Preben Clausen

Regime shift in a coastal marine ecosystem

We demonstrate changes in ecosystem stable states in a coastal lagoon that are consistent with what a regime shift would hypothesize. In the nutrient-stressed Ringkøbing Fjord, Denmark, a small change in one variable (salinity) facilitated by a change in sluice management, caused a sudden regime shift from a bottom-up controlled turbid state, into a top-down controlled clear-water state. The change in dominating pathway of organic matter production from pelagic turnover to benthic-pelagic coupling was facilitated by new recruitment and growth of existing suspension-feeding clams, Mya arenaria. With the invasion of clams, benthic grazing became the key feature of the biological structure. Phytoplankton composition and zooplankton abundance were also affected by the change in biological structure. The small, but sudden, increase in salinity caused by the change in sluice management led to a dramatic reduction in biomass and coverage of benthic vegetation and thus affected herbivorous waterbird populations. In recent years, plant coverage has been increasing, as can be expected with increased water transparency. The regime shift has some major implications for coastal water management and revealed some conflicts between different types of nature and environmental protection legislation.

The scientific basis for a new and sustainable management of migratory European ducks

Abstract It is an axiom in ecology that knowing the sheer number of individuals in a population is of very little help if the objective is to understand future and past changes in population size. Yet, this is exactly how migratory European ducks are monitored, many of which are important quarry species in several countries. We argue that present monitoring is insufficient to address objectives of wise use andsustainabilitysuchasthoseemphasisedinrecentmanagementdirectivesand multilateral international agreements. The two main problems are the almost total lack of reliable data on recruitment and mortality. We advocate a pan-European monitoring system based on undisputed scientific principles; i.e. a long-term, coordinated and standardised scheme that produces data about vital rates of duck populations as well as about harvest size. Data from such a scheme can be used by game biologists to produce predictive tools, thus providing a functional basis for management decisions for adaptive harvesting and conservation alike.

Effects of Water Level Management on Autumn Staging Waterbird and Macrophyte Diversity in Three Danish Coastal Lagoons

Many Danish wetlands frequently experience algae blooms and oxygen deficiencies because of eutrophication caused by enhanced nitrogen and phosphorous levels. As a consequence, wetland managers have focused on improving water quality, but often without considering the suitability of wetlands for waterbirds. In this study, managers improved water quality in two Danish lagoons by opening a floodgate to the sea. We studied the responses of autumn staging waterbirds and submerged vegetation, to resultant higher water replacement rates and water levels. A third lagoon with no change in management was studied for comparisons. Lagoons with rise in water levels experienced declines in bird species diversity, a decline in benthivore species abundances and an increase in herbivore species abundances. Macrophyte biomass increased but seagrass diversity was low due to high salinity. The lagoon with no changes in management control had high and stable waterbird diversity and bird-days spent was increasing during the study period. Explanations for this were diverse typography and low water levels. Furthermore, the macrophyte community was more diverse due to low salinity. In order to improve both water quality and to increase waterbird diversity in the managed lagoons, we suggest water levels should be managed actively during peak migration in autumn. This could secure more shallow-water areas to waders and create better congruence between appropriate water levels and timing of peak bird migration in autumn. The rest of the year the floodgate should be left open in order to secure a high water quality.

Establishing a reserve network for waterfowl in Denmark : a biological evaluation of needs and consequences

Abstract Denmark is of major international importance for a number of waterfowl populations during migration and in winter. As part of the revision of the Danish Hunting and Wildlife Management Act, 1992, it was decided to establish a network of reserves by creation and extension of reserves in 46 EU Special Protection Areas for birds (SPAs). This paper summarises an overall, biologically-based strategy for a reserve network in Denmark, as a basis for sustainable development of wetlands for migratory and wintering waterfowl. As well as securing the future for vulnerable populations, the reserve network should improve conditions for waterfowl staging and wintering in Denmark. The groups potentially most exposed to disturbance from hunting and other leisure activities, i.e. in greatest need of a reserve network, are the geese and the dabbling and small diving ducks. Swans, seaducks, sawbills and coots have less need for such a network. On this basis, deficiencies in the existing system of reserves are identified. Human recreational activities are considered compatible with sustainable development if the use by waterfowl of important wetlands is primarily limited by available food resources. Within each of the principal waterfowl migration routes and wintering areas identified, there should be a network of reserves which provides adequate opportunities for the birds to feed and rest. The proposed network will amount to a total shooting-free refuge area of ca. 650 km2, representing almost a doubling of the refuge area in Denmark. The shooting-free area will be supplemented by ca. 870 km2 with regulation of recreational activities or certain hunting methods. The extension is expected to result in a considerable increase in the numbers of dabbling ducks and geese, with the potential of holding back substantial parts of some populations at a more northerly position on their flyways than before. The area available for hunting in SPAs will be reduced but hunting opportunities in adjacent areas could be expected to improve because the numbers of birds there will also increase.

Population development and changes in winter site use by the Svalbard light-bellied brent goose, Branta bernicla hrota 1980–1994

Abstract The Svalbard breeding population of light-bellied brent goose Branta bernicla hrota , wintering in 4–5 sites in Denmark and at Lindisfarne in north-east England, increased from c. 3500–4000 in the early 1980s to 4000–5800 in the early 1990s. The increase was mainly due to years with good breeding success, but probably also to improved survival in mild winters. Thus, in cold winters it was observed that mortality, of at least first-winter birds, increased. Major changes in winter site use were observed during the 1980s; the use of the Danish Wadden Sea decreased, and in consequence, the brent geese arrived earlier at Lindisfarne and at the main winter site, Mariager-Randers Fjords. In the course of winter the population now disperses to more sites and new wintering sites have been established; in spring the majority of the population is concentrated in two sites in north-west Jutland in Denmark. The changed pattern of movement reflects decreases in distribution of Zostera food resources and depletion of the remaining resources due to waterfowl grazing, which may negatively affect condition of the geese in winter and ultimately survival. On the other hand, the population is now more dispersed during winter and hence less vulnerable to local environmental pertubations. The wider dispersal and changed phenology have several implications for management of the population which are discussed.

Grazing management can counteract the impacts of climate change-induced sea level rise on salt marsh-dependent waterbirds

Summary Climate change–induced rises in sea level threaten to drastically reduce the areal extent of important salt marsh habitats for large numbers of waterfowl and waders. Furthermore, recent changes in management practice have rendered existent salt marshes unfavourable to many birds, as lack of grazing has induced an increase in high-sward communities on former good-quality marshes. Based on a high-resolution digital elevation model and two scenarios for projected rise in near-future sea levels, we employ an ArcMap allocation model to foresee the areal loss in salt marsh associated with these changes. In addition, we quantify the areal extent of inadequate salt marsh management in four EU Special Protection Areas for Birds, and demonstrate concurrent population dynamics in four species relying on managed habitats. We conclude by investigating potential compensation for climate change–induced salt marsh losses by means of more efficient management. Our models indicate that by the end of this century 15·3–43·6% of existent salt marshes will be flooded due to rising sea levels, and that inadequate managed salt marsh presently makes up around 51·1% of total marshes. Thus, re-establishing extensive areas of well-managed marshes might counterbalance the loss expected from rising sea levels during the next century. In addition to positive effects on plant diversity, this will benefit energetically challenged herbivorous waterfowl such as light-bellied brent geese Branta bernicla hrota L. and increase potential recovery of wader populations with unfavourable conservation status such as black-tailed godwit Limosa limosa L., dunlin Calidris alpina L. and ruff Philomachus pugnax L. Synthesis and applications. Implementing environmentally friendly management schemes based on extensive grazing (around 1 cow per hectare) is an important initiative to counteract the accelerating climate change–induced habitat loss in near-coastal areas across the globe, and to secure priority salt marsh habitats that support internationally important populations of breeding, wintering and staging waterfowl. However, this may only be a temporary solution that will have to be supplemented by increased reintegration with the sea and managed retreat of seawalls or near-coastal agricultural areas to effectively safeguard the future salt marsh biome.

Response of breeding waders to agri-environmental schemes may be obscured by effects of existing hydrology and farming history

The initiation of agri-environmental schemes (AES) to improve habitats for breeding waders may not show expected benefits in all situations. We show this, using results from a study of breeding waders on 469 grassland fields in the Danish Wadden Sea area. Numbers of breeding waders were not restored to levels of the recent past, even though Lapwings Vanellus vanellus, Black-tailed Godwits Limosa limosa and Redshanks Tringa totanus were drawn to wet permanent grasslands with AES. We had expected that more waders would be attracted by improved surface water retention during the breeding period due to the AES. However, AES were not introduced to all fields, and this measure did not dramatically change the water regime in part of the breeding fields. Furthermore, dry grasslands and fields that had been cultivated or drained before did not attract more waders, despite the introduction of the AES. The results suggest that the interplay between surface water retention and former farming history are important determinants of the habitat choice for breeding waders. The study emphasises the need for pre-defined success criteria, thorough analysis of the nature of the habitat and consideration of population processes before initiation of an AES, so that realistic management goals are defined and expected benefits can be achieved.

Assessment of Blue Mussel Mytilus edulis Fisheries and Waterbird Shellfish-predator Management in the Danish Wadden Sea

We assessed the blue mussel Mytilus edulis fishery management scheme introduced in 1994 in the Danish Wadden Sea that regulate fishing vessels, fishery quota, set-aside for mussel-eating birds and established zones closed to mussel fishery. The results showed (i) a reduction in the blue mussel biomass and mussel bed areas in zones closed to fishery, (ii) decrease in eiders Somateria mollissima numbers and increase or stable numbers for oystercatcher Haematopus ostralegus and herring gull Larus argentatus and (iii) that energy estimations based on ecological food requirements for the mussel-eating birds should be at least three times larger, than the amount set-aside in the mussel management scheme. It is concluded that the mussel management scheme had been unable to stabilize or increase the blue mussel stocks and to secure stable or increasing numbers for all target bird species. Thus, it is recommended to revise the present blue mussel management scheme in the Danish Wadden Sea, to continue and improve mussel stock and bird surveys, and to consider novel studies of the mussel-eating birds’ energetics for improved set-aside estimates and future assessments.

Current and Potential Threats to Nordic Duck Populations — A Horizon Scanning Exercise

We review the current and future threats to duck populations that breed, stage, moult and/or winter in the Nordic countries. Migratory duck species are sensitive indicators of their changing environment, and their societal value confirms the need to translate signals from changes in their distribution, status and abundance into a better understanding of changes occurring in their wetland environments. We used expert opinion to highlight 25 major areas of anthropogenic change (and touch briefly on potential mitigation measures through nature restoration and reserve management projects) that we consider key issues likely to influence Nordic duck populations now and in the near future to stimulate debate, discussion and further research. We believe such reviews are essential in contributing to development of successful management policy as well as stimulating specific research to support the maintenance of duck species in favourable future conservation status in the face of multiple population pressures and drivers.

The feeding ecology and distribution of Common Coots Fulica atra are affected by hunting taking place in adjacent areas

Capsule Birds using a shooting-free area are markedly influenced by close-by shooting. Aims To examine changes in distribution, feeding ecology and food supply of Common Coots before and after the beginning of hunting on adjacent salt marshes. Methods Common Coots were counted and mapped in October and November, i.e. before and after the start of hunting. In both months, activity budgets for the Coots were determined. To quantify the food supply, macrophyte biomass samples and depth measures were taken along four parallel transects in the areas used by the birds for foraging. Results Before the start of hunting in October, 45% of all Coots were observed within 400 m of the hunting area. In November (when hunting had started) the birds redistributed themselves away from the hunting area, reducing the number of birds within 400 m to 5%. Because of this redistribution, Coots were observed in deeper-water areas of the lagoon during November compared to October, and foraging by diving increased significantly from 14 to 35%. The average biomass of submerged macrophytes in the lagoon was 155.01 g dry weight/m2, which corresponds to a total of 302 250 kg in the whole lagoon. The food requirement of each Coot is 52.3 g/day, which is equivalent to 104 kg/day for the autumn peak number at this lagoon (1989 Coots). Conclusion Hunting disturbance outside the shooting-free area displaced Coots from their preferred feeding grounds inside the reserve, causing their redistribution into sub-optimal habitat at greater water depths where Coot feeding costs were higher. Submerged macrophyte biomass showed that food was abundant in the area and therefore unlikely to be a limiting factor that influenced the redistribution of Coots during the shooting season. We suggest that at this particular site the feeding ecology of Coots could be improved by incorporating an appropriate buffer zone between the protected area and the hunted area. This may be of value not only to Coots, but also for other bird species that exploit shallow water.