In this first study of adult Black Guillemot Cepphus grille diet in the Norwegian mainland, 62 of 63 birds caught incidentally in Lumpfish Cyclopterus lumpus gill nets in the north of the country in April-June 2012 and 2013 contained food remains in their stomachs. Fourteen prey taxa were identified and crustaceans were present in 90% of the stomachs containing food, with squat lobsters (sub-order Anamura) being most common (in 79%). Rock Gunnel Pholis gunnellus was present in 17 (27%) of the samples. The only other prey types to be found in more than 20% of the samples were sculpins (Cottoidei, probably Cottidae and Agonidae, in 13 samples = 21%) and remains of Caridean shrimps (also in 13 samples). There were no differences in the frequencies of taxa in the diets of immature and adult birds or of males and females.

Seabirds are integral components of marine ecosystems and good indicators of changes in the marine environment (e.g. Furness & Monaghan 1987, Furness & Camphuysen 1997). For example, seabird data, including diet, give early indications of fluctuations in fish stocks and oceanographic conditions (Montevecchi 1993, Frederiksen et al. 2004). Conventional studies of seabird diet are often carried out during the breeding season when birds are at the nest and easiest to sample in a non-destructive manner. As a result, overall documentation of food preferences for a given species tend to be biased in space and time, often with a paucity of data from outside the breeding season when birds are not restricted to feeding close to the nest site. Samples collected during the breeding season may consist of prey remains in stomachs of birds collected at sea, food brought to chicks or undigested remains in pellets or faeces. Collecting birds at sea is, of course, possible at any time of year but most practical when birds are concentrated in predictable areas such as on or near the breeding site. Due to ethical considerations and the fact that the method rarely achieves a representative picture of the spatial and temporal variation in diet, the direct killing of birds as a sampling tool for diet studies alone is, however, discouraged (Barrett et al. 2007). Alternative sources such as birds accidentally killed in fisheries, oil spills or wrecks are, instead, preferred. Biochemical methods such as stable isotope or fatty acid analyses have allowed us to explore further adult diets non-destructively. They have certain advantages, such as integrating diet composition over space and time, and new methods such as the use of environmental DNA (eDNA) are constantly under development. These must, however, be considered as important supplements to, rather than replacements of, traditional studies of ingested prey (Bearhop et al. 2001, Karnovsky et al. 2012).

Of the millions of auks (Alcidae) that breed in the North Atlantic, the Black Guillemot (or Tystie, Cepphus grille) is among the least studied, probably because of its habit of breeding widely dispersed along coastlines at sheltered, often very difficult to access nest sites. In addition, their distribution is biased towards the (high) Arctic (Gaston & Jones 1998). As a result, diet studies are scarce. In the Arctic, they feed both inshore and offshore, among ice floes and along the ice edge where they forage both on the sea bottom down to ca. 30 m or dive under the ice to catch ice-associated (sympagic) crustaceans and fish (e.g. Mehlum & Gabrielsen 1993, Divoky et al. 2015). In sub-arctic and boreal regions, Black Guillemots feed on a wide variety of fish and invertebrates caught in the water column and on the seafloor. They usually forage within a few kilometres of the coast, often around rocky islands and over submerged reefs in sheltered waters with strong tidal currents and frequently among kelp (Laminariales) (Gaston & Jones 1998).

In Norway about 5,000 pairs of Black Guillemot breed on Svalbard and about 35,000 pairs on the mainland, nearly all north of the Stad Peninsula (62⁰ 11.7’N, 5⁰ 5.9’E), i.e. along the coasts of the Norwegian and Barents Seas. The Norwegian population constitutes 9‒13% of the North Atlantic (and hence nearly the World) population of 260,000‒410,000 pairs (Mitchell et al. 2004, Barrett et al. 2006, Fauchald et al. 2015a). Because of an evident decline in numbers, the species is ‘red listed’ as Vulnerable on the Norwegian mainland (Barrett et al. 2006, Kålås et al. 2015).

While predation by feral American Mink Neovison vison has been a significant cause of the Black Guillemot population decline in Norway, drowning in fishing nets, especially those set for Lumpfish Cyclopterus lumpus is possibly an important contributor, especially in the north of the country (Barrett & Anker-Nilssen 1997, Fangel et al. 2015). A reversal of the population decline requires effective management based on knowledge of extrinsic and intrinsic factors that affect Black Guillemot population dynamics, including improved knowledge of diet preferences (Fauchald et al. 2015b). Few studies, however, have addressed the diet of Black Guillemots in Norwegian waters. Chick diet has been documented in three colonies on the mainland (Larsen et al. 1989, Larsen & Sæter 1989, Larsen 1990, Barrett & Anker-Nilssen 1997, Anker-Nilssen 2010) and three studies have documented summer diet of adults on Svalbard (Hartley & Fisher 1936, Lønne & Gabrielsen 1992, Mehlum & Gabrielsen 1993). This study is the first to address the diet of adult Black Guillemots on the Norwegian coast.

The Black Guillemots used in this study were collected during a study of the bycatch of seabirds in Norwegian coastal commercial fisheries, funded by the Norwegian Environment Agency. We are grateful to the fishermen who participated in the data-collection and to the contact-persons responsible for forwarding the frozen birds to us. We also thank Prof. Torstein Pedersen, University of Tromsø for sharing his insight into squat lobster distribution with us.

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