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Oystercatcher winter mortality in The Netherlands: the effect of severe weather and food supply
Abstract and Figures
Wintering Oystercatchers in The Netherlands are concentrated in the Wad-den Sea (c. 200000), with suhstantial numbers in the Delta area (c. 90 000). Only 1 % of the total wintering population is normally found along the North Sea coast. Cold-rushes under severe winter conditions lead to a reduction of wintering numbers in the Wadden Sea, and to increases in the Delta and along the North Sea coast. The mortality of wintering Oystercatchers in The Netherlands was studied on the basis of beached bird surveys along the coast of the North Sea and the Wadden Sea between 1969 and 1996. On the whole, the pattern corresponded well to annual mortality estimates from population studies on the Wadden Sea islands of Schiermonnikoog and Texel. Peak numbers of dead Oystercatchers were found in most severe winters, but also in some moderate winters such as in 1976 and 1991. In mild winters, rather small numbers of dead Oystercatchers were recorded. For 1986-1996, annual mortality along the coast ranged from a c. 500 in the mild winters of 1989 to c. 10000 individuals in 1987 and 1996. Multiple regression analysis of the number of dead Oystercatchers in winter 1975-1996 on the number of cold days and the biomass of benthic prey revealed an explained variance of 66% (n = 21). Both the number of cold days and prey biomass had a significant effect. The results explained the relatively high mortality among Oystercatchers in the moderate winters of 1976 and 1991 (very low food stock), and the low mortality in the severe winter of 1982 (large food stock). The extremely high mortality in 1987 was due to a combination of low food stock and severe weather The depletion of Cockles and Mussels in the Wadden Sea in the early 1990s, due to a combination of over-fishing and failing reproduction, triggered a southward mass movement of Oystercatchers and substantial mortality in the Delta area in the winter of 1990-1991. The absence of a peak in the mortality among adult breeders in the population studies for that particular winter is the major discrepancy between these population studies and the beached bird surveys. It is an indication that primarily non-local, and therefore probably less dominant, Oyster-catchers suffered from the food shortage in the Wadden Sea. The results suggest that for Oystercatchers the scarcity of their principal prey due to over-fishing has taken its toll. Had this season also been a cold winter; the results suggest that the mortality would have been unprecedented.
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... De langetermijn trends zijn daarom berekend over een periode van 35 opeenvolgende seizoenen (1975/76-2008/09). Oliebevuilingspercentages werden zoals gebruikelijk weer logitgetransformeerd, waarna trends door middel van lineaire regressie konden worden berekend (Camphuysen 1995(Camphuysen , 1996. ...
... Sula 9(special issue): 1-90, I-XX. Camphuysen C.J. 1996. Tellingen van olieslachtoffers langs de Nederlandse kust: project "Beached Bird Survey II". ...
... Ook de steltlopers Scolopacidae en Charadriidae zijn vogels die vooral in strenge winters massaal op het strand gevonden worden. Scholeksters Haematopus ostralegus zijn daar een bekend voorbeeld van (Swennen & Duiven 1983, Hulscher 1989, Camphuysen et al. 1996 Over een lange reeks van jaren neemt het percentage olieslachtoffers in deze groep van vogels af, zodanig dat tegenwoordig van verwaarloosbare oliebevuilingspercentages gesproken mag worden (Fig. 15). Een kanttekening hierbij is dat elke winter, zo ook in winter 2008/09, vooral na perioden met stormachtig weer, langs de kust meeuwen gezien worden die in meer of mindere mate met olie in aanraking zijn gekomen. ...
Beached bird surveys in The Netherlands 2008/2009
This paper presents the proportion of dead oiled birds washed ashore in The Netherlands of the total number of birds washing ashore as a result of beached bird surveys conducted by volunteers of the Dutch Seabird Group (NZG/NSO). Apart from the survey results for winter 2008/09, a summary is provided of data collected in summer 2008. The results are presented in a context of data collected in nearly 40 earlier seasons (1970-2008). The oil rate (fraction of oiled corpses of all birds found dead) is considered an indicator of levels of (chronic) oil pollution in the Southern Bight with mineral oil and other lipophilic substances (Camphuysen 1999). These (species-specific) oil rates are calculated on the basis of hundreds of beached bird surveys between November and April, carefully checking all dead birds found. The results of winter 2008/09 are compared with long-term trends calculated over 1975/76- 2007/08. Along the North Sea coast, over the years, downward trends in oil rates were found in all species and species groups. Most of the recent data fit this pattern. Notable exceptions, however, were rather high oil rates in Common Guillemots Uria aalge (the international indicator species for oil pollution in the Oiled- Guillemot-EcoQO) and Razorbills Alca torda. The higher oil rates in these auks point at illegal oil spils offshore, some distance away from the coast: close enough to provide numerous fresh corpses littering Dutch beaches, but distant enough to prevent the nearshore seabird population to be hit by the oil. In winter 2008/09. Exceptionally high concentrations of foraging seabirds occurred in nearshore waters and none of these seemed significantly affected by the oil. Oil rates in the Wadden Sea area are rather lower than oil rates on North Sea beaches. The results obtained in 2008/09 did fit that picture, at least for bird numerous enough to provide a reliable sample. In winter 2008/09, at least regionally in The Netherlands, rather severe winter weather occurred over a period of several weeks. Along the coast, however, the winter was rather mild and extra, weather-induced mortality was not found or remained at low levels. In January 2009, rather high densities of (dark phase) Northern Fulmars were found and these were part of a wreck that was noted over large parts of the North Sea area. A minority of the wrecked Fulmars was oil-contaminated.
... De lange-termijn trends zijn daarom berekend over een periode van 35 opeenvolgende seizoenen (1975/76-2008/09). Oliebevuilingspercentages werden zoals gebruikelijk weer logit-getransformeerd, waarna trends door middel van lineaire regressie konden worden berekend (Camphuysen 1995(Camphuysen , 1996. ...
... Ook de steltlopers Scolopacidae en Charadriidae zijn vogels die vooral in strenge winters massaal op het strand gevonden worden. Scholeksters Haematopus ostralegus zijn daar een bekend voorbeeld van (Swennen & Duiven 1983, Hulscher 1989, Camphuysen et al. 1996 Over een lange reeks van jaren neemt het percentage olieslachtoffers in deze groep van vogels af, zodanig dat tegenwoordig van verwaarloosbare oliebevuilingspercentages gesproken mag worden (Fig. 15). Een kanttekening hierbij is dat elke winter, zo ook in winter 2008/09, vooral na perioden met stormachtig weer, langs de kust meeuwen gezien worden die in meer of mindere mate met olie in aanraking zijn gekomen. ...
Oiled seabirds on the Dutch North Sea coast, 2008/2009
Executive summary
· This report presents the proportion of dead oiled birds washed ashore in The Netherlands of the total number of birds washing ashore as a result of beached bird surveys conducted by volunteers of the Dutch Seabird Group (NZG/NSO). Apart from the survey results for winter 2008/09, a summary is provided of data collected in summer 2008. The results are presented in a context of data collected in nearly 40 earlier seasons (1970-2008).
· The oil rate (fraction of oiled corpses of all birds found dead) is considered an indicator of levels of (chronic) oil pollution in the Southern Bight with mineral oil and other lipophilic substances (Camphuysen 1999). These (species-specific) oil rates are calculated on the basis of hundreds of beached bird surveys between November and April, carefully checking all dead birds found. The results of winter 2008/09 are compared with long-term trends calculated over 1975/76-2007/08.
· Along the North Sea coast, over the years, downward trends in oil rates were found in all species and species groups. Most of the recent data fit this pattern. Notable exceptions, however, were rather high oil rates in Common Guillemots Uria aalge (the international indicator species for oil pollution in the Oiled-Guillemot-EcoQO) and Razorbills Alca torda.
· The higher oil rates in these auks point at illegal oil spils offshore, some distance away from the coast: close enough to provide numerous fresh corpses littering Dutch beaches, but distant enough to prevent the nearshore seabird population to be hit by the oil. In winter 2008/09. Exceptionally high concentrations of foraging seabirds occurred in nearshore waters and none of these seemed significantly affected by the oil.
· Oil rates in the Wadden Sea area are rather lower than oil rates on North Sea beaches. The results obtained in 2008/09 did fit that picture, at least for bird numerous enough to provide a reliable sample.
· In winter 2008/09, at least regionally in The Netherlands, rather severe winter weather occurred over a period of several weeks. Along the coast, however, the winter was rather mild and extra, weather-induced mortality was not found or remained at low levels.
· In January 2009, rather high densities of (dark phase) Northern Fulmars were found and these were part of a wreck that was noted over large parts of the North Sea area. A minority of the wrecked Fulmars was oil-contaminated.
... De lange-termijn trends zijn daarom berekend over een periode van 35 opeenvolgende seizoenen (1975/76-2009/10). Oliebevuilingspercentages werden zoals gebruikelijk weer logit-getransformeerd, waarna trends door middel van lineaire regressie konden worden berekend (Camphuysen 1995(Camphuysen , 1996. ...
... Tabel 6. Vondsten in de winter van 2009/10 (november-april) en de aanwezigheid van olie op de gevonden kadavers (0= olie onbekend, 1= geen olie, 2= met olie, 3= verstrikt in touw, nylon of vistuig, 4= besmeurd met andere chemische substantie dan olie). De relatief koude winter is maar gedeeltelijk verantwoordelijk voor dit resultaat, want de wintersterfte langs de kust bleef uiteindelijk een vrij onopvallend fenomeen in vergelijking met eerdere koude of zeer koude winters (Leopold et al. 1986, Camphuysen & Derks 1989, Hulscher 1989, Meininger et al. 1991, Camphuysen et al. 1996. ...
+ Dit rapport presenteert de percentages olieslachtoffers onder op de Nederlandse kust aangespoelde (dode) vogels op basis van tellingen van de Nederlandse Zeevogelgroep, werkgroep NSO. Dit rapport bevat naast resultaten van olieslachtoffertellingen in de winter 2009/2010 ook een overzicht van de gevonden vogels in de zomer van 2009. De vondsten worden gepresenteerd in een context van gegevens uit de voorafgaande bijna 40 seizoenen (1970-2009).
+ Het percentage met olie besmeurde vogels wordt gezien als een graadmeter: een maat voor de (chronische) vervuiling van de zee met olie en andere lipofiele stoffen. Elk jaar wordt de (soortspecifieke) fractie met olie besmeurde vogels bepaald op basis van honderden tellingen tussen november en april verspreid over de gehele Nederlandse kust inclusief het Waddengebied (wintertellingen). Het resultaat van de wintersurveys (nu dus 2009/10) wordt elk jaar vergeleken met de op grond van eerdere tellingen berekende lange termijn trends (1975/76-2009/10).
+ Langs het Noordzee strand werden door de jaren heen geleidelijk afnemende trends in oliebevuilingspercentages gevonden bij alle vogelgroepen. Vrijwel alle gegevens uit de winter 2009/10 passen in dit beeld. De afgelopen jaren leek de afname in het bevuilingspercentage bij de Zeekoet Uria aalge, uitgerekend de internationale indicatorsoort voor chronische olievervuiling, te stagneren, maar de in dit rapport gepresenteerde gegevens laten zien dat dit vermoedelijk ene artefact is; het gevolg van een veranderde leeftijdssamenstelling.
+ De hogere bevuilingspercentages van soorten zoals Noordse Stormvogel, Drieteenmeeuw en alkachtigen in vergelijking tot kustgebonden soorten wijzen op illegale lozingen van olie op enige afstand van de Nederlandse kust (offshore), dichtbij genoeg om verse kadavers op het strand terecht te laten komen, maar ver genoeg weg om kustgebonden soorten min of meer ongemoeid te laten.
+ In het Waddengebied worden al jarenlang gemiddeld lagere bevuilingspercentages geconstateerd en sterker afnemende trend. Voor zover voldoende vogels werden gevonden om bevuilingspercentages te berekenen pasten de verzamelde gegevens prima in deze lange termijn series.
+ Winter 2009/10 was een koude winter met op uitgebreide schaal sneeuw. Toch zijn de aantallen vondsten van vorstgevoelige soorten zoals steltlopers en waterwild langs de kust niet bijzonder veel hoger dan normaal geweest.
+ In maart 2010 werd een flink aantal vogels het slachtoffer van een lozing van Polyisobutyleen. Behalve Zeekoeten werden en vooral Noordse Stormvogels getroffen, hetgeen wijst op een spoor van vervuiling op grote afstand tot de kust. Tot in de Duitse Bocht (Mellum) werden slachtoffers gevonden.
+ In april 2010 werden kolonies Kleine Mantelmeeuwen op Texel en Vlieland getroffen door ene olie-incident. Strandtellingen in die periode leverden geen verhoogde dichtheden olieslachtoffers op.
... For example, the decline in the numbers of oystercatchers in eastern England and the netherlands has been attributed to the overexploitation of mussels (e.g. Camphuysen et al., 1996;Atkinson et al., 2003;Verhulst et al., 2004). ...
Understanding ecological factors and processes affecting the abundance of species of conservation concern is key to ensuring their conservation. The Eurasian Oystercatcher Haematopus ostralegus is a ‘Near Threatened’ wader species that depends on coastal zones during the non-breeding season. However, these habitats constitute focal points of human settlement and suffer from intensified human activities. I used survey data from the central part of the Gulf of Gabès, Tunisia, to assess the significance of ecological and anthropogenic factors as predictors of the density of wintering oystercatchers. Using generalized linear mixed models, Oystercatcher density was positively correlated with sediment mud content and negatively correlated with seagrass cover. Density was also positively correlated with distance from human settlements and negatively correlated with the densities of recreationists and artisanal fishermen. Overall, oystercatchers avoid intertidal areas near human settlements. This study provides critical information on where conservation and research efforts should be focused to preserve this wader species. As a priority, investigations on flight initiation distances are needed to determine at which spatial level it would be beneficial to create buffer zones or walkways to minimise the negative impact of human disturbance on this wintering population.
... Scientific name De relatief koude winter is maar gedeeltelijk verantwoordelijk voor dit resultaat, want de wintersterfte langs de kust bleef uiteindelijk een vrij onopvallend fenomeen in vergelijking met eerdere koude of zeer koude winters (Leopold et al. 1986, Camphuysen & Derks 1989, Hulscher 1989, Meininger et al. 1991, Camphuysen et al. 1996. ...
Beached bird surveys in The Netherlands, 2009/2010
This paper presents the proportion of dead oiled birds washed ashore in The Netherlands of the total number of birds washing ashore as a result of beached bird surveys conducted by volunteers of the Dutch Seabird Group (NZG/NSO). Apart from the survey results for winter 2009/10, a summary is provided of data collected in summer 2009. The results are presented in a context of data collected in nearly 40 earlier seasons (1970-2009). The oil rate (fraction of oiled corpses of all birds found dead) is considered an indicator of levels of (chronic) oil pollution in the Southern Bight with mineral oil and other lipophilic substances. These species-specific oil rates are calculated on the basis of hundreds of beached bird surveys between November and April, carefully checking all dead birds found. The results of winter 2009/10 are compared with long-term trends calculated over 1975/76-2009/10.
Along the North Sea coast, over the years, downward trends in oil rates were found in all species and species groups. Most of the recent data fit this pattern. In the past few years, however, it seemed that the initial decline in oil rates of Common Guillemots Uria aalge (the international indicator species for oil pollution in the Oiled-Guillemot-EcoQO) had more or less come to a halt. New data show that this is probably an artifact, caused be a change in agecomposition of the Guillemots at sea. The higher oil rates in these auks point at illegal oil spills offshore, some distance away from the coast: close enough to provide numerous fresh corpses littering Dutch beaches, but distant enough to prevent the nearshore seabird population to be hit by the oil. In winter 2009/10.
Oil rates in the Wadden Sea area are rather lower than oil rates on North Sea beaches. The results obtained in 2009/10 did fit that picture, at least for birds numerous enough to provide a reliable sample. Winter 2009/10 was the first cold winter for years. Unexpectedly, however, densities of species that are sensitive to cold weather (waders and waterfowl) hardly increased. Entanglements in fishing gear and plastics and mortality from oil pollution are currently almost equally important as a cause of death for Northern Gannets. In March 2010, numerous seabirds were affected by a spill of Polyisobutylene. Common Guillemots and Northern Fulmars were primarily affected, indicating the offshore nature of the spill. Some of the Fulmars were also oilcontaminated. The event was also noted in the German Bight (Mellum). In April 2010, colonies of Lesser Black-backed Gulls at Texel and Vlieland were hit by an illegal spill of oil somewhere at sea. Beached bird surveys at the time did not produce unusual numbers of oiled dead seabirds on Dutch beached. The European Commission wrote a 'Commission Decision', in which "Occurrence, origin (where possible) and scale of significant spills (for example of oil or oil products) and the effect on marine biota" is described as an indicator. The beached bird surveys are an important contribution to this indicator.
... During winter waterbirds can be placed under extra stress due to the joint demands of increasing thermoregulation and foraging in areas of diminishing food reserves (Goss-Custard et al., 1977;Clark, 2002). In severe winters this can lead to the depletion of fat reserves and eventually to death (Camphuysen et al., 1996;Suter & Vaneerden, 1992;Davidson & Evans, 1982). Disturbance of these birds, either by predators (Cresswell & Whitfield, 2008;Whitfield, 2003;Hilton et al., 1999) or human activity (Gill et al., 2001;Tamisier et al., 2003;Burton et al., 2002;Burger, 1998) can act to exclude birds from their feeding areas, as well as causing them to expend energy in their escape flights, potentially leading to greater risks of mortality in these populations. ...
Management of waterfowl shooting during periods of severe weather in the UK.- During prolonged periods of severe weather waterfowl habitats are affected by snow or ice. Recreational users of the countryside can move waterbirds off prime feeding areas, potentially resulting in reductions in body condition, at a time when energy reserves are key to overwinter survival and subsequent breeding success. Over the last 30 years the British Association for Shooting and Conservation has been closely involved, along with the government and other conservation NGOs, in developing and implementing a criteria-driven process for defining severe weather and managing waterfowl shooting during it in order to minimise unnecessary disturbance to overwintering waterbirds. The system works well and is widely respected and applauded by conservation agencies. There are increasing efforts to bring more countryside activities, including dog walking, bird watching/ringing and other wetland users, into the system so as to reduce further unnecessary waterfowl disturbance during difficult winter weather.
... Enhanced mortality during this period, partially, but not always enforced by human activities, has been described for many waterbird species (e.g. Meininger et al. 1991;Camphuysen et al. 1996). This might be particularly so for birds which are living in variable winter climates such as the Baltic Sea (Rheinheimer 1996). ...
Fjord, Baltic Sea, was assessed from pellets collected in a very cold winter (1995-96), in a very mild winter (1997-98) and in spring 1996 for comparison. Bivalves (mainly Mytilus edulis) were the most frequently occurring food item in all three periods. Gastropods (chiefly Littorina spec.) and crustaceans (mainly Carcinus maenas) were following next. Stones and different types of plant material were also quite frequently present in the pellets. Differences between the three periods were not very marked. Crustaceans, algae, grass and stones were most abundantly found in the cold winter. Oligochaetes occurred only in spring. Stones were quite common in the pellets both by frequency and by mass. In the cold winter 1996, mean stone mass comprised 47% of total pellet mass. It is concluded that Herring Gulls did not alter their diet in the cold winter to a major extent.
rom the examples of annual, seasonal and spatial variation in seabird diets provided in this chapter it may seem that we have a reasonable overview of its variability in most common species of seabirds in the
ICES area. In fact, this is not the case. Of 767 studies in which the study season was specified, 64% were conducted during the breeding season or in summer. Only 8% of all studies were conducted in the prebreeding season, 12% during post-breeding (early autumn) and 15% in winter. Logistic problems have prevented large scale studies of the diets of most pelagic seabirds outside the breeding season, simply because most birds are `out of reach' (away from land). From the examples given earlier and from many published papers on variability in seabird diets, it should be emphasised that the results obtained in one area, in one season, in any one year are not necessarily valid with that same predator species in other circumstances. However, on the larger scale it will soon be possible to come up with some generalisations. There is no need to become side-tracked as a result of the immense variation in prey, since most items form only a very small part of the diet. Rather few species/types are `preferred' prey for seabirds while very
many should be labelled `occasional prey'. It is very important, however, that additional information is collected on seabird prey preferences, particularly outside the breeding season and away from the colonies.
A second point which should be highlighted, particularly from the fisheries point of view, is that very few studies known resource of potential prey. There are very obvious methodological problems involved with the assessment of food resources (a function of prey stock size, suitability and availability) for piscivorous seabirds, but in the absence of any insight it remains speculative why certain seabirds rely on sandeels in one year and perhaps clupeids in the next. Size selection (e.g., Swennen and Duiven, 1977, 1991; Camphuysen et al., 1995), differential selection of prey of a certain `quality' or calorific value (e.g., Harris and Hislop, 1978, Wright and Bailey, 1993) and prey choice or dietary shifts in relation to the prey stock (e.g., Doornbos, 1979; Vader et al., 1990) are very important aspects which all deserve a lot more attention in future studies.
Dit rapport presenteert de percentages olieslachtoffers onder op de Nederlandse kust aangespoelde (dode) vogels op basis van tellingen van de Nederlandse Zeevogelgroep, werkgroep NSO. Dit rapport bevat naast resultaten van olieslachtoffertellingen in de winter 2010/2011 ook een overzicht van de gevonden vogels in de zomer van 2010. De vondsten worden gepresenteerd in een context van gegevens uit de voorafgaande seizoenen (1959/60-2009/10).
Het percentage met olie besmeurde vogels wordt gezien als een graadmeter: een maat voor de (chronische) vervuiling van de zee met olie en andere lipofiele stoffen. Elk jaar wordt de (soortspecifieke) fractie met olie besmeurde vogels bepaald op basis van honderden tellingen tussen november en april verspreid over de gehele Nederlandse kust inclusief het Waddengebied (wintertellingen). Het resultaat van de wintersurveys (nu dus 2010/11) is voor Zeekoeten vergeleken met de op grond van eerdere tellingen berekende lange termijn trends (1975/76-2009/10).
Langs het Noordzee strand werden door de jaren heen geleidelijk afnemende trends in oliebevuilingspercentages gevonden bij alle vogelgroepen. Alle gegevens uit de winter 2010/11 passen in dit beeld. Helaas was het aantal strandingen van indicatorsoort Zeekoet Uria aalge nogal klein de afgelopen jaren, maar de gevonden bevuilingspercentages zijn betrouwbaar genoeg om de afnemende trend te bevestigen.
Het relatief wat hogere bevuilingspercentage bij indicatorsoort Zeekoet (vooral in vergelijking met meer kustgebonden oliegevoelige soorten zoals Eidereenden en zee-eenden) toonde opnieuw aan dat er op grotere afstand tot de kust nog steeds geregeld olievervuiling voorkomt. Het gevonden percentage olieslachtoffers was echter het op twee na laagste ooit.
In het Waddengebied worden al jarenlang gemiddeld lagere bevuilingspercentages geconstateerd en sterker afnemende trend. Voor zover voldoende vogels werden gevonden om bevuilingspercentages te berekenen pasten de verzamelde gegevens prima in deze lange termijn series.
Winter 2010/11 was net als het daaraan voorafgaande seizoen opnieuw een vrij koude winter. Toch zijn de aantallen vondsten van vorstgevoelige soorten zoals steltlopers en waterwild langs de kust weer niet veel hoger dan normaal geweest.
De Kaderrichtlijn Mariene Strategie (KRM) is bedoeld om in 2020 een goede milieutoestand (GMT) van de Europese zeeën te behalen. De Europese Commissie heeft daartoe een 'Commission Decision' geschreven, een richtlijn voor lidstaten hoe de KRM geïmplementeerd moet worden. De volgende indicator van belang: "Voorkomen, oorsprong (waar mogelijk) en omvang van significante ernstige verontreinigingen (bijvoorbeeld vlekken van olie en olieproducten) en hun effect op biota die fysisch door deze verontreiniging worden getroffen (8.2.2)". De graadmeter op basis van olieslachtoffertellingen is daaraan een belangrijke bijdrage.
Executive summary
This report presents the proportion of dead oiled birds washed ashore in The Netherlands of the total number of birds washing ashore as a result of beached bird surveys conducted by volunteers of the Dutch Seabird Group (NZG/NSO). Apart from the survey results for winter 2010/11, a summary is provided of data collected in summer 2010. The results are presented in a context of data collected in over 50 earlier seasons (1959/60-2009/10).
The oil rate (fraction of oiled corpses of all birds found dead) is considered an indicator of levels of (chronic) oil pollution in the Southern Bight with mineral oil and other lipophilic substances (Camphuysen 1999). These (species-specific) oil rates are calculated on the basis of hundreds of beached bird surveys between November and April, carefully checking all dead birds found. The results of winter 2010/11 are compared for Common Guillemots with long-term trends calculated over 1975/76-2009/10.
Along the North Sea coast, over the years, downward trends in oil rates were found in all species and species groups. The most recent data fitted nicely in this pattern. Numbers of Common Guillemots Uria aalge (the international indicator species for oil pollution in the Oiled-Guillemot-EcoQO) washing ashore have been fairly small in the past few seasons, but the calculated oil rate was reliable enough and declined according to expectation based on the long-term trend.
The rather higher oil rate found in Common Guillemots (particularly in comparison with more coastal seabirds such as Common Eiders and scoters) confirmed an earlier finding that at greater distances to the coast, oil pollution is still a relatively common issue. The oil rate found, however, was the third lowest ever recorded in Dutch coastal surveys.
Oil rates in the Wadden Sea area are rather lower than oil rates on North Sea beaches. The results obtained in 2010/11 did fit that picture again, at least for bird species that were numerous enough to provide a reliable sample.
Winter 2010/11 was again a fairly cold winter, (2009/10 was the first cold season for years). However, densities of species that are sensitive to cold weather (waders and waterfowl) did again hardly increase.
The Europese Commissie wrote a 'Commission Decision', in which as an indicator: "Occurrence, origin (where possible) and scale of significant spills (for example of oil or oil products) and the effect on marine biota (8.2.2)". The beached bird surveys are an important contribution to this indicator.
Thirteen years of monthly counts of Oystercatchers from the Oosterschelde and Westerschelde, SW. Netherlands, contained approximately 13% missing values. Several log-linear Poisson models which assumed independence of the data were used to fill in (impute) these missing values. Yet these models either filled poorly (when only main effects year, month, and site were considered) or were too complicated (when first order interaction effects were also taken into account) to provide a reliable description. Furthermore, the errors appeared to be dependent. Consequently, regression models which consider simple deterministic functions of time and temporal dependence between the errors, were used for a more parsimonious description of the underlying process. The model which assumed that after some point in time all monthly means are increased or decreased by some factor yielded the best fit. This factor differed among months. Using this model, a sudden change was estimated to have occurred in June/July 1983, three years before the completion of hydrotechnical works that resulted in a loss of about 17% of the intertidal area in the Oosterschelde. Winter and spring numbers have decreased, while summer and autumn numbers have increased.
The average body weight of Oystercatchers capture in the Dutch Wadden Sea and adjoining breeding areas varies between 520 g in mid summer and 620 g in mid winter, but the individual differences remain large: between 450 and 650 g in summer and 500 and 725 g in winter. A part of this variation can be explained by body size. Yearlings weigh, on average, 30 g less than adults and 23 g less than subadults. However, after body weights are standardized for birds with the same body size, defined as the same wing length, yearlings actually weigh more than adults. Besides season and age, four variables explain a further part of the variation in the standardized body weight: (1) ♀ ♀ are 20 g heavier than ♂ ♂; (2) during the breeding season, inland birds weigh 20 g less than coastal birds; (3) adults captured on their nest weigh 25 g less than non-breeding adults, and (4) in late summer, birds undergoing wing and body moult are 15 g heavier than non-moulting birds. Since the majority of the birds are sedentary, there are no premigratory peaks in weight. Carcass analysis shows that the seasonal weight variation may be attributed to a < 10 g increase in lean dry weight in winter, and to a large seasonal difference for fat; coastal birds have, on average, 45 g of fat in summer compared with 100 g of fat in winter. Yearlings weigh more in winter than adults because they store more fat. Also ♀ ♀ deposit more fat than ♂ ♂ in winter as well as in summer. The variation in nutrient stores is thought to be a compromise between avoidance of predation and avoidance of starvation. This explains the seasonal variation in body weight, but also the individual and year-to-year variation, as follows: (1) Body weight in Oystercatchers wintering in the tropics is very low compared to conspecifics wintering in the temperate zone, where a high body weight may serve as insurance against cold spells; (2) body weight in birds breeding inland is lower than in coastal birds, presumably because grasslands offer a more stable, predictable food resource than tidal flats that are exposed, and thus available as feeding area, during a variable part of the day; (3) the large year-to-year variation in the body weight of coastal breeding birds can be attributed to variation in the food supply; if the adjoining feeding areas are poor and the birds have to feed in the lower tidal zone, the nutrient stores need to be larger; (4) body weight is relatively high in estuaries with a small tidal range where due to wind, there is a larger daily variation in the length of time for which feeding areas are exposed; (5) yearlings deposit more fat than (sub)adults and ♀ ♀ more than ♂ ♂, because adults are dominant over yearlings and ♂ ♂ over ♀ ♀ and the subordinate categories suffer more when food resources are limited due to ice covering most of the tidal flats. Despite laying down more stores, mortality during severe winters is higher in yearlings than in adults. Winter mortality does not differ much between the sexes, but since more ♂ ♂ than ♀ ♀ leave the Wadden Sea at the onset of a cold spell, more ♂ ♂ than ♀ ♀ are killed by French hunters.
Oiled seabirds on the Dutch coast as indicators of levels of chronic marine oil pollution
Since the end of last century, oil pollution of the open seas and coastal waters has become obvious mainly because of the frequent mass strandings of heavily oiled seabirds. In contrast to what is generally believed, oil incidents play a rather insignificant role in this form of pollution. Operational discharges by ships and frequent leakages of oil by ships and offshore installations are the main sources of oil washing ashore beached and found on oiled, beached birds. The oil pollution problem has been recognized as a significant threat to the marine environment, and several measures were taken to reduce the amount of oil which is released into the sea.
Beached bird surveys (BBS) have always been used as an aid to demonstrate the impact of oil pollution on the marine environment, but BBS results have played only a minor role in the assessment of the scale of and trends in marine oil pollution. Weather and wind ar normally said to influence the data so much, that the outcome is of limited value or very difficult to interpret at best. However, the use of an oil rate (the fraction of birds oiled out of the total number of birds washing ashore) to demonstrate the level of oil pollution in different sea areas is relatively new. Total numbers of birds washing ashore, usually expressed as densities (number per km surveyed), are now considered of secondary importance and these figures may only be used to examine the (local) impact of a given oil incident. Oil rates were found consistent in different species and in different areas. It is now believed that BBS results are quite useful indicators of the occurrence of marine oil pollution.
On the third North Sea Ministers Conference in 1990 it was concluded that the possible use of Beached Bird Surveys was to be investigated, as an indicator of the effectiveness of actions taken to reduce oil pollution of the seas. Following a report on 'The Value of Beached Bird Surveys in monitoring oil pollution', published in 1992, it was concluded on the interim Ministers Conference in Copenhagen in December 1993 that "In 1995 it should be possible to assess the effectiveness of the measures already agreed, and an assessment should be made available to the Fourth International Conference on the Protection of the North Sea. The Monitoring of oiled seabirds should continue as a useful indicator of the effectiviness of these measures".
In The Netherlands, BBS were an activity of volunteers during the last three decades. Now that BBS results were considered of interest to monitor trends in oil pollution rather than the effect of oil on (sea-) birds, the Directorate-General of Shipping and Maritime Affairs (DGSM) initiated the continuation of Beached Bird surveys in the Netherlands in the form of a research project to evaluate its own, national 'Milieubeleidsplan voor de Scheepvaart' (environmental policy plan for shipping). In this project, (1) 10 years of BBS data were computerized and analysed, (2) the statitistical validity of the information collected during beached bird surveys was evaluated by means of a power analysis and (3) the surveys were continued in 1994/95. The Institute of Forestry and Nature Research was ordered to produce a report on these matters, based on data collected by the Dutch Seabird Group, and CSR Consultancy acted as a sub-contractor to perform the project. In this report, the results of beached bird surveys over 1986-95 are summarized (chapter 2), it provides the results of a power analysis (chapter 3) and discusses the use of BBS results for policy makers (chapter 4).
BBS results 1986-95: In 1986-1995, the highest oil rates were found in divers, grebes, Gannet, scoters, Kittiwake and auks (table 4). Oil rates were significantly higher in winter (November-April) than in summer and it was concluded that these data sets should not be mixed in further analysis. In this report, 'winter oil rates' were provided, unless otherwise stated. A clear exception is the comparison of oil rates found in 1969-85 and 1986-95 (tables 8 & 14), because in the former period 'winter surveys' could not easily be separated from summer surveys. The oil rate found in 1986-95 was lower than the oil rate found in 1969-85 and this was concluded for all species and species groups of birds. Compared to other North Sea countries, the oil rate in The Netherlands is still very high. Most of the oil found on Dutch beaches and stranded birds in The Netherlands originated from operational discharges by ships (bilge oil and engine-room residues); crude oil was rarely encountered. Numbers of seabirds washing ashore are subject to massive fluctuations from year to year and month to month, caused by a variety of factors including variable bird densities at sea, residual currents, prevailing winds, and several mortality factors. The variation in oil rates, specific for species, groups of birds and certain areas, is minimal compared to the variations in overall numbers. The oil pollution of beaches showed the same seasonal pattern as oil rates in stranded birds (figure 2) and the frequency by which polluted beaches were reported has not changed since registrations began in the early 1980s.
Recording trends in marine oil pollution: using oil rates: One objective of the Beached Bird Survey (BBS) is monitoring the amount of oil pollution of the sea by assessing the fraction of oiled objects on a beach. BBS results are a derivative of a direct census of the occurrence of oil, with some very strong points because of its scale (all Europe), cost (with partly volunteer schemes rather low budgets are possible) and the length of its time series. In most countries, data are available over the last two or three decades, with unchanged methods, forming a unique data set which can readily be explored and which may form an additional source of information to other, perhaps more direct measurements. Ideally, an experiment would be set up in which clean pieces of cloth or whatever were released into the sea in huge numbers, to be recovered on the beach. The fraction (%) of oiled objects, the oil rate, would represent the chance for the pieces of cloth to become oil contaminated in that particular sea area. The same experiment in The Netherlands and in Shetland would result into a totally different oil rate (very low in Shetland, very high in The Netherlands). It has been suggested, that the recovery of beached birds is in fact such an experiment because the frequency of oiling of stranded seabirds is a reflection of the chance to become oil contaminated. However, if birds would ónly die at sea becaused of oil, the oil rate on the beach would be meaningless. If birds would never die because of oil, but get oil in their feathers while dead and afloat, the oil rate would be precisely what was wanted. Assuming that, generally speaking, a minority of the birds recorded on beaches died because of oil and considering that there is a linear relationship between the desired oil rate 'r' and the oil rate recorded on the beach 's' (figure 12), the BBS will serve as an accurate tool to measure trends in oil pollution, but a less accurate tool to work out 'true' levels of oil at sea. If methods within countries remain unchanged also in the future, results of trends in different schemes can readily be compared in space and time.
Results of the power analysis: The assumption is made that the fraction of all beached birds that is oil contaminated is in someway related to oil-pollution. This leads to the question: is there a significant trend over years in the fraction of oiled birds (and hence in oil pollution). This note is concerned with the statistical power of appropriate trend tests. The power (1-) is the probability that a trend, if present, will be detected as statistically significant. It depends on the size of the trend, the error variance, the number of years (n), and the size of the test (formula). Presumably the fraction of oiled birds (y) has some s-shaped relation with some index of oil-pollution (x) (figure 11). A widely used mathematical representation of such s-shaped curve is the logit function: formula (1). The analysis focuses on this index of oil pollution, which equals (as follows from (1)): formula (2). Figures 14 (Guillemot) and 15 (Razorbill) show time series of the observed index x and the fitted linear trends (by least-squares estimation) for several countries (h The Netherlands, d Denmark, g Germany, n Norway, s Shetland). Table 10 gives the residual mean squares, which can be used as estimates of the error variances. These residual mean squares are in the same order of magnitude for the various countries and do not show any relationship with the size of the average index. This >homogeneity of variances' is a desirable property as it is one of the assumptions of the underlying regression model. The untransformed data, i.e. the fraction of oiled birds do not show this property. For the Guillemot the error variance is about 0.49, i.e. an error standard devaition of about 0.7. Table 10 also gives the estimated slopes and the accompanying P-values. If, as a side-step, we consider the case that the true x=0, which implies that the true y= 0.5. Then, an error standard deviation of 0.7 for the observed x is equivalent to an error standard deviation of 0.175 (0.7/4) for the observed y, as formula (3). If the error variance would be solely due to a binomial sampling error (which equals formula (4), where is the independent probability that a bird is oiled, i.e. the true y), then such error variance would be obtained by sampling only 8 birds (which follows from formula (5). In practice the number of birds that have been observed is much larger. Hence, this little excercise showed that the observed error is probably not due to sampling error but to >real' deviations of the >true' yearly means from the linear trend. It supports our choice for the use of a least-squares approach. As the test of the regression slope is, in fact, a one-sample t-test, the power can be relatively simply calculated by using the cumulative Student's t-distribution function (tcf, with n-2 degrees of freedom), where the effect size d is expressed as the size of the trend (the slope of the regression) divided by its standard error (which follows from the estimated error variance and the number of years that will be sampled). Hence the power equals formula (6). Figure 16 gives as an example the power as a function of the number of years for slope=-.11 (h) and slope=-.24 (g) with an error variance of 0.49 (as is about true for the Guillemot). It says that a decrease in oil-pollution as observed in Germany (-.24) will be detected with a probability of 90% after 12 years.
The same procedure was followed using data collected in The Netherlands during 1986-95 (figures 17-20, tables 11-12) and using a slightly longer set of data which was available for Noord-Holland, a small part of the country (figures 21-22, table 13). The results showed declines in oil rates all over, and the probablity to find significant results with a certainty of ca. 75% within 13-17 years. The longer data set used illustrated that this was indeed the case: all delines were significant trends. The conclusion from the analysis was that BBS results are sensitive and useful to detect even minor trends in the frequency of occurrence of oil on the corpses.
Conclusions and recommendations for further research: Oil rates in beached birds in the Netherlands have consistently declined over the last 10 years and are now lower than before (table 14). The trends found over the last decade were quite weak and not significant, but can be expected to be so over a slightly longer period. The trends in different groups of birds (estuarine, coastal and offshore species; figure 27) run more or less parallel. If the oil rates found represent the chance for (corpses of) birds to become oil contaminated, and if this chance is mainly affected by the amount of oil at sea (number of slicks, densities, quantity of oil released), than a decline in oil rates on the beach would imply a decline in the amount of oil at sea. If we assume that other factors influencing the chance for birds to become oiled are (on average) constant, than, on the basis of beached birds, the amount of oil released into the southern North Sea would have declined by at least 20% since 1986.
Future research will have to focus on several species and/or groups of species simultaneously to avoid problems caused by certain mortality incidents in individual species. Densities will have to be measured to enable a fair judgement of drops or jumps in oil rates. At the same time, background information needs to be collected for all species used in the monitoring programme, again to make sure that the oil rate found is not influenced by circumstances which are particular for any of the individual species. key species in future monitoring in The Netherlands would be Guillemot, Razorbill, Kittiwake, Fulmar, Gannet, scoters and Larus-gulls.
It is concluded that more historical data will need to be computerized to enable further analysis of trends in oil rates, including information collected prior to the date when MARPOL Annex I was effectuated (October 1983). A continuation of the monitoring programme will focus on the winter period (November-April). It is strongly recommended to include a sampling programme to assess the different types of oil on beaches and beached birds. Such a programme would also provide information on the occurrence of other chemical substances and non-mineral oils.
Seabird strandings in The Netherlands: monitoring a 26 year period (1965-1991)
National beached bird surveys in February have been conducted annually in The Netherlands since 1965, with the exception of 1974. Between 1965 and 1978, the surveys were organized by The Netherlands' Youth Organisation for Nature studies (NJN). Since 1979, the working group beached bird surveys (NSO) was responsible. Slight differences in methodology between the two schemes have made that the oil rates were different (generally slightly lower since 1979). Volunteers were asked to search for corpses at the high water mark. During 1965-78, the main attempt was to cover as many kilometres as possible, regardless where. Since 1979, 6 subregions (fig. 1) were "sampled" (at least 10% surveyed was considered a reliable sample). The usual output of the national surveys, a "national" density (N/km) was considered unreliable, particularly because some of the subregions were not visited every year (tab. 1). A re-analysis of the data was performed in order to assess densities in each of the subregions and to provide baseline data for a future monitoring programme for beached birds in The Netherlands. Proportions oiled were calculated using all birds in a survey, to avoid small samples (national index); oil rates were calculated only when samples contained at least 10 corpses of a species or group. Trends in oil rates were tested using Spearman's Rank Correlation Coefficient for 1965-78 and for 1979-91 separately (tab.2). Correlations between oil rate and density, and between an index of winter severity (Ijnsen index) and density were tested A using linear regression analysis, differences in species composition were assessed using the G-test (Sokal & Rohlf 1981).
Of 531 divers found dead, 8.1% remained unidentified. Red-throated divers predominated (75.6%, N= 488). Ca. 90% of the divers were oiled, while densities were slightly lower in recent years than previously (fig. 2). Great Crested Grebes were the most numerous grebes (84.6%, N=1929), followed by Red-necked Grebes (9.0%). Massive mortality was recorded in most cold and all severe winters (fig. 3), when oil rates were comparatively low. Fulmars were found in small numbers in most years (N= 473), but an increase was found in recent years (fig. 4, appendix 1,2). Mass strandings in 1981(95), 1984(54), and 1988 (83) included 91.7,90.2 and 67.2% with oil on the feathers. Gannets were found in small numbers, and generally, over 75% were oiled. Several oil incidents involving Fiders and scoters were recorded (fig. 5, 6). In Eider, oil rates declined, while other mortality factors became more prominent. Particularly so in the 1990s, when acute food shortages due to overfishing of shellfish in the Wadden Sea led to starvation and mass mortality. Oil was the most important cause of death in scoters (tab.2). Scaup, Goldeneye, Goosander and Red-breasted Merganser were the most common species in a group named "other sea-duck". Severe winter conditions can force these birds to leave the IJsselmeer and Wadden Sea to enter the coastal marine environment and mass mortality occurred particularly in cold and severe winters (tab. 3). Other waterfowl and waders also were mainly encountered in beached bird surveys in cold winters (appendix 1, 2). Relatively few were oiled, and this proportion declined (tab. 2). Herring, Common, Black-headed and Great Black-backed were the commonest of the Larus-gulls. Some 50% of these gulls were oiled and the proportion declined slightly, but not significantly (fig. 7, tab. 2). Slightly higher numbers were found in cold winters. Of Kittiwakes, a common species only in the early 1980s (fig. 8), the majority were oiled. Wrecks were recorded during 1981-84, with oil rates above average. The Guillemot was the most numerous oiled bird on Dutch beaches in the 1980s and early 1990s. A sudden increase in numbers was found from 1981 onwards (fig. 9), remarkably few were found in 1987. Oil rates were always high (tab. 2). As in the next species, very few were found in the 1970s. Also the Razorbill was found in large numbers in the early 1980s, but numbers fell after 1984 (fig. 10). A wreck was recorded in 1990. Very high numbers were oiled (tab. 2). The value of national surveys is discussed. Important features to be derived from these data are: (1) winter mortality, particularly amongst species not normally occurring in Dutch coastal waters (grebes, seaduck, waterfowl, waders), (2) local oil incidents, (3) a decline in the oil rate in several of the coastal species, (4) very large proportions oiled in pelagic species, and (5) mass strandings of pelagic species in the 1980s. The auk 'wrecks' are discussed in more detail. Patterns found in The Netherlands were in fact found all along the North Sea, including very small numbers in 1987 (and 1992). The availability of food in winter is probably the key factor, leading to changes in winter distribution. The oil-induced mortality in our waters, clearly shows that pollution of the North Sea by oil is still at an unacceptable level.
This book reviews what is known about the behaviour and population ecology of a popular shorebird, from a scientific conservation perspective. The plight of this bird highlights the many conflicts of interest in coastal zones, between human activities such as shellfishing, land reclamation, barrage construction, and industrial pollution, and the needs of wildlife for food and suitable habitats. As well as detailing Oystercatcher natural history - including the well-known specialization in feeding technique shown by individuals - the authors use their field studies of individual variations in behaviour to produce population models. This novel approach provides tools for predicting how populations will respond to the many environmental changes to which the coastal zone is subject. It thus can play a role in coastal management schemes that seek to balance the needs of people and wildlife, and suggests that the same methods can be applied in other situtations. The volume contains fifteen well-integrated chapters by an international team of contributors, and is fully referenced.
In the mild winter of 1976 a spell of frost of only 11 days caused high (3-4%) mortality among Haematopus ostralegus wintering in the western part of the Wadden Sea. Mean weight of the 380 victims studied was 38% less than the mean normal winter-weights of the age classes distinguished. Anatomical deviations were found in 61.3% of the frost victims, especially on bill and legs. Over 19% of the dead oystercatchers older than yearling had not yet finished primary moult, which in normal birds is completed already 2 months earlier. Juveniles and males could resist the cold stress somewhat less than adults and females. The high mortality can be related to the unusually frequent raising of the sea level by high winds in the 2 months before the frost, by which feeding had already been prevented frequently. Handicapped birds were unable to build up a sufficient energy reserve to survive the cold stress.-from Authors
Study of Haematopus ostralegus on the Dutch Wadden Sea suggests that competition occurs on migration and on the wintering grounds. Oystercatchers feeding in poor areas suffered higher mortality than those roosting near good feeding areas. All roosts were near to each other so competition probably occurred. The low status of some individuals may be related to severe abnormalities.-after Authors
Coastal seas and estuaries have been used by mankind for many centuries and fulfilled many functions. In many parts of the world sustainable use of these areas is essential for survival of local people. Everywhere these areas are important for a number of reasons (Groot 1992). For example, they are important as nurseries for fish, function as shipping lanes, provide recreational facilities, feed many birds, act as a place to discharge sewage and provide for both commercial and recreational fishing. Over the last decades the commercial value of estuaries has shifted from fisheries production to a variety of interests among which recreation has increased significantly. The majority of estuarine functions are based on natural values; therefore it is essential to maintain these natural values. If one user is allowed to significantly alter the system, it may destroy the options for other or future users. In view of the changes in the past decades, one can conclude that it is difficult to predict what level of importance we will give to the different functions in twenty or more years from now. Therefore it is essential not to aim for a maximisation of one particular function but to aim for optimisation of a great number of functions. This places a burden on science to provide policy-makers with data that can be used to evaluate options.