Zeehonden in de Oosterschelde, fase 2; effecten van extra doorvaart door de Oliegeul

De westelijke Oosterschelde, met name het Oliegeulgebied, is belangrijk voor de kleine zeehondenpopulatie in het Deltagebied. De recreatiesector heeft de overheid gevraagd om in de zomer door dit nu gesloten gebied te varen. De effecten van een toename in doorvaart zijn onderzocht. Er kon geen periode in het getij worden aangewezen waarin de zeehonden het gebied minder gebruiken. Door extra doorvaarten trekken de zeehonden vaker weg uit het gebied. Een populatiemodel laat zien dat de beheersdoelstelling "het realiseren van een leefbare populatie" zonder extra bescherming niet zonder meer gehaald kan worden. Extra doorvaart zal dit herstel nog eens vertragen

On the causes of the decrease in the harbour seal (Phoca vitulina) population in the Dutch Wadden Sea : population dynamics and tissue levels of heavy metal and organochlorine compounds

The harbour seal population in the Dutch Wadden Sea decreased significantly during the last decades. Calculations based on bounty data revealed that the population decreased from about 2700 in 1950 to about 900 in 1959. Annual aerial surveys - which were carried out since that time - showed a slight increase due to the stop of hunting. However, after 1964 a new decline occurred and since 1974 the population stays at a level of about 450 specimens.During 1974 to 1978 the population dynamics of the population have been studied. Frequent aerial surveys provided data on the size of the population. These data have been used in a simulation model and the birth rate and the initial juvenile mortality were calculated. During boat trips an indication of the age composition was obtained by measuring track widths. By comparing these results with similar results from a stable population in Schleswig Holstein it appeared 1) that juvenile mortality in the first weeks in the Dutch population is higher than that in Schleswig Holstein but the overall mortality of pups in both areas in their first three months of life is of the same order; 2) that pup production in the Dutch population is low compared to the population in Schleswig Holstein; 3) that apparently immigration from elsewhere occurs.To investigate the contribution of environmental pollution to the decline of the Dutch seal population tissues of dead, stranded animals originating from Schleswig Holstein, Denmark and The Netherlands were collected and analysed for PCB's, o,p'-DDT, p,p'-(DDT, DDE, TDE), dieldrin, aldrin, endrin, endosulfan, α, β, γ, -HCH, HCB, QCB (pentachlorobenzene), HEPO, total mercury, methylmercury, selenium and bromium.Tests on possible differences in residue levels between Schleswig Holstein plus Denmark on one hand and The Netherlands on the other revealed that especially PCB levels were significantly higher in Dutch adult seals. Considering epidemiological and experimental data an the effects of PCB's on mammalian reproduction, strong support is obtained for the hypothesis that PCB's are responsible for the decreased reproductionin seals from the Dutch Wadden Sea.If PCB's are responsible for the decrease of the Dutch seal population, measures of all North Sea countries are urgently required because these PCB's may not only be a hazard to seals but also to other animals and even man. Production and use of PCB's should be limited and used PCB's should be collected and destroyed.However, even if an immediate ban on PCB's would be effected, the effects of PCB's - because of their persistent character - are likely to last many years. For that reason it may be assumed that the seal population in the western Wadden Sea still faces a difficult period in the years to come. Presently this population shows an insufficient reproductive rate and is partly dependent on immigration from other populations. In order to lead this population through this difficult period all possible measures enhancing its continued existence should be taken. Since the seals in the Wadden Sea form one population maximum result may be expected from measures applying to all seals in the Wadden Sea. Thus international cooperation is necessary.The following measures are proposed:- continued interdiction of hunting in order to obtain a maximum number of animals taking part in reproduction.- establishment of seal reserves and, additionally supporting seal nursery stations in order to guarantee a maximum reproductive output.However, it has to be stressed that these measures are not effective if at the same time the main cause of the decrease is not found and under control.<p/

Earlier pupping in harbour seals, <i>Phoca vitulina</i>

The annual reproductive cycle of most seal species is characterized by a tight synchrony of births. Typically, timing of birth shows little inter-annual variation. Here, however we show that harbour seals Phoca vitulina from the Wadden Sea (southeast North Sea) have shortened their yearly cycle, moving parturition to earlier dates since the early 1970s. Between 1974 and 2009, the birth date of harbour seals shifted on average by -0.71 d yr-1 , three and a half weeks (25 days) earlier, in the Dutch part of the Wadden Sea. Pup counts available for other parts of the Wadden Sea were analysed, showing a similar shift. To elucidate potential mechanism(s) for this shift in pupping phenology, possible changes in population demography, changes in maternal life-history traits and variations in environmental conditions were examined. It was deduced that the most likely mechanism was a shortening of embryonic diapause. We hypothesize that this could have been facilitated by an improved forage base, e.g. increase of small fishes, attributable to overfishing of large predator fishes and size-selective fisheries

Grey seal tracking reveals different behaviours of resident and transient population components

Grey seals recently recolonised the Dutch coasts. The exponential population growth is fuelled by immigrating grey seals from the UK, who subsequently breed in the Netherlands. In addition, population survey data suggested that a number of grey seals use the Dutch area to forage, but return to the UK to breed. Although such population surveys can provide insight into discrepancies and regional imbalances in the number of seals observed on land, the data are insufficient to measure the actual underlying migration processes. Using an extensive data set on individually tracked grey seals, we study the movement of grey seals in relation to where they forage and breed. The tracked seals of different age and sex classes were recognised to be either resident or transient based on where they went during and outside the breeding season. The grey seals breeding in the Netherlands also only haul-out in the Netherlands throughout the period between the moult and next breeding season. Only animals that bred elsewhere (respectively 50% of the adult females and 67% of the adult males tracked in this study) visited regions other than Dutch waters during the rest of the year. Some however, remained in Dutch waters continuously between the breeding trips to other countries. The tracking data suggest a higher number of grey seals might be visiting the Dutch waters than was estimated in a population analysis based on aerial surveys. These results might also have consequences for the population modelling used to estimate the grey seal population size in the UK. As eastern Atlantic grey seals in the North Sea are a mixed meta-population, coordinated, international collaborations are needed to assess their status and trends. Since transient seals might be driven by intraspecific competition, ecological studies on for example, the role of grey seals as a top predator, need to consider the entire North Sea grey seal population

Grey seals (Halichoerus grypus) in the Dutch North sea: population ecology and effects of wind farms

This study was setup to gain an understanding of the possible effects of large-scale development of wind farms in Dutch waters on grey seals (Halichoerus grypus). This should be considered a first step in doing so as up until now relatively little was known about the species in Dutch waters. The study was carried out in the framework of WE@SEA a foundation aimed at acquiring knowledge in the field of offshore wind energy

Harbour porpoises (Phocoena phocoena) and wind farms: a case study in the Dutch North Sea

The rapid increase in development of offshore wind energy in European waters has raised concern for the possible environmental impacts of wind farms. We studied whether harbour porpoise occurrence has been affected by the presence of the Dutch offshore wind farm Egmond aan Zee. This was done by studying acoustic activity of porpoises in the wind farm and in two reference areas using stationary acoustic monitoring (with T-PODs) prior to construction (baseline: June 2003 to June 2004) and during normal operation of the wind farm (operation: April 2007 to April 2009). The results show a strong seasonal pattern, with more activity recorded during winter months. There was also an overall increase in acoustic activity from baseline to operation, in line with a general increase in porpoise abundance in Dutch waters over the last decade. The acoustic activity was significantly higher inside the wind farm than in the reference areas, indicating that the occurrence of porpoises in this area increased as well. The reasons of this apparent preference for the wind farm area are not clear. Two possible causes are discussed: an increased food availability inside the wind farm (reef effect) and/or the absence of vessels in an otherwise heavily trafficked part of the North Sea (sheltering effect