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Biological Conservation 81 (1997) 97-102 © 1997 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0006-3207/97 $17.00 + 0.00

THE I M P O R T A N C E OF THE N O R T H SEA FOR WINTER DISPERSAL OF H A R B O U R SEALS Phoca vitulina FROM THE

W A D D E N SEA

Mardik F. Leopold,* Bert van der Werf, Edith H. Ries & Peter J. H. Reijnders

Institute for Forestry and Nature Research, PO Box 167, 1790 AD Den Burg, Texel, The Netherlands

(Received 27 October 1995; accepted 23 March 1996)

Abstract Harbour seals Phoca vitulina were routinely recorded during ship-based seabird counts in the eastern North Sea in winter. This area borders on the Wadden Sea, a major breeding ground o f the species in Europe. Line transect theory was used to estimate total numbers o f seals at sea. A total estimate o f 2200 seals was obtained (c.v. 35%; 95% confidence interval =855-3785). Stratification o f the data according to geographical subregion did not improve precision o f this estimate. Our estimate is uncor- rected for animals missed on the tracklines and is thus a minimum. Considering this, at least 20% o f the estimated Wadden Sea summer population is to be found in the adjacent North Sea in winter. This implies that seal con- servation policy needs to consider this wintering range in the North Sea, in addition to the existing management plan that protects the breeding and moulting areas in the Wadden Sea. © 1997 Elsevier Science Ltd

Keywords: Seals, line transect, Wadden Sea, North Sea and population size.

I N T R O D U C T I O N

The harbour seal Phoca vitulina is a priority species of nature conservation in the Wadden Sea and as such, their numbers have been monitored for decades (Reijn- ders, 1981, 1992; Tougaard, 1989; Schwarz & Heide- mann, 1994). The population size is assessed in summer by counting all animals during low tide when they aggregate on specific tidal haul-out sites. These counts provide a minimum number, as some animals may remain in the water, which makes them unavailable for counting. The proportion of animals missed is not known, but could be as high as 30% of the total (esti- mate by S. Tougaard in Reijnders, 1992). Using this figure, the population size in 1992-93 would have been

*To whom correspondence should be addressed. 97

around 10500 animals (calculated from Schwarz & Heidemann, 1994).

In winter fewer seals haul-out (Drescher, 1979; Reijnders et al., 1981) and numbers in the water must thus be comparatively large. Little is known of the whereabouts of these seals. Telemetric studies (Nor- gaard et al., 1992; Ries, 1993) suggest that they often use the adjacent North Sea in winter. This is reflected in the distribution of haul-out sites: in winter more seals are found at sites near the North Sea (Tougaard, 1989) and on the offshore island Helgoland in the central German Bight of the North Sea (Fig. 1). It has been argued that the coastal waters of the eastern North Sea are important to the seals, as they are to other "Wadden Sea" species that show regular migration between the two seas (Leopold et al., 1993). Based on such consid- erations, the governments of the three Wadden Sea countries (the Netherlands, Germany and Denmark) decided in 1994 to move the legal boundary of the Wadden Sea seaward by three nautical miles, to include the inshore North Sea waters.

Although the harbour seal is not a rare species in Europe, its conservation status is unfavourable. In the Wadden Sea, the seals have been subjected to a variety of threats and the population size has been significantly impacted by hunting, pollution, a virus epidemic and to a lesser extent by incidental catches and disturbance (Reijnders et al., 1993; Brasseur & Reijnders, 1994). All these issues have been subsequently dealt with, and seal conservation is now embedded in a comprehensive seal management plan, drafted jointly by the three Wadden Sea states (Common Wadden Sea Secretariat, 1996). However, despite all this conservation effort, the present population size is still estimated at only one-third of the carrying capacity of the Wadden Sea in a pristine state (Reijnders, 1992).

Seal conservation at present is mainly focused on protecting major haul-out sites in summer where the seals breed and moult. Harbour seals have never been surveyed outside the Wadden Sea and the relative importance of the adjacent North Sea for the seals is thus not known. Should the seals rely on the North Sea

98 M . F . Leopo ld et al.

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Fig. 1. Numbers of harbour seals (monthly maxima) hauled- out on Helgoland in 1993 (11) and 1994 (0) . Data courtesy

Dr Ommo Hiippop, Vogelwarte Helgoland.

°N

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54

53

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Fig. 2. Transect lines sailed and geographical strata (I-V) used in this study. Hinterland seal density was based on seal counts in the Wadden Sea, divided in 21 parts (A-U, see Table I).

Inset, RV Navicula. . ..... 20 m isobath.

as well as on the Wadden Sea, for instance in seasons other than the breeding or moulting periods, this area should be included in future management. In order to substantiate the suggestion that large numbers of har- bour seals use the Nor th Sea in winter, data from sea- bird surveys that included seal sightings in the area have been used to estimate the number of seals present in the coastal Nor th Sea.

M E T H O D S

Ship-based counts were conducted in January/February 1992 and 1993 in the coastal, eastern Nor th Sea, bor- dering on the Wadden Sea. The inner limit of the study area was taken as the shortest line between neighbour- ing barrier islands, or in the inner German Bight where such islands are lacking, the line between neighbouring fiats (the 0 m isobaths). The outer border was set at the shipping lane in the area west of the river Elbe (nearly equivalent to the 20 m isobath). In the waters north of the river Elbe the 20 m line is situated much further offshore and here the outer limit was set half-way between the islands and the 20 m isobath (Fig. 2).

The transect lines ran from the inner to the outer border in a zig-zag fashion, in order to make the best use of the available ship time. Each leg was treated as a single sampling unit (n--85; Fig. 2). The platform used was the 21-m long research vessel Navicula . This ship has a draft of only 1 m, allowing access to all parts of the shallow inshore waters. The observations were made from the top of the bridge, at a height of 7 m, by a team of four observers. Seals were counted on both sides of the ship on most lines, but occasionally sun glare or a

combination of wind and rain prevented observations on one side. In that case, the measure of effort, kilo- metres sailed, was halved in the analyses (cf. Buckland et al., 1993).

In total, 1452.5 km of transects were sailed. Line transect methodology was used to estimate numbers of seals in the area. This method takes into account that animals at larger distances from the observation plat- form have a lower probabili ty of being detected (Buck- land et al., 1993). All seals observed were assigned to one out of five distance bands (parallel to the trackline), with cut-off points at 0, 50, 100, 200, 300 and 700 m. Estimates of perpendicular distances were made by eye, aided by a simple range finder (Heinemann, 1981), set at distances of 200 and 300 m. These were used for our seabird counts, the principle subject of the surveys (Skov et al., 1994). Observations beyond 700 m were not used, and neither were observations of seals hauled-out on the seaward side of islands or on sand-banks between islands.

A detection function, relating the probability of see- ing a seal to perpendicular distance, was derived to correct for animals missed at greater distances (Fig. 3). Program DISTANCE, version 2.1 (Laake et al., 1994) was used to estimate the detection function, and total numbers of seals in the area. A hazard-rate model without adjustments was selected from this package, as this gave the best fit in terms of the Akaike information criterion (AIC), without constraining the parameters used in the model. AIC provides an objective and quantita- tive method for model selection, much like a likelihood ratio test. The criterion AIC = -2[loge(~')-q], where loge(~ ) is the value of the log-likelihood function evaluated at the maximum likelihood estimates ( y ) of

North Sea dispersal of harbour seals 99

~ 1.0 -~

g c -

O 0.5

D

i i 1 i

0 200 400 600 800 Perpendicular distance (m)

Fig. 3. Detection curve for harbour seal fitted to the data. A hazard-rate model without adjustments was used for grouped data (five distance bands), truncated at 700 m. Effective strip

width was 136 m, for all data pooled.

the model parameters and q is the number of para- meters used in the model. AIC is computed for each candidate model, and the model with the lowest AIC is selected (see Buckland et al., 1993).

Two potential stratification factors were considered: geographical subregion and Beaufort seastate during data collection. These two factors probably interacted, but sample size considerations precluded a simultaneous stratification by both factors. Analyses of total numbers were therefore done for the whole dataset and with the dataset divided into either geographical strata or into subsets of data collected at similar seastates.

Table 1. Numbers of seals (n) counted in the Wadden Sea in 21 subareas (A-U: see Fig. 2) in summer 1992 (data: Peter Reijnders, the Netherlands; H. yon Bothmer, Niedersachsen; J. Schwarz and G. Heidemann, Schleswig-Holstein; S. Tougaard, Denmark; aerial surveys). The size of each subarea was measured as the length of seaward coastline of the barrier

islands, including the tidal inlets (km)

Subarea Stratum km n n/km Average

A I 42.3 78 1-8 2.9 B I 37.0 83 2.2 C I 28-6 136 4.8 D I 27.5 94 3.4 E II 33.8 439 13.0 18.3 F II 23.3 605 26.0 G II! 19.0 49 2.6 5-2 H III 20.1 71 3.5 I III 12.7 85 6-7 J Ill 11-6 127 10-9 K IV 20.1 872 43-4 24.0 L IV 24.3 604 24.8 M IV 19.0 256 13.4 N IV 29-6 594 20.1 O IV 18-0 497 27.6 P IV 27-5 857 31.2 Q IV 29-6 486 16.4 R IV 30.7 523 17.1 S IV 11.6 366 31.5 T V 20.1 100 5.0 6.0 U V 26.4 179 6.8

Stratification into geographical subregions was based on the relative numbers of seals in the adjacent parts of the Wadden Sea in summer. To this end we used the results of the synoptic counts in the Wadden Sea of summer 1992 (P. Reijnders, S. Tougaard, J. Schwarz and H. von Bothmer, unpublished results; Table 1). The "hinterland seal density" was expressed as the number of seals counted per km of outer "coastline", for 21 subareas (Table 1). Based on an arbitrary cut-off point of 10 seals per km of coastline, five different strata were defined, three with low seal densities and two with high seal densities (Table 1; Fig. 2). High densities were found from Schiermonnikoog to Juist (stratum II) and from Wangerooge to Mano (stratum IV). For calcula- tions of encounter rate and density of seals, the data for different sub-areas were used separately. Total numbers of seals in the study area were estimated by performing 1000 non-parametric bootstraps of the density estima- tion procedure.

Stratification by seastate was based on differences in encounter rates (Table 2) and effective strip widths. The data suggested a grouping into seastates 0-1, 2 4 and 5-7 Beaufort. The wind strata were weighted by relative effort in each Beaufort catagory.

R E S U L T S

During the surveys 101 seals were recorded. Their dis- tribution was apparently non-random (Fig. 4). Clusters of high densities were found off the River Ems estuary (at the Dutch/German border) and off Schleswig- Holstein, Germany. Moreover, the seals appeared to be clustered around the tidal inlets and, off Schleswig- Holstein, also around the 10 m isobath. Low densities were encountered in the west of the study area, in the inner German Bight, and particularly in Danish waters. North of the island Romo not a single seal was observed.

In total, some 2200 harbour seals were estimated to be present in the eastern North Sea (Table 3). The esti- mates are based on data with a high level of heteroge- neity, as the encounter rate differed considerably between transect lines. Confidence limits are wide as a result. Precision did not increase by using the five geo- graphical strata based on the hinterland model of seal distribution in the Wadden Sea, indicating that the seals range widely when in the North Sea. Apparently, the

Table 2. Numbers of harbour seals seen in relation to Beaufort seastate (Bft). Effort is given as total km sailed per class of Beaufort. n is total number of seals seen in each seastate, nex p is

the expected number, based on relative effort

BR Effort n aex p n/km nexp/km

0-1 200.17 25 14 0-125 0.070 2-4 613.93 22 43 0-036 0.070 5-7 638.42 54 44 0-085 0.070

1 O0 M . F . Leopold et al.

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) Transect line • ~on of s~ sighlJng e n Cluster of ~ sighlJngs

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Fig. 4. Seals (0 ) observed during the survey.

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seals may swim considerable distances from the tidal channel from where they exit. The two high density areas in the Wadden Sea (II and IV: Table 1) are still reflected to some extent in the distribution of the seals in the Nor th Sea in winter, as the highest seal densities and seal numbers were found in these offshore strata. Areas I and V, both with low hinterland densities, had low densities at sea as well. However, area III had low hin- terland densities, but high numbers at sea (Tables 1 and 3). Geographical stratification did not affect the esti- mate of the number seals present in the study area as a whole, since one detection function was used for all data pooled.

Stratification by observation conditions yielded ambiguous results. As expected, effective strip width decreased with increasing seastate, but the encounter rate was lowest in moderate conditions (Table 4). Het- erogeneity in the data for 2-4 Beaufort was too high to perform the boots t rap procedure successfully in the

Table 3. Estimates of mean density (n/km 2) and numbers (n) of seals present in the study area (surface area in km2). Effort gives the total length (km) of transects in each stratum. A separate analysis was done for five different geographical strata (hinter- land model). In both cases the estimates are based on 1000 bootstraps of the density estimation procedure. % CV is given for n. The 95% confidence interval is the 2.5% and 97-5

quantiles of the bootstrap estimate of n

Stratum Area Effort n/km 2 n % CV 95% Conf. Int.

Total 8576 1452.5 0.2562 2 1 9 7 34.97 790-3878 I 1922 329.3 0.1007 194 37 .81 72-342 II 1026 190.4 0.3290 338 47.48 87-700 III 967 160.5 0.2983 288 61.05 0-649 IV 3601 659.1 0.3466 1248 40.28 437-2369 V 1060 113.2 0 0 - - - - Total 8576 1452.5 0.2562 2 1 9 7 34.23 855-3785

calculations. Therefore, although Beaufort seastate is likely to affect sighting probability, the data contain too little evidence that this was important. The different encounter rates at different windspeeds are probably mainly related to an interaction of area and Beaufort, or to chance.

D I S C U S S I O N

The detection function suggests "heaping" near the transect line, with the probabili ty of detection rapidly failing off with increasing distance (Fig. 3). Line transect theory works on the assumption that all objects on the transect line are seen, or that the probability of detect- ing a target at zero perpendicular distance, known as g(0) can be estimated. A steep detection curve makes estimating g(0) difficult and this may result in a biased estimate of density. Therefore, this feature is usually considered undesirable in line transect theory (Buckland et al., 1993). In the present study, the shape of the detection curve may have been partly caused by the behaviour of the seals. In most sightings only the head of the seal was seen, and the animals were almost always eyeing the ship. Telemetric data from the Wadden Sea suggest that harbour seals spend a considerable amount of time underwater when at sea (Ries, 1993; Ries & Traut, 1993). This would make many seals unavailable for counting, leading to a g(0) much less than 1 and an underestimate of density. However, seals swimming at small perpendicular distances seemed to react to the approaching ship by surfacing. We have noted similar responsive surfacing behaviour in the Wadden Sea dur- ing ship-based observations. I f the tendency to surface was relatively strong near the transect line this would result in a steep detection function and a g(o) close to 1. I f seals near the trackline did not always surface when the ship passed, and if they were not all detected at close range, our estimate of total numbers is negatively biased. No correction could be made for observation conditions. There was an indication that effective strip width decreased with Beaufort, but this effect could not be incorporated into the density estimation model. Harbour seals have a tendency to leave their haul-out sites at higher wind speeds (Pauli & Terhune, 1987; Brasseur et al., 1996). This could in theory lead to higher numbers at sea in higher seastates. However, it is unlikely that numbers at sea would drop at windspeeds between 2 and 4 Beaufort, only to increase again at

Table 4. Estimated effective strip widths (ESW) and encounter rates (niL, number of seals seen per km of transect) in different

wind conditions

B~ ESW % C V d.K n/L % C V d.f.

0-1 260 23.5 23 0.124 27.65 12 2-4 122 87.7 20 0.036 27-90 33 5-7 108 21.0 52 0.085 28.43 37

North Sea dispersal o f harbour seals 101

higher wind speeds, and the relatively low encounter rates at moderate conditions (Table 4) is at present best explained by chance.

Considering the uncertainties in estimating g(0) and the effects of seastate, our estimate of 2200 seals for the coastal waters of the Nor th Sea in 1992 and 1993 is clearly a minimum number. Note also that the two winters in which we surveyed the area were relatively mild and that in colder winters larger numbers may leave the Wadden Sea (Reijnders, 1981). Moreover, seals sometimes venture out beyond the seaward limit of the present study area (Ries, 1993) and such animals have obviously not been included in the present estimate for the Nor th Sea. For instance, some 100 seals may be present at Helgoland in mid-winter (Fig. 1) and these have not been included in the total estimate of 2200. As a conservative estimate, our data show that some 20% of the total summer populat ion of the Wadden Sea occurs offshore in the Nor th Sea in mid-winter, at least during the daylight hours. This clearly substantiates the observations in the Wadden Sea that in winter fewer seals are found hauled-out than in summer.

Not only will some animals have been missed during the surveys, the total number of individual animals using the Nor th Sea in the course of the winter must be considerably higher than the number that can be found in the study area at any one time. Telemetric data show that individual seals may cross the boundary of the two seas on many occasions in the course of the winter (Norgaard et al., 1992). At present, the telemetric data are insufficient to calculate turnover rates of seals using the Nor th Sea, but these data suggest movement into the Nor th Sea is a common behaviour in winter (Ries, unpubl, obs.). Although the frequency of this behaviour is yet unknown, it is postulated that a much larger pro- port ion the Wadden Sea seals than the 20% present in the Nor th Sea at a given moment , may go on trips out- side the Wadden Sea. The concentrations of animals near the tidal inlets further endorses the likelihood of frequent travelling f rom the Wadden Sea to the Nor th Sea and vice versa as the tidal inlets are geographical bottlenecks for seals moving between the two seas. Alternatively, these areas could be of special significance for the seals, if for instance feeding conditions are par- ticularly good here.

The motivat ion for venturing out into the Nor th Sea is still unknown. In winter, foraging conditions in the relatively warm North Sea could be better than in the Wadden Sea. The clustering of observations around the 10 m isobath off Schleswig-Holstein suggested that this zone might be an important offshore foraging area. Whatever the reason, seals obviously venture into the Nor th Sea in important numbers in winter. This signif- ies that both the Wadden Sea and the adjacent Nor th Sea are an important part of their habitat.

Considering that the conservation of any species relies on the conservation of all key habitats the animals need to complete their life cycle, we now must gain

insight in the question of what makes the North Sea so important for the seals? The available evidence suggests that the Nor th Sea may provide better foraging oppor- tunities at times when fish leave the Wadden Sea due to low water temperatures. In order to better understand the reasons for these trips into the Nor th Sea, we need studies of seal diets, feeding methods and energetic constraints. In the meantime, the offshore wintering grounds in the Nor th Sea should be considered as an integral part of the seals' habitat, next to the breeding and moulting areas in the Wadden Sea. The coastal Nor th Sea should thus be included in future manage- ment plans. On a wider scale, our findings may also be relevant for harbour seal management in other parts of its range, as well as for other pinnipeds.

ACKNOWLEDGEMENTS

We would like to thank Captain C. Wisse and Captain K. van der Star and their crews of the RV Navicula. This project was partly funded by the European Union (under EU D G XI ACE contract no. 445-45). The field work was conducted by the first author while employed by the Netherlands Institute for Sea Research in a joint project with Ornis Consult, Copenhagen. The observers, Hans van den Berg, Erik Bos, Bernhard Budde, Sina Clorius, Marten Geertsma, Mark Hoekstein, Nick den Hollander, Henk Offringa and Erwin Winter, assisted during the surveys. Henk Offringa made the computer drawing of the Navicula (Fig. 2); Peter van der Wolf and Sophie Brasseur that of the seal (Fig. 4). We are parti- cularly grateful to Mr H. von Bothmer, Giinter Heide- mann, Jochen Schwarz and Svend Tougaard for letting us use the seal counts in the German and Danish Wad- den Sea and to O m m o Hiippop, of Vogelwarte Helgo- land, for the seal counts on Helgoland. The course in " D I S T A N C E " given by Steve Buckland, David Ander- son and Ken Burnham in St Andrews in 1994 was a great help in drafting this paper. Kees Camphuysen, Sophie Brasseur, Bart Ebbinge, Jan Andries van Frane- ker and two anonymous referees read and improved earlier versions of the manuscript.

REFERENCES

Brasseur, S. M. J. M. and Reijnders, P. J. H. (1994) Invloed van diverse verstoringsbronnen op her gedrag en habitat- gebruik van gewone zeehonden: consequenties voor de inrichting van het gebied. IBN Report No. 113. Institute for Forestry and Nature Research, Texel, the Netherlands.

Brasseur, S., Creuwels, J., van der Werf, B. and Reijnders, P. (1996) Deprivation indicates necessity for haul-out in har- bour seals. Mar. Mammal Sci. 12, 619-624.

Buckland, S. T., Anderson, D. R., Burnham, K. P. and Laake, J. L. (1993) Distance Sampling: Estimating Abundance of Biological Populations. Chapman and Hall, London.

Common Wadden Sea Secretariat (ed) (1996) Conservation and management plan for the Wadden Sea seal population

M. F. Leopold et al.

19962000, including additional measures for the protection of the grey seal. CWSS, Wilhelmshaven.

Drescher, H. E. (1979) Biologie, ijkologie und Schutz der Seehunde im schleswig-holsteinischen Wattenmeer. Beitr. Wildbiol. Meldorf 1, l-73.

Heinemann, D. (1981) A range finder for pelagic bird census- ing. J. Wildl. Mgmt. 45,489493.

Laake, J. L., Buckland, S. T., Anderson, D. R. and Burnham, K. P. (1994). DISTANCE user’s guide, version 2. 1. Color- ado Cooperative Fish and Wildlife Research Unit, Color- ado State University, Fort Collins, CO.

Leopold, M. F., Skov, H. and Htippop, 0. (1993) Where does the Wadden Sea end? Links with the adjacent North Sea. Wadden Sea New.& 3, 5-9.

Nerrgaard, N., Ries, E. H., Schwarz, J. and Traut, I. M. (1992) Conservation and management plan for the harbour seal (Phoca vitulina) population in the Wadden Sea based on a joint telemetry study. In Wildlife Telemetry Remote Moni- toring and Tracking of Animals, ed. I. G. Pride and S. M. Swift, pp. 687693. Ellis Harwood, Chichester.

Pauli, B. D. and Terhune, J. M. (1987) Meteorological influen- ces on harbour seal haul-out. Aquat. Mammals 13, 114-l 18.

Reijnders, P. J. H. (1981) Management and conservation of the harbour seal, Phoca vitulina, population in the interna- tional Wadden Sea area. Biol. Conserv. 19, 213-221.

Reijnders, P. J. H. (1992) Retrospective population analysis and related future management perspectives for the harbour seal Phoca vitulina in the Wadden Sea. Proc. Znt. Wadden Sea Symp., 7, Ameland, 1990. Netherlands Inst. Sea Res. Publ. Ser., 20, 193-7.

Reijnders, P. J. H., Drescher, H. E., van Haaften, J. L., Bogebjerg Hansen, E. and Tougaard, S. (198 1). Population dynamics of the harbour seal in the Wadden Sea. In Marine Mammals of the Wadden Sea, ed. P. J. H. Reijnders and W. J. Wolff, pp. 19-32. Balkema, Rotterdam.

Reijnders, P., Brasseur, S., van der Toorn, J., van der Wolf, P., Boyd, I., Harwood, J., Lavigne, D. and Lowry, L. (1993) Seals, Fur Seals, Sea Lions and Walrus. Status Survey and Conservation Action Plan. IUCN/SSC Seal Specialist Group, IUCN, Gland.

Ries, E. H. (1993) Monitoring the activity patterns of free- ranging harbor seals (Phoca vitulina) by means of VHF telemetry. Wadden Sea Newsl. 3, 1 l-14.

Ries, E. H. and Traut, I. M. (1993) Diving patterns of harbour seals Phoca vitulina in the shallow Wadden Sea assessed by VHF-telemetry. Proc. Biennia! Conference on the Biolology of Marine Mammals, 10, Galveston, Texas, p. 90.

Schwarz, J. and Heidemann, G. (1994) Zum Status der Be- stande der Seehund und Kegelrobbenpopulationen im Wat- tenmeer. In Warnsignale aus dem Wattenmeer, ed. J. L. Lozan, E. Rachor, K. Reise, H. v. Westernhagen and W. Lenz, pp. 296302. Blackwell, Berlin.

Skov, H., Durinck, J., Leopold, M. F. and Offringa, H. (1994) Habitats at sea: Action preparatory to the establishment of a protected areas network in the southeastern North Sea and the southern Baltic. European Union Directorate General XI Action Environment contract no. 445-45, Final Report.

Tougaard, S. (1989) Monitoring harbour seals (Phoca vitulina) in the Danish Wadden Sea. Helgoldnd. Meeresunters. 43, 347-356.