Unchanging diet in a stable colony: contemporary and past diet composition of black-legged kittiwakes Rissa tridactyla at Helgoland, south-eastern North Sea

Helgol Mar Res (2009) 63:199206 DOI 10.1007/s10152-009-0149-8

ORIGINAL ARTICLE

Unchanging diet in a stable colony: contemporary and past diet composition of black-legged kittiwakes Rissa tridactylaat Helgoland, south-eastern North Sea

Nele Markones Nils Guse Ommo Hppop Stefan Garthe

Received: 26 August 2008 / Accepted: 17 February 2009 / Published online: 22 March 2009 Springer-Verlag and AWI 2009

Abstract In contrast to the situation at the west coast of the North Sea, the breeding colony of black-legged kittiwakes Rissa tridactyla at Helgoland in the south-eastern North Sea did not exhibit severe declines since 1990 but instead numbers increased and only lately stabilised. Declines at the west coast of the North Sea were attributed to a lower abundance and lower quality of the key prey, sandeels. We hypothesised that kittiwakes at Helgoland do not rely as heavily on sandeels as their conspeciWcs. We analysed stomach contents of nestlings and adults of 2001, 2002, 2004 and 2006. In concordance with earlier studies of the 1980s and 1990s, young whiting Merlangius merlangus was the most important prey species in 2001, 2002 and 2004. Clupeids and sandeels were consumed in lower proportions. While earlier studies suggested whiting to originate from Wsheries discards, evidence now supports that kittiwakes prey upon whiting in areas of hydrographic fronts. No whiting was recovered in samples of 2006 and the proportion of Wsh prey was low. Main prey items were polychaete worms (Nereidae), which were presumably consumed as swarming Heteronereis stages. An observed strong rise in water temperature in summer 2006 might have inXuenced food availability of kittiwakes by inducing

swarming of Nereidae. Overall, kittiwakes breeding on Helgoland showed a positive population trend for several decades while mainly feeding on whiting.

Keywords Black-legged kittiwake Rissa tridactyla Diet North Sea Whiting Merlangius merlangus

Introduction

In the North Sea, breeding numbers of black-legged kittiwakes Rissa tridactyla (hereafter kittiwakes) have declined by more than 50% since 1990 (Frederiksen et al. 2004; Heubeck 2004). In most areas, a distinct decrease of breeding success has been recorded (Wanless et al. 2005; ICES-WGSE 2006), which was probably connected to low food availability and low food quality (Frederiksen et al. 2004; Wanless et al. 2005, 2007). The kittiwakes surface-feeding foraging technique restricts it to prey concentrations at the sea surface and thus renders...