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2001 Northern Gannets found dead in The Netherlands 15
NORTHERN GANNETS MORUS BASSANUS FOUND
DEAD IN THE NETHERLANDS, 1970-2000
KEES (C.J.) CAMPHUYSEN1,2,3
Camphuysen C.J. 2001. Northern Gannets Morus bassanus found dead in The
Netherlands, 1970-2000. Atlantic Seabirds 3(1): 15-28: Northern Gannets are passage
migrants in Dutch coastal waters. Ship-based seabird surveys revealed that Northern
Gannets occur year-round in the Southern North Sea, a finding that has been
supported by beached bird surveys. Most of the beached Northern Gannets in The
Netherlands were either oiled, or entangled in fishing gear. Typical patterns of oiling
(birds seem to hit the oil mainly during take-off) and types of ropes and fishing gear
responsible for most Northern Gannet deaths are described. Oil rates in beached
Northern Gannets declined significantly over time, but are still very high (79% in
adults and immatures, 47% in juveniles). On the contrary, the frequency of entangled
Northern Gannets increased significantly recent years (1977-89 5%, 1990s 7.5%). In
the 1980s, most were entangled in fishnets or in various types of ropes and nylon fibres
from trawlers. In the 1990s most entangled Northern Gannets were killed in nylon fish
line, normally used by sports anglers. Approximately 450 Northern Gannets are
estimated to wash ashore annually. Relatively few juvenile Northern Gannets have
been found and the shift in age distribution through the year reflects the age
composition of Northern Gannets in the Southern Bight. The Atlantic breeding
population has increased during most of the 20th century and in accordance with that,
numbers of Northern Gannets recorded during seawatching have increased over the
past 30 years. These trends are not reflected in Northern Gannet strandings, the
frequency of which seems to have declined markedly after the late 1940s and have been
stable over the last 30 years.
1Dutch Seabird Group, working group Beached Bird Surveys (NZG/NSO), Ankerstraat
20, 1794 BJ Oosterend, Texel, The Netherlands, kees.camphuysen@wxs.nl 2CSR
Consultancy, Oosterend, Texel, 3Netherlands Institute for Sea Research, P.O. Box 59,
1790 AB Den Burg, Texel, The Netherlands
INTRODUCTION
Northern Gannets Morus bassanus are passage migrants in Dutch coastal waters
(Camphuysen 2001). In coastal waters, numbers gradually increase after March
and Northern Gannets reach peak abundance in autumn (Sep-Nov; Camphuysen
& Van Dijk 1983; Platteeuw et al. 1994; Camphuysen 2001). From ship-based
seabird surveys it was demonstrated that Northern Gannets occur year-round in
the Southern Bight, but in variable numbers, with peaks in February/March and
in August-November (Camphuysen & Leopold 1994; Stone et al. 1995).
16 C.J. CAMPHUYSEN Atlantic Seabirds 3(1)
Most of the world population of Northern Gannets nests in Europe
(Tasker 1994) and it may be assumed that substantial numbers migrate through
the Southern Bight towards wintering areas in the Bay of Biscay and along the
51.5N
52.0N
52.5N
53.0N
53.5N
4.0E 5.0E 6.0E
Northern Gannets, 1970-2000
0.05+
0.03 - 0.05
0.01 - 0.03
0.005 - 0.01
less than 0.005
none
1
2
3
4
5
6
7
8
9
27
10
11
12
13 14
15
16
17 18
19
20
21
22
23
24
25 26
NORTH SEA
IJsselmeer
Groningen
Friesland
Noord-
Holland
Zuid-
Holland
Zeeland
BELGIUM
THE NETHERLANDS
SOUTHERN
BIGHT
Delta area
Mainland coast
Wadden Sea isles
*
Hondsbossche
Zeewering
Figure 1. Study area, 27 subregions along the North Sea coast and in the Wadden
Sea area (see Table 1 for site names), as well as the location of the constant
effort site Hondsbossche Zeewering (* near-daily counts 1988-2000) and
average densities of Northern Gannets based on beached bird surveys 1970-
2000.
Figuur 1. Studiegebied, 27 deelgebieden langs de Noordzeekust en in het
Waddengebied (zie Tabel 1), met daarbij de ligging van de Hondsbossche
Zeewering (*) waar tussen 1988 en 2000 vrijwel dagelijks werd geteld en
gemiddelde dichtheden Jan-van-genten op basis van stookolieslachtoffer-
tellingen 1970-2000.
2001 Northern Gannets found dead in The Netherlands 17
African west coast (Cramp & Simmons 1977). It is therefore of interest to
monitor the well being of these Northern Gannets and to identify the scale and
any trends in mortality patterns and causes of death, for example by means of
beached bird surveys (BBS). This paper reviews the results of 30 years of
systematic beached bird surveys in The Netherlands.
METHODS
This analysis includes BBS results collected between 1970 and 2000 on all
Dutch North Sea coasts and in the Wadden Sea (Fig. 1). BBS are conducted on
foot, walking the tide-line with one or more people, recording and describing
every corpse. Standard notes include details on the presence of oil on the
feathers, or other obvious causes of death such as entanglements or injury, and
the state of the corpse (fresh, old, very old, complete or disintegrated; cf.
Camphuysen & Heubeck 2001). To avoid double counts during subsequent
surveys, corpses were marked by clipping the primaries of the wings.
Twenty-seven subregions were used, 15 along the North Sea, 12 within
the Wadden Sea. Within those subregions, data were collected on a smaller
scale (108 standard, numbered, stretches of variable length, mean ± SD length
6.5 ± 3.7 km, max 18 km length). Total coast length was 706 km, of which 381
km (54%) was along the North Sea shore. BBS were organised so that as many
coastal stretches as possible were visited at least monthly. Regional co-
ordinators were responsible for the precise planning of surveys, which were
mainly organised between November and April and with lower frequency in the
rest of the year. Results were usually presented as 'densities' (n found per km
surveyed; n km-1). The data comes from 6533 surveys, of which 5126 were
conducted in winter (Nov-Apr), 1407 in summer (May-Oct; Table 1), and of
which 4732 were performed along the North Sea coast and 1801 in the Wadden
Sea (Table 2). The latter area was not adequately surveyed between 1974 and
1979.
Part of the Noord-Holland mainland coast, an 8 km long stretch of dike
and sandy beach between Camperduin and St Maartenszee, has been surveyed
on a near-daily basis since 1988. This 'constant-effort-site' was used to estimate
the daily stranding rate of corpses km-1 and from that the total number of
Northern Gannets washing ashore per year.
Northern Gannets were aged by using plumage characteristics and
grouped in six (plumage 1-5, and adults) or three categories (first year birds,
immatures and adults). Plumage types 1-5 were taken from Fig. 2 in Nelson
(1978). Of the nine immature stages depicted there, type 1 was represented by
the first, type 2 by the second, fourth and fifth, type 3 by the third, sixth and
18 C.J. CAMPHUYSEN Atlantic Seabirds 3(1)
Table 1. Subregions used for the analysis of beached bird surveys (1-27, see Fig.
1), their location along the North Sea coast (S) or in the Wadden Sea area
(W), the number of standard stretches (n = 108), total coast length in km
(total 706 km), and number of surveys (counts) and coverage (km) in winter
(Nov-Apr) and summer (May-Oct) surveys since 1969.
Table 1. Deelgebieden gebruikt voor de analyse van olieslachtoffertellingen (1-27,
zie Fig. 1), hun ligging langs de Noordzeekust (S) of in het Waddengebied
(W), het aantal standaard-trajecten (stretches; n = 108), de kustlengte per
deelgebied in km (totaal 706 km), en het aantal sinds 1969 uitgevoerde
tellingen (counts) en de daarbij afgelegde afstand (km) in winter (nov-apr)
en zomer (mei-okt).
winter summer
# subregion stretches km counts km counts km
1 Zeeuws Vlaanderen S 1 14 25 192 1 4
2 Walcheren S 6 37 393 2168 53 322
3 Schouwen S 4 24 420 3225 18 112
4 Goeree S 3 16 118 988 7 36
5 Voorne-Maasvlakte S 4 25 176 1151 24 178
6 Zuid-Holland S 5 36 313 2523 48 312
7 Noord-Holland Z S 4 26 421 3241 37 300
8 Noord-Holland M S 6 29 481 2842 69 328
9 Noord-Holland N S 5 31 602 4623 250 1741
10 Texel strand S 6 32 482 2402 104 365
11 Vlieland strand S 5 29 120 862 80 401
12 Terschelling strand S 4 27 86 1064 33 284
13 Ameland strand S 4 27 154 1438 76 373
14 Schiermonnikoog strand S 3 19 102 618 32 153
15 Rottum S 2 9 3 11 4 29
16 Texel wad W 5 25 140 626 32 189
17 Vlieland wad W 5 12 60 219 18 70
18 Griend W 1 6 15 61 16 86
19 Terschelling wad W 5 34 18 88 2 14
20 Ameland wad W 4 22 22 98 8 39
21 Schiermonnikoog wad W 4 20 50 377 14 69
22 Balgzand W 4 23 35 452 11 64
23 Afsluitdijk W 3 31 200 1851 104 1218
24 Friese kust W W 4 38 402 3673 303 2699
25 Friese kust O W 4 34 198 1327 47 254
26 Groninge kust W W 3 42 82 864 13 98
27 Groningse kust O W 4 38 8 67 3 14
108 706 5126 37050 1407 9747
2001 Northern Gannets found dead in The Netherlands 19
Table 2. Annual observer effort (number of counts and total km surveyed) in beached
bird surveys along the North Sea coast or in the Wadden Sea area, 1970-2000.
Table 2. Jaarlijkse waarnemingsinspanning (aantal tellingen en totaal afgelegde afstand
in km) tijdens olieslachtoffertellingen langs het Noordzeestrand en in het
Waddengebied, 1970-2000.
North Sea coast Wadden Sea coast
counts km surveyed counts km surveyed
1970 29 492 13 148
1971 23 508 4 152
1972 17 277 2 114
1973 21 337 11 164
1974 31 181
1975 48 312
1976 31 263
1977 35 335
1978 79 593 1 7
1979 141 886 2 5
1980 178 1185 6 52
1981 372 2489 92 915
1982 256 1645 124 938
1983 385 3014 107 978
1984 285 2046 106 937
1985 304 1849 76 538
1986 154 1074 74 477
1987 150 1120 99 762
1988 227 1737 69 593
1989 198 1279 63 473
1990 232 1484 47 307
1991 218 1406 52 432
1992 152 1053 48 497
1993 124 842 65 659
1994 103 625 69 615
1995 120 702 83 697
1996 113 693 56 472
1997 107 642 67 653
1998 158 843 103 730
1999 225 1234 156 1084
2000 216 1139 206 1118
Totals 4732 32284 1801 14514
sample 31 25*
*1978 and 1979 not recognised as adequate samples
20 C.J. CAMPHUYSEN Atlantic Seabirds 3(1)
seventh, type 4 by the eighth and type 5 by the ninth. Ageing was standardised
only in 1977 and all detailed data on the age of Northern Gannets were derived
from surveys since 1977. The age composition was compared with that found
during seawatching in The Netherlands (1972-93; Camphuysen & Van Dijk
1983; Platteeuw et al. 1994; and NZG/CvZ unpubl. data) and with data
collected during ship-based seabird surveys at sea in Dutch waters (51-56°N, 2-
8°E; Camphuysen & Leopold 1994; ESAS database, unpubl. data).
The most comprehensive data were collected in winter (Nov-Apr).
Trends in annual winter oil rates (fraction of birds oil contaminated of all birds
found) were estimated after logit-transformation by fitting a linear trend on
annual oil rates by least-squares estimation (methods follow Camphuysen 1995,
1997). The significance of r was assessed after converting it to a t value (Fowler
& Cohen 1986). Annual oil rates were calculated only if at least 10 complete,
relatively fresh corpses were found of which the oiling was recorded (29 out of
31 winter seasons since 1969). Trends in the frequency of entanglements were
treated similarly.
RESULTS
Review of strandings, 1970-2000 Between January 1970 and December 2000,
1413 dead Northern Gannets were found. Of all Northern Gannets, 50.4% were
found as 'fresh' or 'rather fresh' corpses, 38.0% were recorded as being 'old' and
8.0% were 'very old'. These Northern Gannets were all intact and suitable for
examination of oiling. In addition, 3.6% were described as incomplete corpses
(often just pairs of wings). Annual fluctuations in densities are minor (mean ±
SD 0.033 ± 0.012 km-1, range 0.016-0.066 km-1, n = 31; Fig. 2) and there have
not been any mass strandings of Northern Gannets in The Netherlands in this
study period. At constant effort site Hondsbossche Zeewering, 93 Northern
Gannets were found between 1988 and 2000, a daily rate of 0.003 km-1, or
approximately one km-1 year-1.
Seasonal pattern and age composition Northern Gannets wash ashore year-
round, but with a slightly higher density between November and January (Fig.
3). Between January and May, over 80% of the Northern Gannets found dead
were in adult plumage (86.3% adult, n = 606 aged individuals). Immatures
increased proportionally in June, peaked in July and August and gradually
declined again during autumn. Juveniles (i.e. black-headed, first year
individuals) were most commonly found between September and November
(17.5%, n = 189) and were near absent in late spring and early summer (Apr-Jun
2.1%, n = 97; Fig. 4). Of 158 immatures aged in further detail, 44.3% were
plumage type 2, 33.5% were type 3, 19.0% were type 4, and 3.2% were type 5.
2001 Northern Gannets found dead in The Netherlands 21
Spatial pattern Along the North Sea coast, in 31 years with adequate survey
results (Table 2), the mean annual density (± SD) of Northern Gannets
amounted to 0.042 ± 0.014 (min 0.023, max 0.087) km-1. Average densities
were highest in the north-west of the country (westernmost Wadden Sea islands
and northern half of the mainland coast), but overall the differences are rather
small and this species may be considered evenly distributed over most of the
Dutch North Sea coastline (Fig. 1; Table 3). In the Wadden Sea area, in 25 years
0,00
0,01
0,02
0,03
0,04
0,05
0,06
0,07
1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999
Year
N/km
Figure 2. Annual fluctuations in densities of beached Northern Gannets, all subregions
combined.
Figuur 2. Jaarlijkse fluctuaties in dichtheden gestrande Jan-van-genten op de
Nederlandse kust, alle deelgebieden gecombineerd.
0,00
0,01
0,02
0,03
0,04
0,05
0,06
JASONDJFMAMJ
Figure 3. Seasonal pattern in densities (monthly average, n km-1 ± SE) of beached
Northern Gannets, all subregions combined.
Figuur 3. Seizoenspatroon in dichtheden (maandgemiddelde, n km-1 ± SE) gestrande
Jan-van-genten op de Nederlandse kust, alle deelgebieden gecombineerd.
22 C.J. CAMPHUYSEN Atlantic Seabirds 3(1)
with sufficient data (Table 2), the mean annual density was 3.86x lower with
0.011 ± 0.014 (min 0.0, max 0.061) km-1 (Table 3).
Tabel 3. Regional densities of Northern Gannets in winter and summer, 1970-2000.
Tabel 3. Regionale verschillen in dichtheden Jan-van-genten in zomer en winter, 1970-
2000.
Winter Summer
Subregions km Gannets n km-1 km Gannets n/km-1
S Delta area 1-3 5584 199 0.04 438 7 0.02
N Delta area 4-5 2140 58 0.03 213 7 0.03
S mainland coast 6-7 5764 183 0.03 612 25 0.04
N mainland coast 8-9 7465 326 0.04 2069 91 0.04
W Wadden Sea isles 10-12 4329 238 0.05 1049 62 0.06
E Wadden Sea isles 13-15 2066 65 0.03 555 18 0.03
W Wadden Sea area 16-19, 22-25 8297 81 0.01 4592 34 0.01
E Wadden Sea area 20-21, 26-27 1405 16 0.01 219 2 0.01
Totals 37050 1166 9747 246
Oiling A characteristic pattern of oiling found most frequently in Northern
Gannets, is that both wing-tips, the tail and the belly between the feet are
heavily oiled, whereas the rest of the bird is more or less clean. This pattern is
interpreted as representing the effect of a take-off (running and hitting the water
with the wings in an attempt to fly) when facing an 'approaching' oil-slick.
Northern Gannets take wing into the wind and oil driven by wind towards
swimming birds therefore poses a great risk even in case of an attempted escape.
For adults (1970-2000, 79.5% oiled, n = 694) and immatures (79.4%, n
= 126), oil rates are similar. In juveniles, however, overall oil rates are
significantly lower (46.5%, n = 43; Gadj = 21.0, df = 2, P< 0.001). Oil rates in
Northern Gannets found dead in The Netherlands have declined gradually, but
highly significantly, since the early 1970s (t = 5.17, df = 27, P < 0.001; Fig. 2).
In recent years, just over half of all Northern Gannets found were somehow
oiled or contaminated with other lipophilic substances, whereas nearly all birds
found in the early 1970s were oil-fouled.
Other substances causing the death of Northern Gannets included
dodecylphenol (March 1990, one individual), Apron-plus (toxic pesticide;
January 1994, 1), and polyisobutylene (December 1998, 4). Some Northern
Gannets were captured and treated in rehabilitation centres in November 1987
(not included in BBS database) and these were covered in a sticky substance
that was found to contain 90% linseed oil (Engelen 1987a).
2001 Northern Gannets found dead in The Netherlands 23
Entanglements in fishing gear A total of 83 Northern Gannets were found that
were entangled (5.8%, n = 1431). Prior to 1977, it is not certain that all
entangled Northern Gannets were reported as such and these data were therefore
omitted from the trend analysis. The fraction of entangled Northern Gannets has
increased significantly since 1977 (t = 3.85, df = 22, P < 0.01; Fig. 6). Between
1977 and 1989, 5.2% of all Northern Gannets were entangled (n = 692) and
most were entangled in fishing nets (61.1%, n = 36) or in various heavy ropes or
nylon fibres from beamtrawler nets (31.8%; Table 4). In the 1990s, the
incidence of entanglements increased to 7.5% al all Northern Gannets found
dead (n = 600), but half of these were killed in nylon sports anglers fish line
(often with hook and/or sinker still attached; 51.1%, n = 45). The proportion of
Northern Gannets entangled in fishing nets declined from 3.2% to 1.5% of all
birds found dead, whereas that of Northern Gannets in nylon lines increased
from only 0.3% to 3.8%. Eight Northern Gannets found dead had a broken wing
(4), missing wings (or part of wings, 3) or a broken mandible (1), injuries that
were most likely caused when they were pulled out of a net on a fishing trawler.
DISCUSSION
In the first report of beached oiled seabirds in the Netherlands, in 1915, of 18
oiled seabirds found dead, six were Northern Gannets (Verwey 1915). A much
lower proportion has been found in most anecdotal reports and subsequent
systematic surveys conducted since, but Camphuysen (1989) identified the
Entangled Gannet
24 C.J. CAMPHUYSEN Atlantic Seabirds 3(1)
Northern Gannet as being highly vulnerable to oil pollution, given that 86.5% of
561 individuals found dead between 1969 and 1985 were oil-fouled. In this and
in later publications (Camphuysen 1990a, 1994), the incidence of entanglements
in fishing gear in Northern Gannets was highlighted as an additional (unnatural)
cause of death in Dutch waters.
There is no other species in The Netherlands which shows so little
variation in numbers washing ashore, both between years (Fig. 2) and within
one calendar year (Fig. 3). No mass strandings of Northern Gannets have been
recorded on the BBS, either in particularly harsh, or mild winters, in
exceptionally stormy seasons, or in oil pollution incidents. Yet, they were
represented in virtually all local oil pollution incidents, but in relatively small
numbers (e.g. Swennen & Spaans 1970; Engelen 1987ab; Camphuysen et al.
1988; Camphuysen 1989; Leopold & Camphuysen 1992; Camphuysen 1995,
1997; Camphuysen et al. 1999). When extrapolating the results of constant
effort site Hondsbossche Zeewering over the entire 381 km long North Sea
coast, approximately 360 Northern Gannets have washed ashore annually in the
1990s. Considerably lower densities in the Wadden Sea (see below) would
suggest that another 80 Northern Gannets may wash ashore on the 325 km long
Dutch Wadden Sea shores. These figures are not unrealistic given the annual
coverage in beached bird surveys and the c. 53.4 ± 19.6 Northern Gannets
actually found dead each year since 1988.
Estimates of total numbers of Northern Gannets in the Dutch sector of
the North Sea based on densities derived from ship-based surveys ranged from a
minimum of 3700 birds December-January, via 18 800 in February-March,
7200 in April-May and 5040 in June-July, to peak numbers in autumn with
16 700 in August-September and 28 700 in October-November (Camphuysen &
Leopold 1994). In October-November, 4.0% of the East Atlantic breeding
population may be found in Dutch waters (cf. Lloyd et al. 1991). These
estimates are in fact misleading, because the turnover (caused by migratory
movements through this sea area) is substantial, albeit very difficult to quantify.
Strandings of some 450 Northern Gannets on an annual basis are clearly no
reason for immediate worry, even if not all corpses of Northern Gannets dying
at sea are likely to wash ashore.
The seasonal pattern of stranding (n km-1) is clearly not representative
of seasonal changes in relative abundance at sea (Camphuysen & Leopold
1994). The age structure of beached birds, however, although with an
understandable one month delay, mirrors the offshore situation quite precisely
(Fig. 4). So, the relative abundance of each of the age-classes in beached birds
seems to be a direct result of differences in the relative abundance of different
age groups at sea. A closer look shows that there are subtle differences. Because
of the greater frequency of strandings in winter than in summer, a relatively
2001 Northern Gannets found dead in The Netherlands 25
greater proportion of Northern Gannets found dead were adult (71.8%, n =
1094) than could be expected from year-round abundance at sea (51.8%, n =
9624). In summer, however, the proportion of adults beached (10.3%, n = 204)
was lower than expected on the basis of ship-based surveys (28.1%, n = 4447).
In winter, both the proportion of adults (78.0%) as well as the proportion of
juveniles (6.5%, n = 890) were significantly higher than in ship-based surveys
(adults 72.3%, juveniles 4.5%, n = 5177; Gadj 31.3, df = 2, P < 0.001).
Differences in the relative abundance of each of the immature stages are
difficult to explain, but could be due to different skills of observers in either
scheme. During ship-based surveys, of 3255 aged immatures, 54.2% were
plumage type 2, 21.7% were type 3, 14.8% were type 4, and 9.3% were type 5.
Particularly the latter type may be easily overlooked at sea (mis-identified as
adults), but this fraction is distinctly greater than in beached bird surveys, where
a single remaining black feather would be noticed immediately.
Only 3.6% of the Northern Gannets were found as incomplete corpses
('wings'). This percentage is considerably lower than for example in other
pelagic seabirds such as Northern Fulmars Fulmarus glacialis (10.8%, n =
4557), Great Black-backed Gulls Larus marinus (21.0%, n = 2300) or Black-
legged Kittiwakes Rissa tridactyla (24.6%, n = 10 571), with a similar
distribution in the Southern Bight (Camphuysen & Leopold 1994). However,
this percentage is in accordance with a negative relationship between body mass
(g) and the fraction of corpses recorded as wings (fraction as wings = -0.134Ln
(body mass, g) + 1.1097; r² = 0.48; calculated over 81 species; NZG/NSO
unpubl. data): large birds tend to be found whole.
The oil-rates in Northern Gannets in The Netherlands are still very
high, indicating that these aerial seabirds are perhaps more vulnerable to oil
pollution than generally assumed. A recent study of the activity budget of
Northern Gannets in the breeding season showed that these birds spend
considerable time swimming at sea (Garthe et al. 1999). This, combined with
their heavy weight and difficulty in taking flight, could pose the greatest risk for
these birds to get in contact with oil, even if their escape behaviour would be to
fly off near an approaching oil slick. The age dependent differences in oil rates
indicate differences in the risk to become oiled for juvenile versus immature and
adult (combined) birds. This must be a different risk due to age-specific
behavioural differences at sea or age-specific mortality rates and therefore
points to pre-mortal oiling only.
In comparison with historical beached bird surveys in The Netherlands,
Northern Gannets seem to have declined. Brouwer (1953), reviewing
observations from the first half of the 20th century, suggested that 7% of all birds
killed by oil along the Dutch coast were Northern Gannets (1970-2000 only
0.7% of all birds found dead, n = 192 450, 1.5% of all intact corpses reported as
26 C.J. CAMPHUYSEN Atlantic Seabirds 3(1)
oiled, n = 63 271; NZG/NSO unpubl. data). Mörzer Bruijns (1959) reported on
the results of beached bird surveys in The Netherlands between 1948 and 1958
(1018 km surveyed, year-round programme) and found 133 oiled Northern
Gannets (4.0% of all birds found). His overall density (0.13 km-1) was four
times higher than our present figure and even more than twice the highest
density found in any one year since 1970. In a follow-up study, numbers seemed
to have declined quite markedly (1958-62 0.031 km-1), in fact to similar levels
as reported in the present study, and Northern Gannets formed only 1.5% of all
birds reported as oil victims (Tanis & Mörzer Bruijns 1962). Although the
evidence has a slender factual basis (relatively few data compared to the present
day situation), these publications suggest that Northern Gannets have declined
as beached birds both in relative terms and in absolute terms. This trend is
completely opposite to the marked population increase over most of the last
century (Nelson 1978; Wanless 1987; Lloyd et al. 1991; Tasker 1994) and
suggests a reduced risk for Northern Gannets to fall victim to oil pollution, or a
markedly changed at-sea distribution.
The overall proportion of evidently entangled individuals is higher than
in any other species found in beached bird surveys in The Netherlands and is
increasing (Camphuysen 1990a, 1994). Previous studies have pointed at these
entanglements, that are not only common in the Southern North Sea (Schneider
1991; Hartwig et al. 1992), but also known from breeding colonies
(Montevecchi 1991; Camphuysen 1990b) and winterquarters (Leopold 1993).
Northern Gannets are probably the most vulnerable seabirds to get trapped in
trawls, due to their habit to hammer into gear hauled into the boat. Most trapped
Northern Gannets will be pulled out of the net by fishermen and be thrown into
the sea, without obvious indications of the cause of death visible on the corpse.
Yet, the available evidence points at a recent reduction in both the relative and
in the absolute importance of deaths in trawl nets, whereas the contrary was
observed in entanglements in nylon lines and hooks (sports anglers fishing
gear). Future surveys will have to prove whether these trends are consolidated.
ACKNOWLEDGEMENTS
Hundreds of volunteers have participated in beached bird surveys in The Netherlands and my
warmest thanks are in the first place to all those (listed in Camphuysen 1989, 1995 and 1997). Ruud
Costers, Arnold and Rineke Gronert have conducted near-daily surveys in constant effort site
Hondsbossche Zeewering since 1988. Danish, German, British, Belgian and Dutch partners in the
European Seabirds at Sea database are thanked for permission to use data on the age of Northern
Gannets at sea, the Dutch Seabird Group, working group CvZ is thanked for permission to use data
on the age of Northern Gannets migrating through Dutch coastal waters. NZG/NSO is financially
supported by the North Sea Directorate of the Ministry of Transport, Public Works and Waterways.
Martin Heubeck and Mardik Leopold kindly commented on a draft version of this contribution.
2001 Northern Gannets found dead in The Netherlands 27
VONDSTEN VAN JAN-VAN-GENTEN LANGS DE NEDERLANDSE KUST, 1970-2000
Jan-van-genten zijn doortrekkers in Nederlandse kustwateren, waarvan de grootste aantallen tijdens
zeetrektellingen in de herfst (september-oktober) worden gezien. Tellingen vanaf schepen wezen uit
dat Jan-van-genten gedurende het gehele jaar in de Zuidelijke Bocht voorkomen, een bevinding die
bevestigd werd door de resultaten van olieslachtoffertellingen op de Nederlandse kust. Van de in
Nederland aangespoelde Jan-van-genten was een groot deel of met olie besmeurd, of in vistuig
verstrikt geraakt. Een karakteristiek patroon van oliebesmeuring bij de Jan-van-gent, olie aan de
vleugelpunten, tussen de poten en aan de staart (de vogels komen kennelijk vaak tijdens het
opvliegen met olie in contact en 'rennen' als het ware door de olie) wordt beschreven. De meest
voorkomende vormen van vistuig waarin Jan-van-genten verstrikt raken zijn allerlei visnetten en
nylon vislijnen van sportvissers. De oliebevuilingspercentages bij de Jan-van-genten (jaarlijkse
fractie met olie besmeurde exemplaren, gemeten op grond van tenminste 10 complete kadavers in
het winterhalfjaar) vertonen een gestage afname in de tijd, maar zijn nog steeds relatief hoog (79%
bij adulte en onvolwassen exemplaren, 47% bij juvenielen). Daar staat tegenover dat tegenwoordig
steeds vaker in vistuig verstrikte Jan-van-genten worden gevonden (1977-89 5%, 1990-99 7.5%).
Zowel de afname van het percentage olieslachtoffers als de toename van het percentage
verstrikkingsslachtoffers zijn significant. In de jaren tachtig vielen de meeste verstrikkings-
slachtoffers in allerlei touwen en visnetten. Sinds 1990 worden de meeste verstrikte Jan-van-genten
echter in vislijnen van sportvissers aangetroffen, vaak met de haken en het gewicht er nog aan. In
totaal spoelen er jaarlijks in Nederland ongeveer 450 Jan-van-genten aan. Hieronder zijn
betrekkelijk weinig juveniele exemplaren (die een hogere jaarlijkse sterfte hebben dan oudere
vogels) en de maandelijkse veranderingen in de leeftijdsopbouw van gestrande vogels zijn een
nauwkeurige afspiegeling van de leeftijdsopbouw die bij de Jan-van-genten tijdens tellingen vanaf
schepen op zee wordt gevonden. De Noord-Atlantische populatie van de Jan-van-gent is in de 20e
eeuw enorm toegenomen en de soort vestigde zich in tal van nieuwe kolonies. Zeetrekwaarnemers
zien dan ook meer en meer Jan-van-genten langs de Nederlandse kust langstrekken. Op het strand
wordt deze trend echter niet gevonden. Na een opvallende afname in het aantal aangespoelde Jan-
van-genten sinds de jaren veertig is het aantal strandingen opmerkelijk stabiel gebleven.
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