Abstract
Background
The MONS research programme was initiated by the North Sea Agreement and aims to guide policy decisions relating to the transitions in energy, food supply and nature in the Dutch North Sea. The programme includes studies of coastal and offshore seabird species that are sensitive to the impacts of these transitions. This project contributes to answering key questions in the MONS-research programme about the consequences of the energy and food transition on seabirds, the distribution of seabirds, and the environmental factors that determine seabird distribution in the North Sea. The overall project includes three work packages (WPs) on seabird ecology: distribution (WP2), diet (WP3) and population studies (WP4), and a final work package with an overview of knowledge gaps and recommendations for a research plan, including field studies (WP5).
Aims and research questions
The aim of Work Package 2 (ID 60) is to review existing knowledge on the foraging areas of 12 selected seabird species in the (Dutch) North Sea; these areas are crucial for the survival of individual seabirds and conservation of the target seabird populations. The main research topics for WP2 are:
(1) What are the most important resting and foraging areas in the (southern) North Sea for the focus species of three functional species groups?
(2) What are the knowledge gaps with respect to the environmental factors which determine these foraging areas?
(3) Recommendations are provided for follow-up studies (ship-based, aircraft, digital aerial surveys) of the selected seabird species, their prey and core foraging areas in the Dutch North Sea.
Selected seabird species
Twelve seabird species were selected; these selections were made based on their abundance in the (Dutch part of the) North Sea, importance of the North Sea for international populations of those species, inclusion of both wintering and breeding birds, dependence on offshore vs. coastal food sources, and diversity in feeding strategies and functional groups. These species included red-throated diver, northern fulmar, northern gannet, black-legged kittiwake, great black-backed gull, European herring gull, lesser black-backed gull, Sandwich tern, common tern, common guillemot, razorbill, common scoter.
Survey and tracking data
To date, the number of environmental factors used in the analysis of survey data of seabirds to construct distribution maps is very limited. Therefore, this report begins with a literature review of studies with tracking data (based on GPS-transmitters and geolocators) of the target species in the North Sea area. Although a substantial number of covariates were included in these studies to explain seabird distribution, very few studies included the actual abundance of forage fish, an important prey for the majority of the selected seabird species.
Seabird distribution in the North Sea with survey data
Density distribution maps of the selected species were generated with ESAS/MWTL-data and 11 different covariates, including abiotic factors (mainly proxies for food availability), distance to nearest breeding site, distribution of other species, and factors such as disturbance (e.g. wind farms) or attraction (fishing vessels) . The results show substantial variation in observed seabird densities over space and time, and an absence of strong effects of static environmental variables. Two to four covariates were retained in the final models, depending on species and season, and only a small proportion of the variation in seabird densities in the Dutch North Sea could be explained by the available environmental factors. Possible causes for the low explanatory power of the covariates, such as unavailability of data and incomplete knowledge (e.g. abundance and availability of forage fish) are discussed.
Distribution of foraging seabirds
The occurrence of actively foraging seabirds was studied using ten years’ of aerial surveys on the Dutch part of the North Sea based on a statistical model to estimate foraging probability. Since 2014, the behaviour of seabirds and their association (if any) with marine mammals has been recorded according to standardised protocols. The estimated fraction of actively foraging birds ranged from very small (<1%; in common guillemot and razorbill) to 22% in northern fulmar and 58% in common/Arctic tern.
Knowledge gaps and recommendations
Tracking and survey studies
Although tracking and survey studies have contributed significantly to our understanding of seabird distribution in the North Sea, a range of knowledge gaps have been identified with respect to this study’s research questions. Most utilised data on environmental factors in the tracking and survey studies include proxies for fish abundance and availability, such as abiotic factors (e.g. water depth, sea surface temperature, stratification, wave activity, wind, tides, fronts), biotic factors (e.g. chlorophyll-a, diet, prey length), and human activity (wind farms, fishing activity).
An important knowledge gap in both survey and tracking studies is the direct relationship between seabird distribution and density and the availability of forage fish. Other environmental factors that influence food availability and potentially seabird distribution are ocean currents, temperature anomalies (e.g. heatwaves, nutrient upwelling, turbidity, moon phase, prey species behaviour, multiple species foraging associations, and shipping lanes. Knowledge gaps specifically for survey studies are the time periods between the current six arial surveys, breeding site fidelity, interactions with other seabirds and cetaceans, and the distribution of seabirds outside the Dutch continental shelf.
Availability of forage fish
The distribution of forage fish (herring, sprat and sandeel) and their importance for coastal and offshore seabirds is briefly reviewed. These forage fish species occur throughout the North Sea, including the Dutch part. The majority of the 12 selected seabird species in this study depend on these forage fish species, especially in the breeding season, for survival and reproduction. Understanding the factors that affect coastal and seabird foraging therefore requires insight into the availability of forage fish for breeding and visiting bird populations in the (southern) North Sea. Although forage fish species are monitored in relation to international stock management, the availability of forage fish stocks for seabirds is an important knowledge gap. More detailed spatial and temporal monitoring of these stocks with respect to year-round availability to seabirds is required to understand their foraging distribution and the influence of various environmental factors and human activities. The recommended follow-up studies will help provide additional data necessary for effective seabird conservation.