Hentati-Sundberg et al.: Largest seabird colony in the Baltic Sea 61 Marine Ornithology 46: 61–68 (2018) FISH AND SEABIRD SPATIAL DISTRIBUTION AND ABUNDANCE AROUND THE LARGEST SEABIRD COLONY IN THE BALTIC SEA JONAS HENTATI-SUNDBERG 1,2 , THOMAS EVANS 3 , HENRIK ÖSTERBLOM 2 , JOAKIM HJELM 1 , NIKLAS LARSON 1 , VIDAR BAKKEN 4 , ANDERS SVENSON 1 & OLOF OLSSON 2 1 Department of Aquatic Resources, Institute of Marine Research, Swedish University of Agricultural Sciences, Turistgatan 5, Lysekil SE 45330, Sweden (jonas.sundberg@slu.se) 2 Stockholm Resilience Centre, Stockholm University, Stockholm SE 106 91, Sweden 3 Department of Biology, Centre for Animal Movement Research, Lund University, Lund SE 22362, Sweden 4 Ombustvedtveien 20, 1592 Våler i Ø., Norway Received 15 December 2016, accepted 11 December 2017 ABSTRACT HENTATI-SUNDBERG, J., EVANS, T., ÖSTERBLOM, H., HJELM, J., LARSON, N., BAKKEN, V., SVENSON, A. & OLSSON, O. 2018. Fish and seabird spatial distribution and abundance at the largest seabird colony in the Baltic Sea. Marine Ornithology 46: 61–68. We studied the at-sea distribution of two auks (Common Murre Uria aalge, Razorbill Alca torda), two gulls (Lesser Black-backed Gull Larus fuscus, Herring Gull Larus argentatus), and Great Cormorant Phalacrocorax carbo during the peak breeding season of 2014 around Stora Karlsö, the main Baltic Sea seabird colony. Simultaneously, we quantified forage fish abundance and distribution using hydro-acoustics and pelagic trawling. The auks and gulls had a roughly similar distribution, foraging mainly about 40 km west-northwest from the colony. Great Cormorants were found only in inshore areas, close to the colony. Sprat Sprattus sprattus and herring Clupea harengus biomass was, respectively, 1.38 and 2.68 mt/km 2 averaged over the whole study area. These estimates represent a total biomass for small pelagic fish of 17 900 t in the 4 408 km 2 study area. The estimated prey consumption over the breeding season was 2 310 t for Common Murre and Razorbill combined. Thus, auks may have a non-negligible impact on their prey sources in the region. Common Murres foraged closer to the colony (median 36.3 km) than Razorbills (median 41.1 km), but we found no significant correlation between auk at-sea numbers and fish densities. We discuss how new technology can contribute to detailed monitoring of the interactions between seabirds and fish at different spatial and temporal scales, with the ultimate aim of providing a scientific basis for ecosystem-based management. Key words: Alca torda, ecosystem-based management, ecosystem surveys, forage fish, Larus argentatus, Larus fuscus, Phalacrocorax carbo, Uria aalge 61 is a key aspect in the ongoing move from sectorial management to ecosystem-based management (Jennings & Quesne 2012, Link & Browman 2014). In the Baltic Sea, the problem of gillnet bycatch of seabirds and marine mammals has been extensively studied, eliciting far- reaching action to mitigate the problem (such as a ban on driftnet fishing in 2008) (Hentati-Sundberg et al. 2015, Žydelis et al. 2009, Österblom et al. 2002). Less is known about the ecological interaction among small pelagic fish, fishing, and seabirds. Earlier studies have indicated that Common Murre Uria aalge productivity and chick mass at fledging are correlated with the condition of its main forage species: sprat Sprattus sprattus (Kadin et al. 2012, Österblom et al. 2006). However, in the Baltic Sea, birds and fish have so far been compared only on coarse temporal (annual) and spatial scales, making it difficult both to disentangle the detailed ecological relationship and to devise concrete management actions. Further, these earlier studies considered only one species (Common Murre), for which there were long-term monitoring data, making it difficult to draw general conclusions on the links between seabirds and fish stocks in the area. In this study, we investigated finer-scale spatio-temporal dimensions of seabirds and fish stock interactions around Stora Karlsö, the largest seabird colony in the Baltic Sea (57°17′1N, 17°58′2E; Fig. 1). INTRODUCTION Interactions between marine top predators (e.g., seabirds) and fisheries involve a trade-off between conservation and resource harvesting in marine systems (Lescroël et al. 2016). Seabird- fish-fisheries interactions include direct mortality due to bycatch (Lewison et al. 2014), seabirds foraging on fishing discards (Bartumeus et al. 2010, Votier et al. 2004), and resource competition (Cury et al. 2011, Furness & Camphuysen 1997). Identifying, analyzing, and negotiating such trade-offs between ecosystem uses Fig. 1. Map of the study area with bathymetry indicated by colors.