca (Balearic Islands). This information contributes to our knowledge of the diet of this species in the Mediterranean Sea.

Acknowledgements

We would like to express our gratitude to Dr Daniel Oro, Dr Alejandro Martínez- Abraín and Dr Damià Jaume for valuable comments, and to Silvia Pérez and Itzíar Álvarez for their help and useful technical recommendations. This paper is included as a colateral research in the DGICYT Research Projects CGL2012-38089 and MINECO Research Project CTM2010-19701. Dr. Victor M. Tuset has a contract JAE-DOC (CSIC) co-funded by the European Social Foundation. Finally, we thank Dr David Grémillet and an anonymous reviewer for their useful comments and improvements, and Martin Heubeck and Andy Webb for their editorial improvements.

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