ESTIMATING PUP PRODUCTION OF HARP SEALS, PAGOPHILUS GROENLANDICUS, IN THE NORTHWEST ATLANTIC

Photographic and visual aerial surveys to determine current pup production of Northwest Atlantic harp seals were conducted off Newfoundland and in the Gulf of St. Lawrence during March 1999-Photographic surveys were conducted on all whelping concentrations between 14 and 24 March, whereas a visual survey was made of the southern Gulf concentrations on 14 March. Pup production was estimated to be 739,100 (SE = 96,300, CV = 13.0%) at the Front, 82,600 (SE = 22,500, CV = 27.2%) in the northern Gulf, and 176,200 (SE = 25,400, CV = 14.4%) in the southern Gulf (Magdalen Island) for a total of 997,900 (SE = 102,100, 10.2%). Changes in aerial survey estimates indicate that pup production has increased since 1994. A new method to correct for the temporal change in the proportion of pups present on the ice was examined by fitting the percentage of pups observed in three age-dependent stages to a Normal distribution. The results were compared to those obtained from a more complex model used previously. The Simple model produced slightly higher, and hence more conservative, estimates of the proportion of births that had occurred before the time of the survey than the Complex model. When using the Simple model fewer assumptions regarding the start date of pupping and the proportion of older pups remaining on the ice were required, the herd had to be followed for a shorter period, and a more convenient means of calculating confidence limits was available.     

Distribution and abundance of spotted seals Phoca largha and ribbon seals Phoca fasciata in the southern Sea of Okhotsk

The distribution and abundance of spotted seals (Phoca largha) and ribbon seals (Phoca fasciata) were assessed in March and April, 2000, by aerial line-transect surveys along the southern edge of the pack ice off the coast of Hokkaido (southern Sea of Okhotsk), Japan. Five hundred and seventeen spotted seals and 107 ribbon seals were found on the total 2944 km survey line. Total abundance was estimated to be 13 653 spotted (95% CI = 6167–30 252) and 2260 ribbon seals (95% CI = 783–6607) in March, and 6545 spotted (95% CI = 3284–815 644) and 3134 ribbon seals (95% CI = 1247–17 802 512) in April. The pack ice area off Hokkaido had higher densities (0.54 seals km^–2 and 0.58 seals km^–2 in March and April, respectively) of spotted seals than those reported in eastern Sakhalin, whereas densities (0.09 seals km^–2 in March and 0.28 seals km^–2 in April) of ribbon seals were lower than those in eastern Sakhalin. The large number of spotted seal pups suggests that the study area is an important breeding center. A greater number of female spotted seals with pups tended to be found in the center of larger and rougher floes than in other categories, and they were more abundant in stable pack ice areas. Observations of ribbon seals were limited because the survey period preceded the peak of pupping season. Ribbon seal surveys were also hampered by the inability to fly over the main breeding area between the Shiretoko Peninsula and Kunashiri Island.     

Panmictic population structure in the hooded seal (Cystophora cristata)

Two putative populations of hooded seals (Cystophora cristata) occur in the North Atlantic. The Greenland Sea population pup and breed on the pack ice near Jan Mayen ('West Ice') while the Northwest Atlantic population is thought to pup in the Davis Strait, in the Gulf of St. Lawrence (the 'Gulf'), and off southern Labrador or northeast Newfoundland (the 'Front'). We used microsatellite profiling of 300 individuals at 13 loci and mitochondrial DNA sequencing of the control region of 123 individuals to test for genetic differentiation between these four breeding herds. We found no significant genetic differences between breeding areas, nor evidence for cryptic nor higher level genetic structure in this species. The Greenland Sea breeding herd was genetically most distant from the Northwest Atlantic breeding areas; however, the differences were statistically nonsignificant. Our data therefore suggest that the world's hooded seals comprise a single panmictic genetic population.     

Hematology and serum chemistry of harp (Phoca groenlandica) and hooded seals (Cystophora cristata) during the breeding season, in the Gulf of St. Lawrence, Canada

Standard hematologic and serum chemistry parameters were determined from 28 harp seals (Phoca groenlandica) and 20 hooded seals (Cystophora cristata) sampled from 6 March 2001 to 13 March 2001 during the breeding season. Whole blood was collected immediately postmortem from harp seal mother-pup pairs and from six hooded seal pups, and from live-captured adult hooded seals and three hooded seal pups; blood was analyzed within 24 hr at a local human hospital. A certified veterinary laboratory validated subsamples of whole blood and analyzed all serum chemistry parameters. Significant interlaboratory differences in mean values of packed cell volume (PCV) and mean cell volume (MCV) were found. Significant differences were found between samples from the five seal groups (adult male hooded seals, lactating female hooded seals, unweaned hooded seal pups; lactating female harp seals, and unweaned harp seal pups) for hematology and most serum chemistry parameters. In general, age-class influenced mean values of PCV, hemoglobin (HB), red blood cell (RBC) counts, MCV, mean cell hemoglobin (MCH), mean cell hemoglobin concentration (MCHC), and nucleated red blood cell (NRBC) counts per 100 leucocytes, but most age-related variations were species specific. Harp seal pups had significantly lower mean values of HB, PCV, MCH, and MCHC than did other seal groups, and significantly lower mean RBC counts than did hooded seal pups. Mean NRBC counts per 100 leukocytes were more than three times higher in harp seal pups than in hooded seal pups, but this difference was not statistically significant. Mean MCV were significantly lower in harp and hooded seal pups compared to those of adult harp and hooded seals. Differences in hemograms between pup species were likely because of the precocious development of hooded seal pups, which are weaned within 4 days, compared to 12 days for harp seal pups. Among adult seal groups, male hooded seals had significantly higher mean values of PCV and HB than did female harp and hooded seals, and significantly higher mean RBC counts than did adult female hooded seals. Among adult females, mean values of MCH and MCHC were statistically higher in hooded seals than in harp seals. Adult female harp and hooded seals did not differ significantly in other RBC parameters and mean leukocyte counts. Mean values of glucose, blood urea nitrogen, total bilirubin, alanine aminotransferase (ALT), alkaline phosphatase (ALP), total protein, and albumin showed species-specific variations between adults and pups. Except for ALP, few significant differences in mean enzyme activities of aspartate aminotransferase (AST), ALT, creatine kinase and gamma-glutamyltransferase were found between seal groups. Mean concentrations of electrolytes (calcium, phosphorus, sodium, potassium, chloride, magnesium, and total carbon dioxide) varied with age class, but variations in potassium and magnesium were species specific. Harp seal pups had significantly higher mean phosphorus and potassium levels compared to other seal groups.     

Paternity exclusion by DNA fingerprinting,and mate guarding in the hooded seal Cystophora cristata

Hooded seal Cystophora cristata trios consist of an adult female, her pup, and an attending adult male. Using DNA fingerprinting, we excluded the possibility that the attending males within hooded seal trios were the fathers of the pups, proving that these hooded seals did not remain paired from one breeding season to the next. Behavioural observations of the trios after capture and release revealed that male hooded seals displace one another in attending nursing females. Mate guarding appears to be the preferred mating strategy available to male hooded seals given intense competition for females, a very brief nursing period, and oestrus occuring soon after weaning, but its effectiveness remains unclear.     

Seasonal movements of hooded seals tagged in the Gulf of St. Lawrence,Canada

Hooded seals were tagged as pups (N = 1186) in the Gulf of St. Lawrence between 1975 and 1984. A total of 76 tags have been recovered or resighted. These recoveries indicate a dispersal from the Gulf in the spring to Greenland waters during the summer, and confirm that some seals marked as pups in the Gulf return as adults to this area during the whelping season.     

Estimation of In-Water Density and Abundance of Harbor Seals

Ecologists and managers require accurate population estimates of marine mammals to assess potential anthropogenic threats to these animals. We present estimates of in-water density and abundance of a distinct stock of harbor seals (Phoca vitulina richardii) in Hood Canal, Washington, USA. We used aerial line-transect survey data collected from 2013 to 2016 to directly estimate harbor seal density and abundance in the waters of Hood Canal, a deep-water fjord in the Salish Sea. We estimated a correction factor for trackline detection probability from dive and surface time data gathered from regional seal tagging studies, and applied this factor to correct for seals missed on the trackline during surveys. We applied conventional and multiple covariate line-transect approaches in the analysis. The resulting best estimate of in-water density of harbor seals in the Hood Canal study region was 5.80 seals/km², with an estimated abundance of 2,009 seals. We did not derive a correction factor to account for the number of seals on land (i.e., hauled out). Therefore, these estimates do not reflect total stock size but provide a starting point to evaluate potential influences of anthropogenic activities, particularly those involving underwater noise, on this marine mammal stock. © 2021 The Wildlife Society.     

Distribution and diving behaviour of harp seals (<Emphasis Type="Italic">Pagophilus groenlandicus</Emphasis>) from the Greenland Sea stock

The distribution and diving behaviour of 16 adult harp seals (Pagophilus groenlandicus) from the Greenland Sea stock were studied in 1993 and 1999, using satellite-linked dive recorders (SDRs). The seals remained near the pack-ice edge in the Greenland Sea between breeding and moulting (April/May 1993; 6F) and during the first 7 weeks after moulting (June/July, 1999; 4F, 6M), there diving to depths of <100 m. In mid-July 1999, seven out of eight seals with active SDRs migrated into the Barents Sea, there diving to <400 m and sharing feeding grounds with the Barents Sea harp seal stock. Between September and December, six of these seals joined the eighth seal in the Denmark Strait until March 2000, there diving to depths of 100–400 m. Overall, dives were significantly deeper in the day and in winter than at night and in summer, with some regional differences. Harp seals are considered pack-ice-associated seals, but our tagged seals spent a considerable proportion of their time in open water, their distribution largely overlapping with that of capelin (Mallotus villosus).     

Population status and breeding season chronology of Heard Island fur seals

Fur seals were eliminated by sealers at Heard Island soon after its discovery in the 1850s. The first recorded breeding of Antarctic fur seals (Arctocephalus gazella) since sealing was reported in early 1963 (two pups). The most recent survey of the Heard Island fur-seal population was undertaken between November 2000 and March 2001, when 1,012 Antarctic fur-seal pups were born. This represents a fourfold increase since the last complete census in 1987/1988 (13 years), when 248 births were recorded. Pup estimates and counts available for eight breeding seasons since 1962/1963 suggest the population has been increasing at between 12 and 20% per year. Based on pup production, the breeding population is estimated to number approximately 4,100 seals. The number of fur seals on Heard Island peaked in late February/early March at 29,256 indicating that, in addition to the breeding population, a significant number of seals born elsewhere haul out on the island. Most of these are moulting sub-adult and adult males. As in 1987/1988, only one subantarctic fur-seal pup (A. tropicalis) was observed, suggesting this species is not colonising the island, as has been speculated.     

Remarkable development of diving performance and migrations of hooded seals (Cystophora cristata) during their first year of life

Newborn hooded seals (Cystophora cristata) have smaller weight-specific oxygen stores than adults, but nothing is known about how this affects their diving behaviour. Here, we present data on the diving behaviour and migrations of seven weaned hooded seal pups of the Greenland Sea stock during their first year of life, as collected by use of satellite telemetry. The pups started diving 1–2 days after tagging, and during a tracking period of 25–398 days they dispersed over vast areas of the Greenland and Norwegian Seas in a manner similar to adults. The initial development of diving depths and durations in April–May was rapid, and pups reached depths of >100 m and dived for >15 min within 3 weeks of age. During early summer (May–June) this development was temporarily discontinued, to be resumed throughout autumn and winter, during which time maximum depths and durations of >700 m and >30 min, respectively, were reached. Depths and durations were significantly related to age/season, location and time of day. The dive behaviour in early summer, with relatively shallow and short dives without diurnal variations, resembles that of adults and probably reflects the vertical distribution of prey rather than physiological constraints. Dives of pups were nevertheless shallower and shorter than those of adults, but relative to body mass both hooded seal pups and adults display a remarkable diving capacity which makes the species particularly suited for studies of defence mechanisms against hypoxia insult in mammals.     

Diets of hooded seals (Cystophora cristata) in coastal waters and drift ice waters along the east coast of Greenland

To provide data on the diets of hooded seals (Cystophora cristata) in the Greenland Sea, seals were collected for scientific purposes on expeditions conducted in the pack ice belt east of Greenland in September/October 1999, 2002 and 2003 (autumn), July/August in 2000 (summer), and February/March in 2001 and 2002 (winter). The results from analyses of stomach and intestinal contents from captured seals revealed that their diet was comprised of relatively few prey taxa. The squid Gonatus fabricii and polar cod (Boreogadus saida) were particularly important, whereas capelin (Mallotus villosus) and sand eels (Ammodytes spp.) occasionally contributed more. These four prey items constituted 60-97% of the diet biomass. Gonatus fabricii was the most important food item in autumn and winter, whereas polar cod dominated the summer diet, with important contributions from G. fabricii and sand eels. The latter was only observed on the hooded seal menu during the summer period, whereas polar cod, which was an important component during the autumn survey, was almost absent from the winter samples. During the latter survey, capelin also contributed to the hooded seal diet. Samples obtained from hooded seals in more coastal waters indicated a more varied diet based on fish such as polar cod, redfish (Sebastes sp.) and Greenland halibut (Reinhardtius hippoglossoides).     

Ringed and bearded seal densities in the eastern Chukchi Sea, 1999–2000

Aerial surveys were conducted in 1999 and 2000 to estimate the densities of ringed (Phoca hispida) and bearded (Erignathus barbatus) seals in the eastern Chukchi Sea. Survey lines were focused mainly on the coastal zone within 37 km of the shoreline, with additional lines flown 148–185 km offshore to assess how densities of seals changed as a function of distance from shore. Satellite-linked time-depth recorders were attached to ringed seals in both years to evaluate the time spent basking on the ice surface. Haulout patterns indicated that ringed seals transitioned to basking behavior in late May and early June, and that the largest proportion of seals (60–68%) was hauled out between 0830 and 1530 local solar time. Ringed seals were relatively common in nearshore fast ice and pack ice, with lower densities in offshore pack ice. The average density of ringed seals was 1.91 seals km⁻² in 1999 (range 0.37–16.32) and 1.62 seals km⁻² in 2000 (range 0.42–19.4), with the highest densities of ringed seals found in coastal waters south of Kivalina and near Kotzebue Sound. The estimated abundance of ringed seals for the entire study area was similar in 1999 (252,488 seals, SE=47,204) and 2000 (208,857 seals, SE=25,502). Bearded seals were generally more common in offshore pack ice, with the exception of high bearded seal numbers observed near the shore south of Kivalina. Bearded seal densities were not adjusted for haulout behavior, and therefore, abundance was not estimated. Unadjusted average bearded seal density was 0.07 seals km⁻² in 1999 (range 0.011–0.393) and 0.14 seals km⁻² in 2000 (range 0.009–0.652). Levels of primary productivity, benthic biomass, and fast ice distribution may influence the distributions of ringed and bearded seals in the Chukchi Sea. Information on movement and haulout behavior of ringed and bearded seals would be very useful for designing future surveys.     

Harbor seal use of glacier ice and terrestrial haul‐outs in the Kenai Fjords,Alaska

Harbor seals, Phoca vitulina, use diverse haul‐out substrates including ice calved by tidewater glaciers. Numbers of seals at glacial and terrestrial haul‐outs on the southeastern Kenai Peninsula, Alaska, were assessed using aerial, vessel, and video surveys. Mean annual abundance at glacial and terrestrial haul‐outs differed temporally. From 2004 to 2011, numbers of seals counted during the molt increased 5.4%/yr at glacial haul‐outs and 9%/yr at terrestrial haul‐outs while numbers of pups increased 5.0%/yr at glacial sites and 1.5%/yr at terrestrial sites. Numbers of seals without pups counted during pupping increased 7.96%/yr at glacial sites and 5.1%/yr at terrestrial sites. Results indicate that pupping and molting locations are not equivalent and population monitoring during the molt does not necessarily reflect habitat association of pupping seals. Ratios of pups to total seals counted during pupping and the subsequent molt were used to contrast habitat use. Low proportions of pups at terrestrial haul‐outs, relative to most glacial haul‐outs, indicate an overall preference for pupping in glacial haul‐outs. High proportions of pups at most glacial sites (during pupping and molting) suggest reduced use of tidewater glacier habitats by nonbreeders and molting seals. Results suggest more seals associate with glacial haul‐outs than currently estimated.     

Contrasting trends in gray seal (Halichoerus grypus) pup production throughout the increasing northwest Atlantic metapopulation

The northwest Atlantic subspecies of gray seal (Halicheorus grypus grypus) has been increasing for more than a half century and has reestablished breeding colonies in Canadian and US waters. In 2016, visual, oblique, and vertical large-format digital photographic surveys were conducted at all known breeding colonies in the northwest Atlantic. Total pup production in the northwest Atlantic was estimated to be 109,000 (SE = 17,500) pups. At 87,500 (SE = 15,100) pups, Sable Island accounts for 80% of total pup production. Regional differences in pup production trends are evident. Pup production in the Gulf of St. Lawrence and along the eastern shore of Nova Scotia has been relatively stable. Since 2004, the rate of increase in pup production at Sable Island has slowed to about 5%–7% per year, while the newer colonies in southwest Nova Scotia and the northeastern United States are increasing rapidly. In 2016, the Muskeget Island (MA) breeding colony produced 3,900 (SE = 200) pups, making it the third largest breeding colony in the northwest Atlantic. This southward shift in production may reflect climate-mediated changes in population growth as well as reestablishment of colonies throughout the former range associated with increased protection.     

THE ROLE OF PREDATION IN THE ECOLOGY OF THE RINGED SEAL IN BARROW STRAIT, NORTHWEST TERRITORIES, CANADA

Predation on ringed seals ( Phoca hispida ) was examined in Barrow Strait between March and May 1984 to 1986. Polar bears were the most important predator. Evidence of bear predation was observed at 18–30% of the ringed seal subnivean structures we located. Ten to 24% of predation attempts were successful, with pups making up 75% to 100% of the seals killed. Bears killed an average of 0.08 to 0.51 seals/km², which comprised 8 to 44% of the estimated annual pup production. Bears were successful on average in 11.3% of their attempts to kill pups hidden inside birth lairs. On southeast Baffin Island where snow was soft and pups were exposed, bears were successful in 33.5% of their attempts to kill a seal. Negative correlations were found between mean snow depth and predation by polar bears ( r = -0.896, P = 0.04, n = 5) in 1985, and between snow depth and the number of predation attempts ( r = -0.613, P = 0.02, n = 14) in 1986.     

Rapid recovery of Dutch gray seal colonies fueled by immigration

Gray seals were first observed breeding in the Dutch Wadden Sea in 1985, after centuries of absence. The breeding colony there is now the largest on the European continent. We describe the changes in gray seal numbers and their geographical expansion, and estimate how these processes were influenced by immigration from other colonies. Counts of hauled out animals were carried out between 1985 and 2013, monitoring three different periods of the seals' annual cycle. Using priors determined for the UK population, a Bayesian demographic model was fitted to pup numbers to estimate the population parameters driving the growth. This included immigration of subadults into the breeding population, which contributed to an average growth rate in the pup counts of 19%/yr, much higher than expected in a closed population. This immigration may account for approximately 35% of the total annual growth. In addition, at least 200 gray seals from the UK visit the area temporarily. Recovery of the population in the Netherlands occurred more than 50 yr after gray seals were protected in the UK. These time scales should be taken into account when studying long living marine mammals, e.g., in impact and conservation studies.     

Annual distribution of hooded seals (Cystophora cristata) in the Greenland and Norwegian seas

 Nineteen hooded seals (Cystophora cristata) were tagged with satellite-linked platform terminal transmitters (PTT) on the sea ice near Jan Mayen. Fifteen were instrumented after completion of the moult in July 1992 (five males, ten females, at 71°N, 12°W), and four during breeding in March 1993 (four females, at 69°N, 20°W). Sixteen of the seals were tagged with Satellite-Linked Time-Depth-Recorders (SLTDR), yielding location, dive depth and dive duration data. The average (±SD) longevity of all PTTs was 199±84 days (n=19; range: 43–340 days), and they yielded 12,834 location fixes. Between tagging in July 1992 and pupping in March 1993, two seals remained in or near the ice off the east coast of Greenland for most of the tracking period. However, most of the seals made one or several trips away from the ice edge, mostly to distant waters. These excursions had an average (±SD) duration of 47±22 days (n=46; range: 4–99 days). Eight seals travelled to waters off the Faeroe Islands, three to the continental shelf break south of Bear Island, and three to the Irminger Sea southwest of Iceland. Eleven seals were tracked in the period between breeding (March/April) and moulting (July). Several of these spent extended periods at sea west of the British Isles, or in the Norwegian Sea. Received: 3 August 1994/Accepted: 4 July 1995     

Breeding biology of weddell seals (Leptonychotes weddellii) at Drescher Inlet,riiser larsen ice shelf,Antarctica

Summary The breeding biology of Weddell seals (Leptonychotes weddellii) was studied at Drescher Inlet (Antarctica) during the austral late winter and spring 1986. The whelping season was estimated to last about 40 days, with a peak in pupping at late October. No post natal mortality was observed during the whole study. The attachment between parous females and their pups was strong during the first five weeks after parturition. From the third week on mothers spent more time in the water than the pups. Data on daily peak haul-out time of pups, post natal presence of pups on the ice and distribution of births during the whelping season, indicate that censuses should be carried out between 12.00 hours and 16.00 hours local time and during three weeks after the peak of the pupping season. Pups weaned between 6 and 7 weeks of age. Hormone concentrations (oestradiol-17 and progesterone) indicated that females did not come into oestrus during lactation. Throughout the whole lactation period the mother dominates the relationship with her pup. Growth of the pups was linear during the first 5 weeks post partum, during which period birth weight (x=29.1kg) increased more than 3 1/2 times. Until the 6th week of age the pups gained on average 2.6 kg/day, the respective weight loss in cows was 5.8 kg/day. The latter represents an average loss of nearly 38% of the initial weight at parturition. During the course of lactation, suckling frequency decreased, whereas the length of each bout increased resulting in an approximately constant total suckling duration per week. Of the energy used by females during the first 5 weeks, 52% was consumed by pups. Pups only took milk and the cows did not feed at all during the whole period of observation. Therefore the realized growth in pups, 10 g/min of suckling, was totally derived from energy stored by the cows.     

Development of myoglobin concentration and acid buffering capacity in harp (<Emphasis Type="Italic">Pagophilus groenlandicus</Emphasis>) and hooded (<Emphasis Type="Italic">Cystophora cristata</Emphasis>) seals from birth to maturity

Pinnipeds rely on muscle oxygen stores to help support aerobic diving, therefore muscle maturation may influence the behavioral ecology of young pinnipeds. To investigate the pattern of muscle development, myoglobin concentration ([Mb]) and acid buffering ability (β) was measured in ten muscles from 23 harp and 40 hooded seals of various ages. Adult [Mb] ranged from 28–97 to 35–104 mg g tissue⁻¹ in harp and hooded seals, respectively, with values increasing from the cervical, non-swimming muscles to the main swimming muscles of the lumbar region. Neonatal and weaned pup muscles exhibited lower (~30% adult values) and less variable [Mb] across the body than adults. In contrast, adult β showed little regional variation (60–90 slykes), while high pup values (~75% adult values) indicate significant in utero development. These findings suggest that intra-uterine conditions are sufficiently hypoxic to stimulate prenatal β development, but that [Mb] development requires additional postnatal signal such as exercise, and/or growth factors. However, because of limited development in both β and [Mb] during the nursing period, pups are weaned with muscles with lower aerobic and anaerobic capacities than those of adults.     

Maximum likelihood population size estimation of harbour seals in the Dutch Wadden Sea based on a mark–recapture experiment

1. The harbour seal population Phoca vitulina in the entire Wadden Sea was severely depleted due to a virus-epizootic during 1988. A comprehensive study on the population biology and activity patterns was subsequently initiated to design a management and conservation plan. The main objective of this study was to estimate harbour seal abundance in the different regions of the Wadden Sea.
2. We investigated the potential of a mark–recapture experiment using VHF radio-tags in combination with repeated aerial surveys to estimate the number of harbour seals in the Dutch part of the Wadden Sea. The number of harbour seals hauled-out and the presence of any radio-tagged seals was monitored during seven aerial surveys of all known haul-out sites in the Dutch Wadden Sea over the 1994 breeding season.
3. A maximum likelihood (ML) estimator was developed to infer the rate of tag-loss and the size of the local prepupping population.
4. The ML estimate of the number of harbour seals in the Dutch Wadden Sea was 1536 (95% confidence limits were 1225 and 2200). The corresponding maximum proportion of seals hauled-out was 68%.
5. The use of VHF radio-tags which can be monitored from the air provides a way of correcting aerial survey counts for the proportion of harbour seals hauled-out during the surveys. Since haul-out behaviour may be influenced by local conditions, such as exposure time of sand banks, we recommend this technique be repeated in other areas of the Wadden Sea rather than using the estimates from the current study in other areas.