Investigating a seabird hotspot : factors influencing the distribution of birds in the southern Baltic Sea

The southern Baltic Sea in northeastern Europe has been described as one of the most important areas for seabirds in the Western Palaearctic, based on extraordinary high numbers of birds occurring in the area particularly during the winter half-year. Most studies, however, only comprise information about the distribution of birds, while the factors and mechanisms causing the observed distribution patterns are only seldom addressed. Prey distribution and abundance and foraging strategies are certainly the most decisive factors determining the distribution and habitat choice of seabirds at sea, but diet preferences are unknown for many species. However, an extensive understanding of the functionality between seabirds and their marine environment is essential not only to be able to assess natural changes in numbers and distribution, but to evaluate the risks posed by anthropogenic activities as human pressure on marine ecosystems has increased enormously over the last decades. This thesis overcomes the lack of comprehensive knowledge about the ecology of birds in the brackish environment of the southern Baltic Sea and provides basic information on bird-habitat-relationships, on interspecific interactions and on diet preferences of selected species. Furthermore, set net fisheries as a particularly high threat to birds in the southern Baltic Sea are exemplarily outlined in this thesis. The methods to address the study objectives comprise ship-based Seabirds at Sea surveys to reveal spatial and temporal distribution patterns, mathematical modelling to analyse these patterns with regard to habitat parameters and species’ interactions, analyses of stomach contents for dietary studies and the development of an approach to evaluate the conflict between birds and human impacts. The avifauna of the brackish Baltic Sea strongly differs from truly marine systems: while pelagic species are only represented by auks, the bird community is dominated by species that breed in (arctic) freshwater habitats and occur in the southern Baltic Sea only during winter and migration. The different species are not evenly distributed, but show clear preferences for certain areas and habitats, depending on their diet and foraging strategies. Birds feeding on or near the sea bottom, like sea ducks and grebes, are restricted to areas with low water depth and, according to the substrate preference of their main prey species, occur in waters over specific bottom sediment types. Species that mainly forage on pelagic fishes, like auks, are not restricted to shallow waters but also occur in deeper offshore areas. Surface feeding species are mainly represented by gulls. Especially the large Larus-species have strongly adapted to the use of discards and offal from fishing vessels and distribution patterns are influenced by the occurrence of human fishing activities accordingly. Detailed ecological questions are addressed in two case studies on grebes that are represented in the study area by three species and more than 10 000 wintering birds. The habitat choice of Slavonian Grebes is influenced by water depth and bottom sediment type: they prefer shallow waters of 4 to 14 m depth and occur only over sandy sediments. Both habitat factors can be linked to diet preferences: while the diving depth of endothermic animals is limited due to energetic constraints and thermoregulation, sediment type is regarded to be a proxy for food choice. The diet of Slavonian Grebes mainly consists of gobies that frequently occur over sandy bottom substrates. Great Crested and Red-necked Grebes prefer waters up to 26 m and 30 m, respectively. Within the areas suitable with regard to water depth, the three grebe species exhibit distinctive distribution patterns with only low spatial overlap. Furthermore, with increasing abundance of one grebe species, the abundance of the others significantly decreases. As the diet of the three grebe species differs only slightly, they seem to enable coexistence by spatial segregation rather than by ecological separation via different dietary niches. This indicates that interspecific competition, well known to influence seabird communities during the breeding season, can also act as structuring mechanism in densely populated wintering areas. The most prominent threat to the highly abundant diving bird species are set net fisheries, extensively carried out in the southern Baltic Sea and leading to the entanglement and drowning of birds foraging underwater. Two indices are developed to overcome the lack of comprehensive knowledge on bird bycatch: a spatial overlap approach indicates the potential of conflict for diving birds in relation to set net fisheries, based on data of bird abundance and fishing activities, while a vulnerability index based on relative bird abundance indicates the maximum susceptibility of birds towards drowning mortality. Vulnerability and potential conflict exhibit spatial and temporal variations and are particularly high in coastal areas and on shallow offshore grounds during winter and spring. Large areas of the German Baltic Sea have been designated as protected areas and the implementation of management objectives is currently under process. The presented approach involving an improved understanding of the spatio-temporal occurrence of potential bycatch events and the assessment of bird vulnerability towards the impact provides a valuable tool for such management purposes. By addressing various aspects of the ecology of birds at sea, this thesis fills considerable gaps in the knowledge about one of the most important areas for seabirds and waterbirds in the Western Palaearctic. It provides basic information on the relationship between birds and their biological and physical environment, which is fundamental to evaluate the consequences of human impacts and to set up appropriate conservation measures.