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Iriternational Couricil for
the Exploration of the Sea
Shellfish Committee
CM. 1995/K: 44
Aerial surveys, biomass estimates, and elimination of the musseI popuiation
(Myti/us edulis L.), in the Danish Wadden Sea, 1991-1994.
by
Per Sand Kristerisen
Danish Institute for Fisheries Research, Charlottenlund Castle, DK-2920 Charlottenlund.
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Abstract
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In 1991 and 1993, the Porest änd Nature Agericy and Danish Institute for Fisheries
Research, perfonneci aeroplane photo survey cf all the gullies in the Danish Wadden Sea,
to identify areas with rllUssels (M. edulis). The main areas with musseIs were fciund in Ho
Bight in the northern part of the Danish Wadden Sea~ From 1991 to 1993 a decrease of mtissel
bed areas were obSeivect. The total area with rllUssel bects registered in 1991 in the Wadden Sea
was 10.92km2 , arid in 1993 only 8.67 krit2 • The subtidal areas increased from 1.8 km2 in
1991, to 2.5 km2 in 1993, and the intertidai area decreased from 9.1 km2 in"1991, to 6.2 km2
iri 1993. .
To verify the aeroplane observation, simples of musseIs were collectoo in the beds in
1991, 1992 and 1993. The densities (nci./m2) of musseIs in Ho Bight varied considerably
thl-oughout the investigation between 0 and 3 800 mussels/m2, and the biomass varieci between
okg and 63.9 kg per m2• The biomass in the sampIes was in 1991 in average 7,1 ±6,0 kg per
m2 (mean +2 SE; n ,14). The biomass in the sampIes in 1992 was in average 17,0 ±10,3 kg
per m2 (mean +2 SE) and in 1993 in average 19,5 ± 7,8 kg per m2 (meaß ±2 SE; n=32).
The total biomass of musseIs in the Dänish Wadden Sea increased over seven years
from 15500 metric tonnes in 1988, to 90 000 me~c tonnes in September 1993. Iri September
1994 the standing stock (subtidal and iritertidal) iri the Danish Wadden Sea was 117 000 metric
tonnes.
In the period 1991-1994, 4 628 metrlc tonneS of musseIs (~ 5 cin in shelllength) were
fished anilUally in the Danish Wadden Sea~.
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Eiders, herring gulls änd oystercatchers in the Danish Wadden Sea feect primarilyon
musSeIs and rriay consume up to 16000 metric tonnes of musseIs annually (equal to ;,,'17 * 10&
bird days)~
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The elimiriaticin of musseIs (Z=M+ F) from 1991 to 1993 is estimated to be as high
. as 60 000 metric tonnes, or 20000 memc tonnes annually. 3/4 consuITloo by the birds (M) and
1/4 catched by the fishermen (F).
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Introduction.
In 1961-65, Theisen (1968) performed an experimenW traIlsplantatIon of musseis in
the Danish Wadden Sea, similar to what has been undertakeri in the Dutch and German
Wadden Sea for decades. His investigations did not result in culture of musseis in the Danish
Wadderi Sea. So, today transphmtation arid culture of musseis or other marine bivalve species
is prohibited according to the legislation for protection of the Danish Wa.dden Sea. Only fishing on the riatural stockS of mussels may take place in the Dariish Wadden Sea. .
The Danish \Vadden Sea haS been 'exploited by oommercial fishirig of musseis (Mytilus
edulis L.) for the last 16 years (Fig. 1). The landings from 1979-1994 varied annually between
a few hundred tennes (1979-1982) to more than 27000 metric tonnes in 1985. Unefficial
figures for the landings in 1985 iridicated values of 75000 metric toimes (Krlsterisen, i994,
information given by mussei fishermen). A total of 90 000 metric torines of musseis were
larided from the Danish Wadden Sea between 1983 and 1987.
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,The intensive fishery of musseis, and the destruction of a large number of intertidal
mussei beds during the ice winter 1986/87 resulted in a collapse of the mussei stock in the
Danish Wadden Sea in spring 1987. During the summer and.autu.mn 1987 a large spat-fall took
place iri the Ho Bight (Mtinch-Petersen and Kristensen, 1987 andMunksgaard, 1989). The 87cohort ofmussels in the subtidal bects had reached the fish~ng size in the summer 1990 (shell
length ~ 5 cm; arid > 90% of the musseis suitable for fishirig)~
The fishery of musseis in the Danish Wadden Sea hasali the years beeri secondary
compäred to the main fishing area for iriusse~s in Denmark, Limfjorderi, where the annual
laridings the last five years have beeri around 100 000 metric tonnes (Kristerisen, 1994).
The mussei stock in the Danish Wadden Sea was assessed for the first time in the
autuIrin 1986 (Munch-Petersen and Kristensen, 1987), followiiig the comprehensive fishery in
the years 1983-1986 (Fig. 1). The areas with mussei beds were identified by use of aerial
photos (black and white pictures). Since the photos were not taken .w~th the purj>ose to distinguish mussel beds, problems with the interpretation arose (Munch-Peü~rsen and Kristensen;
1987 arid Munksgaard; 1989). .
, In 1988, the Darush Institute for Fisheries and Marine Research irivestigated the use
of aerial photos iri coleurs io. quantlfy areas with inteItidal musseI beds in the Danish Wadderi
Sea (Munch-Petersenand Kristensen, 1989)~ The pictüres were" analysis, and the biomass
. (kg/m2) of musseis within the mussei beds eStlmatect. There was a weak correlation between
the grey tone vä!ues, and the biomass of musseis in the investigated area. However, use.of
aerial photos in colour to detect the mUssei bects and their size was more useful, than the aerial
photos in black and white used in the assessment in 1986 (Murich-Petersen and Kristensen,
19~9).
In the alltumn 1990 it was decided to peiforril an investigatiori of the mussei stock,
and to ascertain the impact on the musseI stock from mussel fishing in Ho Bight (iIjeIting
Stream only). The purpose waS to fuld the siZe of the ßmssel beds by aenal photo surveys, arid
to assess the biomass of musseis in the nmsseI beds. Secondly, to evaluate the annual
production (P) of musseIs in the Danish Wadden Sea, for which figures from the literature
(Asmus, 1987) was üsed. The estimate was based on the estimated stariding stock (B) in this
investigation. For the third, to evaluate the importance of F (the fishing inortality) and M (the
bird predation, assumed to be 50% of the totaI mörtaIity) for declineslincreases in the standing
stock of musseis in the Danish Wadden Sea~
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l\laterial and methods.
In Ho Bight, with the two tidal gullies Hjerting Stream and Hobo Deep several mussei
spats settled in the summer 1987. (1:funch-Petersen.and.Kristensen, .1987 and Munksgaard,
1989). .
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In Hjerting Stream, the. rilUssel. areas were divided into three seetors with a total
biomass ofmussels ofaround 15700 rrietrie tonnes in the May.1991 (Tab. 3). The three areas
were different in extent, and in size of the mussei stoekS (Fig.2 and .Tab. 3).
Seetor 1:
Seetor 2:
Seetor 3:
3.1.
Fishing: 1. Oetober - 31. April eaeh year•.
Fishing: 15. July - 1. Oetober each year.
". . .Fishing: No fishirig
Estimating the size of the musseI beds.
All gullies in the Danish.Wadden Sea were photographed in 1991 and in 1993. The
intertidal mussei beds, which were farthest away from the gullies, were not photographed, because they are of no interest in the management of the fishery of musseis.
The aeroplane photo survey was carried out by a eonsultant company (Jan Kofod
Winther, Luftfoto). Photographing was only successful at low tide, when there were no wind,
no clouds, and only if the sun was high on the sky, whieh only happens few times during the
summer time.
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A Cessna 172 plane was used. The flying altitude was 4 500 feeds (-1 350 m). A
camera model 500 ELX Hasselblad was used (ler:ts: 40 mm, Zeiss, distagon). The films used,
were kodak aeroehrome or kodak ectochrome. Slides were made from the negatives; 56 mm
x 56 mm. Afterwards, paper pictures were made of the slides, 200 mm x 200 mm. . .
An overView picture was eonstrueted by use of all the paper pictures, to form a map
ofthe Wadden Sea , and to distinguish the potential mussei beds. Areas with mussei beds were
distinguished by their eolour (dark areas), from areas with sand (light coloured). Sampling of
musseis on the assumed mussei beds in the feIt was conclusively used to verified the aeroplane
observations.
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Bach aerial photo covered about one minute of are = 1 852 m. Bach sUde or paper
picture therefore coveci 1 852 * 1852 m2 = 3429904 m2 • Both in 1991 and1993 a'building
of specifie size could be distinguish on the paper pietures. From those, the scale in 1991 was
estimated to 1:10000, and in 1993 to 1:9481.
From the map constructed from the paper pictures, the feit verified areas with mussei
beds, were cut out in tracing paper. The pieees representing mussei beds were all weighed on
a Mettler PM 4600 Delta Range weight (accuraey: 0.01 g equal to one heetare (10 000 m2
Two cut pieces of tracing paper (10 * 10 cm2 for the 1991 pictures, and 9.5.. * 9.5 cm2 for the
pictures from 1993) were used to transform one m2 of mussei bed to weight.
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3.2.
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oe musSeIs (no.lrit% and kg/ni~.
. Mtissels were sarnpled from 32 different sampling stations regularly spaced and stratified in the three sectors 1, 2 and 3 (Fig.2). Within a frame of 0.25 ri12 all musseis and shells
of mussels were collected by a diyer. At sampling, the diver judged the area covered with
musseis within the vicinity of the sampling station (- 100 x 100 m) in the intertidal mussei
beds. The sampies were weighed on asteelyard weight (accuracy: 0.25 kg). A raridom
subSampie arOund 3 kg (at least 150 live musseis) was talcen from the gross sampie and frezen
for later analysis. .
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In the laboratory the subsampies were sorted in fractions of live musseis, and shells
of different bivalves and gastroPcids, and invertebrntes. The fraction of live musseIs in the sampleswaS determined~ and used to estimate the density of musseis on the sampling station; in
niIinbers, and the biomass of live wet weight of mussels1m2• The shelliength of the mussels
was measufed in mm. Data analysis was carried out in EXCEL 5.0 and SAS (plot and'GLM. '
routines).
The sampling stations in the subtidal mussel beds were spaced randomly arid repreusoo
serited both areas with, and without musseis, arid the average density of musseis
. directly, to estimate the biomass of musseis in the various beds.
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The density of musseis measured for the musSeI beds in the Hjerting Stream have been
uSed to estimate the stock size in the three other tidal areas iri the Danish Wadden Sea (Knude
Deep, Juvre Deep and Lister Deep).
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3.3
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Density and biomass
Sampling by dredge.
In Septeinber 1994 all musse! beds identified in the 1993 aeroplane photo suivey in
the three tidal areaS in the Danish Wadden Sea were drerlged by a commerdal vessel to collect
sarriples of inussels. The purpose was to determine the biomass (kg/m2) or"the mussel stocks
in the entire Danish Wadden Sea. Secondly, to compare the dredging technique with the diver
sainpling technique used in, the stock sUrVey the previous years in Hjerting Stream. Drooge
sampIes were talcen at 79 different stations (Tab. 6). Fifty two samples were laken in Grey
Deep at Esbjerg, nine in Knude Deep, and twelve in Lister Deep. It was not possible to take
dredge samples in Juvre Deep in September 1994.
The hauUng speed was two knot, and the hauling time varled between 4 and 159
seconds (Tab. 6). The commercial dredge is 1.8 m wide; arid a haul could therefore be
estimated to cover between 7.408 m2 and 294.47 m2• The catch in each haul was jUdgect by the
skipper and the research assistant (if there was disagreement between their tWo judgementS an
average figure was chosen). A subsample was taken from the gross'catch and frozen to be
analySed iri the laboratory. The number oflive musseIs iri the frozensubSample was mcilsured.
By unfreezing, the musseIs loase in 3;verage 20% of their wet weight, which has to be added
to the musseI weight before the fmctiori of live musseis in the catches can be estimated. The
. density (kg) af mussels in total wet weight per m2 was estlmated for each sampIe laken by
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dredge.
. The biomass of the standing stock of musseIs in each tidal area(4) was estimatoo by
rriultiplying the average biemass cf mussels (kg/m2) with the total area cf mussel boos (m2) in
each tidal area determined on the aeroplane pictUres from 1993. To estimate the standing stock
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of mussels in Juvre Deep, the average density-estimates for the mussel beds in Grey Deep,
Knude Deep and Lister Deep was used.
3.4.
Estimation of food demancIs
oe birds•..
The mussel stock in the Danish Wadden Sea is consumed by a number of bird species.
The most characteristic mussel eating birds are the oystercatcher (Haematopus ostralegus), the
eider (Somateria mdllissima), and the herring gull (Larus argentatus).
Based on figures from the literature the three bird populations consumption of musseIs
are estimated. The oystercatcher has a daily average energy demand of 833 kJ/day (GOS5Custrad and Durell, 1987), which is equ3.l to around 38 g AFDW mussel meat (22 kJ/g AFDW
of mussel meat; McLusky, 1989). The eider needs 2732 kJ/day (Swerinen, 1976, Nyström and
Pehrsson, 1988), which is equal to 124 g AFDW of mussel meat. The herring gull has a daily
demand of energy of 1987 kJ/day (Faldborg et al, 1994), which is equal to 90 g AFDW of
mussel meat. The energy demands are used to calculate the number of birds (bird days), that
can be sustained on the annual production of mussels (50% of P) in the Danish Wadden Sea.
4.
Results.
4.1.
Estimating tbe area of tbe different musseI beds.
Estimates of the area of the different mussel beds in the Danish \Vadden Sea deteeted
from the aerial photos are shown in Table 1.
In both 1991 and 1993 the largest areas with mussel beds were found in the Hjerting
Stream (the fishing sectors). The mussel beds respectively represented 38 % in 1991, and 46 %
in 1993 of the total area of mussels in the Danish Wadden Sea (Tab. 1).
On the photos, the mussel beds are spaced in groups. The total area of mussel beds
in the Danish Wadden Sea is estimated to 10.92 km2 in 1991. The intertidal beds covered 9.13
km2, and the subtidal beds 1.79 km2• The total area (8.67 km2) with mussel beds in 1993 was
smaller (21 %) compared to 1991. The size of the intertidal beds was 6.19 km2, and 2.48 km2
were subtidal mussel beds. Conclusively, the total area of intertidal mussel beds deelined from
9.13 km2 in 1991 to 6.19 km2 in 1993, a chance of 32 %. Simultaneously the subtidal beds
grow from 1.79 km2 in 1991 to 2.48 km2 in 1993, and with 38%.
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The deeline of mussel beds in the Danish Wadden Sea is masking over a large variety
of declines and increases in the different tidal areas, and civer differences between the various
intertidal and subtidal mussel beds. The biggest chance was found in Juvre Deep among the
intertidal mussel beds, which declined ca 64% from 2.69 km2 in 19~1 to only 0.96 km2 in 1993
(Tab. 1).
In the fishing seetors in Hjerting Stream; the musseI beds covered 4.14 km2 in 1991
(exclusive the beds in Hobo Deep west of Langli, 1.23 km2) cif which 3.05 km2 were intertidal
mussel beds, and 1.09 km2 were subtidal mussel beds (Tab. 1). From 1991 to 1993, the area
with intertidal mussel bed' deereased from 3.05 to 2.03 km2 or 33 %. Simultaneously, the
subtidal mussel beds increased from 1.09 to 1.98 km2 or 82%. There were not deteeted any
intertidal musseI beds in fishing seetor 3, neither in 1991 nor in 1993 (Tab. 1).
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From 1991 to 1993 the total area with mussei beds in Ho Bight was reduced by 12%.
In tJu~ fishing seetors thereduction wasoilly 3% from 4.14 km2 to 4.01 km2 (Tab.l). In seetor
1, where the fishing from 1991 -1993 annually amounted to 25% of the annual estimated
standing stock, the area with mussei beds increased 10%, from 2.78 km2 in 1991 to 3.07 km2
in 1993 (Tab. 1). In seetor 2 (fishing 10% of the annual standing stock) the area with musseis
decreased 58 %, and in seetor 3 (without fishing) the area with mussei beds increased 78 %
(Tab. 1).
4.2.
Densities (no.lm%).
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1991 the densit)r (range) of rritissels, in the three fishing seetors in Hjerting stream
were between 0 arid 1 000 per m2• The highest density (- 3 800 mussels1m2) of musseis was
found in sainpling station 31 (an intertidal mussei bed) in fishing seetor 1, and in Sampling
station 13 in fishing sedor 3 (a subtidal mussei bed). Both samplirig station 1 and 26 (seetor
1, subtidal beds) showed a density of musseis of max~ 2000 per m2 • I{\ all other stations the
maXimum riumber of musseis was ca 1 500 per m2 •
Iri seetor 1; four sampling stations were without musseis, and iri seetor 2 all sampling .
stations had musseis. In seetor 3, 4 sampling stations were without any rriussels. The smallest
number of mussds per m2 was found in August 1992 and iri July 1993 in sampie station 12
(seeter 3), and in October 1991 in sampling station 22 (sector 1), where only 150 musseis per
m2 were found. The density of musseIs in the mussei beds varied, both from seetor to seetor,
and from sampling station to sampling station, within the seetors. So, there was rio disiinct
pattern for the variation, whether the mussei beds were intertidal or subtidal; fished or'not
. fished.
4.3.
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Biomass (total wet weight)
(kg/m%)~
To estimate the biomass of musseis, it was rieeessary t6 estimate the density (kg) of
musseis + shells per m2 from all the sampling stations, and to estimate the fraction of mussels
in the sanipies. The biomass varied (range) betweeri 0 kg and 63.9 kgtm2 in the 1991, 1992 and
1993 sampies. The targest biomass was found in sampling station 31 in fishing seetor i in
September 1993 (63.9 kg/m 2).
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, The average biomass estimated from each survey in the different fishing seetors are
shown in Table 2. From 1991 to 1993 there was an increase in the average biomass of musseis
in all three seetors. The annual average biomass iri all three sectors was in 1991, 7.1 + 6.0
kglm2 (mean +2 SE). In 1992 the estimated annual average biomass increased to 17.0 + 10.3
kg/m2 (mean ± 2SE)j and in 1993 to ca 19.5 + 7.8 kg/m2 (mean + 2 SE)~
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Table,6 show the biomass estimated from the dredge sampling in all four tidal areas
in the Danish :Wadden Sea. An average biomass figure from Grey, Knude, arid Lister Deeps
is used, to estimate the standing stock of musseis in Juvre Deep. The catch efficiency of tbe
dredge is assumed to be 100% in the biomass estimations~ The biomass in the four different
tidal areas väried bet'Yeen ca 1.6 kg/m2 (in Grey Deep South of Esbjerg) and 14.2 kg/m2 (in
Ho Bight North of Esbjerg, in the fishing seetors) (Tab. 6).
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4.4.
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Estimations of the standing stock.
Estimates of the standing stock is achieved by niultiplying the monthly average
biomass (kg/m2) for each fishing sector, with the estimated area (in m2) of each fishing seetor
determined from the aeroplane pi~tures; in Hjerting Stream. The estimations are shown in .
Table 3.
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The standing stock estimations are based on biomass estimations from between 1 and
16 observations in the different sectors in Ho Bight (not all sampling stations were visited at
each survey)..Monthly figures from the surveyed fishing seetors are used in the none surveyed
fishing sectors to estimate the biomass of musseIs in those sectors (Tab. 3).
The estimations of the standing stock are only valid if the ~mpling stations are'
randomly distributed within the musseI beds deteeted from the aenal photos. Tbis is the case
for the subtidal mussel beds. However, the aSsumption of randomized samplirig can not be
validated for the intertidal beds. Tbe coverage of niussels within beds in the intertidal musseI
beds varied between 5 and 100%. In average 65 % of the beds were covered with musseIs (65
+36%; mean +2 SE; n = 25) from 1991-1993.To compensate for the coverage of musseIs
an average coverage figure of 0.65 was used in the estimate of the biomass in the intertidal
mussel beds, although zero sampling stations was observed. Consequently, the stariding stock
in September 1993 may be reduced, from totally 93 296 metric tonnes to totaily 76 753 metric
tonnes in all three seetors in Hjerting Stream (Tab. 3)... :.'.
In a11 three sectors an increase in the standing stock was observed from 1991 to 1993.
In seetor 1, the standing stock increased, as a year average, from 8 600 metrlc tonnes in 1991,
to 42 000 metric tonnes in 1992, and to 48 500 metric tonnes in 1993 (Tab. 3). The annual
standing stock was in 1993 10 700 metric tonnes in seetor 2, and 7 000 metric tonnes in seetor
3 (Tab. 3).
Estimations of the musseI stocks in the three fishing sectors in Hjerting Stream (exclusive Hobo Deep) based on the dredge surveys in September 1994, and with an assumed catch
efficiency of 100%, indicate a standing stock of musseIs of 56 857 +23 100 metric tonnes
(mean +2 SE) (Tab. 6).
The standing stock of musseIs in the entire Danish Wadden Sea was estimated to 116
879 + 54 894 metnc tonnes (mean +2 SE) in September 1994 (Tab. 6).
5.
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Fishing or' mussels, 1991~1994.
In 1991 and 1992 the annual grass catches in the two fishing seetors (l and 2) were
respectively 6 905 metric tonnes and 6 344 metnc torines. The net catches were respeetively
5 539 metric tonnes and 5 040 metric tonnes (Tab. 4).
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A chance in the regulation in 1993, resulted in a smaller gross catch of only 5031
metric tonnes, and a net catch of only 3 267 metne tonnes (Tab. 4), which only was 60% of
the annual net catches in 1991 and 1992. In 1994 the gross catches increased compared to the
catches in 1993, with ca 1 337 metrie tonnes. to 6 368 metric tonnes (Tab. 4). Tbe net catches
in 1994 were 4 635 metrie tonnes, or 42% higher thari in 1993.
Tbe largest exploitation of the standing stock was in 1991, when 29% of the stock in
Hjerting Stream (sector 1, 2 and 3 combined) was fished. (Tab. 4). The average exploitation
ofthe standing stock in 1992 amounted to only 8% in alt three sectors, and in 1993 only 5%.
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The anmial average exploitation rate from 1991 io 1994 was 11 %of the standing stock in all
three sectors.
The exploitation rate from 1991 - 1994 did not result in a redtiction in the standing
stock of musSeIs in Hjerting Strearn. Contrary, the standing stock increased more then 3 times,
from 16000 metric tonnes in May 1991 to 57 000 metric tonnes in September 1994.
When an average production index (pIß) of 0.4 (Asmus, 1987) for both sub- and
intertidal mussei beds are used (cohort 1987), the standing stock in 1994 of 117 000 metric
tonnes in the entire Danish Wadden Sea produced around 36 500 metric tonnes of imissels.
This is eight times the amount of musseis fished (13% OfPl994) in 1994, which ieaves 16000
metric tonnes of musseis for the birds (44 % of Pl994 ).
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Estimates of bird predation of musseIs in Danish 'Vadden
Sea.
An accesslble biomass of musseis of 16 000 m~mc tonnes wet weight gives 1 440
metric tonnes of AFDW (9 % the wet weight) of mussei meat for birds to feect on in 1994 (Tab.
5).
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An oyste~tcher has a daily demand ~f crinsumption energy of 38 g AFDW of mussei
meat (see chapter 3.4). 1}1is gives a potential number öf oystercatchers, which may be
sustained by the mussei prOduction of38 * 106 bird days, or more then 100 000 oystercatchers
,
per day.
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If dders were the only mussei consuming bird species in the Wadden Sea, there would
be food enough for 11.6 * 106 eider days. Eiders have, a daily energy need, which is more then
. three tlmes as high as for the oystercatcher, or 124 g AFDW of mussei meat. The number of
eiders which daily can nourish on musseis may be 32 000.
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Herring gulls feed on the intertidal mussel beds. They have a daily consumption energy need of 90 g of AFDW of mussei meat, which gives 16 "* 106 bird days annually for
herririg gulls. The daily number of herring gulls irre 44 000 individuals.
The figures above are basect on the assumption that the birds live on musseis alone,
which is not the case. Additional, all three species are present simultaneously in the Wadden
Sea. A separation in the feecting niche can be observed companng eiders, oystereatchers and
herring gulls reducing the competition for musseis.
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The ornithologists have counted the number of all three species of birds in the Danish
Wadden Sea. The latest counts observed, as a monthly mean, 19 500 oystercatchers, and 22
411 eiders (only from August to March=8 month). The counts ofherring gulls observed 13
428 individuals (Laursen et al, 1994 and DAFNA, 1993).
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If, the oystercatcher had to nourish on musseis alone, consumption of 270 metric
toimes of AFDW of mussei meat could be expected annually, equal to 3 005 metric tonnes of
live musseis (assumption: 9% ofmussels are AFDW of meat). The eider stock(both serleritary
and migratory eiders) could consume (m 245 dayslyear) 681 metric tonnes of AFDW of mussei
meat, which is equal to 7 565 metric tonnes of live rilUssels. The herring gulls could consuine
44i metric tonnes of AFDW of mussei meat, equal to 4 901 metric tonnes of live musseis
(Tab. 5). If al1 three species of birds wem living on musseis alone, they could consume 15 471
metric tonnes of live musseis.
ßased on these calculations, the TAC=F=10 000 memc tonnes is an appropriate
amount of musseis to be fished in 1995, leavin"g enough foOd in form of musseis for birds in
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the Danish Wadden Sea, and without a decrease in the size of the standing stock (B).
7.
Discussion and conclusions•
.Not all mussel beds were photographed in'1991 in 1993~ However, all beds important
for the fishery have been photographed in both:years..The.observedchances in the areas
between 1991 and 1993 are based on the mussel beds importantfor the fishery, detected on the
photos...
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The subtidal areas with musseis in the three fishing sectors increased from 1991 to
1993 by 82%. Additional, the intertidal area decreased by 33%. It is difficult, to interpret, if
the fishing had positive or negative impact on the size and distribution of beds in the fishirig
sectors. Other factors, such as predation from different species of birds and invertebrates, and
ice winter destruction of especially the intertidal mussel beds, may be important too.
The large variation in density of musseis observed at the sampling stations, reflects
the variation in number of musseis in place and time, caused. by variation in the fishery
intensity and variation in predation and settling of new spat. Spat fall was observed during the
three years of investigation. However, they "disappeared" shortly after settlement. The densities of mussels can very considerably even within relatively small areas. Laursen and Egerrup
(1992) found densities of musseis between 80 and 180 per square (20x20 cm) within an area
of2 m2, which is equivalent to 2 000 and 4 500 musseis per m2 • Their finding is in accordance
with the densities found in this investigation~ .
The dredge survey technique of the mussel stock (swept area) is just as useful, as using
a diver, sampling the mussels within a frame, and assessirig the coverage proportion of musseis
at each sampling station. There are sources of errors using both techniques.
Measuring the dredge Iength after four seconds of dredging introduce errors. No navigation system to day can record such smalI distances. The biomass determinatioris used in this
investigation could be optimized weighing the gross catch instead ofjudging the weight.
The diver introduce errors in the determination of the coverage proportion, which is
assessed at each sampling station. The sampling has to be raridom, which may be difficult,
when sampling in the intertidal mussel beds. There is always a certain chance of being
subjective, when taking sampies on scattered and exposed intertidal mussel beds.
The dredge survey technique is preferable, because it is Iess time consuming and less
expensive, compared to survey and sampling by
ofa diver. Both teChniques have although,
the same degree of precision.
From 1983-1987 the fishermen landed 85000 metric tonnes, or calculated as annual
average 17 000 metric tonnes, caught in the entire Danish Wadden Sea (no limitations on areas
or on TAC were enforced at that time). If the birds only fed on musseis, they would have eaten
an estiinated amount musse1s of75 000 metric tonnes in the same period of time (1983-1987).
Fishermen and birds have respectively fished and consumed (eliminated) 160 000 metric
tonnes, or annually 32 000 metric tonnes. lh of the stock for the fisherrrien (F) and Ih for the
birds (M) ( Z=M +F > > P). The result was a collapse in the fishable stock of mussels
between 1988 arid 1990. However, the 87-cohort of musseIs settling in the autumn 1987 (15
500 tonnes; Munksgaard 1989) could provide sufficient food for eiders, herring guIIs and
oytercatchers in the Danish Wadden Sea.
The musseI fishery is often criticized for eliminating the food supply for mussel eating
use
9
•
•
..
..
birds like eiders, oystercatchers, and herring gulls. Although, the estimations in this paper
reveal, that the determined regulation of the Danish mussei fishery (ca 11 % of standing stock
as annual average), which has been implemented since 1990, has resulted in an increase of the
standing stock of musseis in the fishing areas in Hjerting Stream from 16000 metric tonnes in
May 1991 (rab. 3) to more than 57000 metric tonnes in September 1994 (rab. 6). The annual
production of musseis has obviously been larger than the fishery mortality (F), the mortality
by predation (Mp), and the natural mortality (MN) by other courses than predation (P > F +
M (M=Mp +MN
It is not in"this investigation possible to analyse, whether the fishery and bird predation, at the present level, have positive or negative impact on the mussei stocks in the Danish
Wadden Sea. Anyhow; the standing stock of musseis has increased more then three times
between 1991 and 1994. Most ofthe intertidal mussei beds have survived due to no appearance
of severe ice winters since 1986/87.
" The regulation of the mussei fishery for 1995 in the Danish Wadden Sea has resulted
in a TAC=10 000 metric tonnes, which is twice as high as the annual TAC for the last three
years. A TAC l99S of 10'000 metric tonnes leaves 19 000 metric tonnes for the bird in 1995
(50% of the production P) before a reduction in the stock may occur. The regulation of the
mussel fishery has therefore the last five years been carried out in agreement with a wise
exploitation ofthe mussei stocks (F=TAC< the annual P), and to secure sufficient food for
the different bird stocks in the Wadden Sea.
».
•
.
I
•
10
·1
6.
.,
Literature.
Asmus, H. 1987.
Secondary production of an intertidal musseI bed community related
to its storage and tumover compartments. Mar. Ecol. Prog. Sero
Vol. 39., p 251-266.' ",
Fuglene i de danske faivande~' Resultaterne af landsdrekkende optrellinger 1987-1991. MiljoministerietlSkov- og Naturstyrelsen.
. . .
... .." ... , .
Faldborg, K., K. Th. Jensen and L. Maagaard~ 1994. Dynamics, Growth, Secondary Production and Elimination by Waterfowl of an Intertidal Population of
Myti!us edulis L. Ophelia Suppl. 6. p 187-200.'
DAFNA, 1993.
~
~
'.
'
Goss-Custard, J.D. and S.E.A. Durell,
1987~
Age-related effects in Oystercatchers, Haema-
topus ostralegus, feeding on musse1s, Mytilus edulis. J. Foraging efficiency and interference. J. Anim. Ecol. 56. p 521-536.
Kristensen, O.S. 1994.
Information on mussellandings from the Wadden Sea in 1987.
Kristensen, P.S. 1994.
Blämuslingebestanden i det danske Vadehav og blämuslingefiskeri
(1991-1993). DFH-rapport nr. 476-1994. pp 56.
Kristensen, P.S. 1994.
Blämuslingebestanden i det danske Vadehav 1994/1995. Note to the
Ministry for Agriculture and Fishery. pp. 6. (unpublished data).
•
Laursen, K., J. Frlkke and J.C. Salvig, 1994. Survey ofwaterfowls in the Danish Wadden Sea
1980-1991. Numbers, pheriology and distribution related
tidal
press).
cycle arid season. Danish Review of Game Biology.
(In
to
Laursen, M.-L. and M. Egerrup. 1992. Aspekter af prredation, vrekst og produktion pä en littoral blämuslingebanke i det danske Vadehav. Thesis-report to the
University of Copenhagen and DIFMAR. pp 78.
McLusky, D.S., 1989. The estuarine ecosystem. Chapman and Hall, NY. 215 pp.
Munch-Petersen, S. and P.S. Kristensen. 1987. Assessment of the stocks of musseIs in the
Danish Wadden Sea. ICES, Shellfish Committee. K: 13. pp 22.
Munch-Petersen, S. andP.S. Kristensen. 1989. On theapplicability'ofaerial survey techniques
for recording and estimating densities of mussei beds. leES,
Shellfish Committee. K:24. pp 16.
Munksgaard, C. 1989.
Naturovervägningsrapport. Undersogelse af blämuslingebestanden
i det danske Vadehav 1988. MiljoministerietlSkov- og Naturstyrelsen. pp 39.
.
11
•
I
•
.'
Nyström, K.G.K. and O. Pehrsson, 1988. Salinity as a constraint affecting food and habits
choice of mussel-feeding diving ducks. Ibis, 130. p 94-110.
Swennen, C., 1976.
Populatiestructuur en voedsel van de Eidereend (Somateria mollissima) in de Nederlandse Waddenzee. Ardea, 64. p 311-371.
Theisen, B. 1968.
Growth and mortality of culture musseIs in the Danish Wadden Sea.
Meddl. Danm. Fisk.- og Havunders0gelser. 6(3): 47-78.
•
•
12
"
7.
.
,
Tables and Figures.. _
.'
2
)
Table 1.
The areas (m km of the musseI beds in each tidal area of the Danish Wad. den Sea. The area in km2 of each fishing seetor 1, 2 and 3, and the fraction
covered with mussei beds in Hjerting Stream are given as weIl.
Table 2.
...The average biomass.(total wet weight) of musseIs in kg/m2 of mussei bed.
- : no sampling~' (n): number of sampling stations.
Table 3.
The estimated standing stock of musseIs (in tonnes) in the three fishing
sectors in Hjerting Stream, 1991-1993. ' X ': Figures between the apo';'
strophes are extrapolated from the estimated monthly stock figures from the
other monthly estimations each particular year. The estimations for the intertidal musseI b~s have been correeted with a coverage factor of 0.65.
Table 4.
Catches and stock of musseIs in the three seetors in Hjerting Stream, 19911993, in tonnes. The annual average figures are from Table 3. The annual
exploitation rates are given. ' X ': The areas in 1992 and 1994 are assumed
equal to the areas the previous years (1991 and 1993 respeetively). (*:
Figures from Munksgaard, 1989).
Table 5.
The annual food demands for oystercatchers, herring gulls and eiders in the
Danish Wadden Sea in fonn of mussei meat. (*: Figures from Laursen et al,
1994 and DAFNA, 1993).
Table 6.
Estimates of the mussei stock (m tonnes) after dredging in the four tidal areas
of the Danish Wadden Sea in 1994.
Table 7.
Estimates of P 94 (production), F (fishery mortality) and M (mortality due to
other causes than fishery and predation in 1994) on basis of the standing
stock of musseIs in 1994 <B9J. Three separate scenarios are given for F, 2xF
and 3xF (F: The average annual net catch between 1991 and 1994).
Figure 1.
The landings, VPA and TAC of musseIs (M. edulis L.) in tonnes for the
Danish Wadden Sea, 1979-1995. The figures from 1995 only covers the 1.
half year of 1995.
Figure 2.
The investigation sector's 1,2 and 3 in the gully, Hjerting Stream, in the Ho
Bight. All sampling stations are shown.
..
13
•
•
..
..'
Table I.
Areas with musseis
in 1991
in km2
intertidal
subtidal
Tidal areas
Area with musseis
in 1993
..
in km2
intetidal
subtidal
Grey Deep
5,01
1,09
3.39
1.98
Knude Deep
0.94
0.64
0.99
0,5
Juvre Deep
2.69
none
0.96-
none
Lister Deep
0,49
0,06
0,85
.-
The Wadden Sea
9,13
1,79
6,19
-
Sector 1 (ca 8 km2)
2,56
0,22
2,03
1,04
Sector 2 (ca 2 km2)
0,49
0,6
none
0,46
Sector 3 (ca 7,3 km2)
none
0,27
none
0,48
Sector 1-3 (17,3 km2)
3,05
1,09
2,03
1,98
..
.
•
none
'
..
-
.
2,48
....,
Table 2.
•
Area
May
1991
August
October
April
Sector 1 (17)
0,7 (4)
6,8 (12)
6.2 (11)
19,0 (14)
Sector 2 (4)
18,7 (4)
-
9,1 (1)
-
Sector 3 (11)
6,8 (6)
1,4 (8)
-
6,0 (8)
14
1992
June
September
1993
September
June
25,1 (13)
18,5 (16)
22,6 (16)
29,3 (2)
26,1 (4)
13.5 (4)
33,1 (4)
7,6 (6)
-
10,9 (7)
18,1 (5)
,,
Table 3.
Area
May
1991
August
Sector 1
1946
18904
Oclober
17236 .
1992
June
September
April
52820
'61299'
1993
September
June
69778
56795
69382
..
Sector 2
20383
'15151'
9919
'30193'
31937
28449
6210
15226
Sector 3
1836
378
'1157'
1620
2052
'1826"
5232
8668
Sector 1-3
24155
'34433'
"2831Z
'84633'
'95288'
'100063'
68229
93296
The standing stock figures below has been corrected with a coverage factor of 0.65 for the intertidal mussei beds
Sector 1·3
'22381'
15701
'18403'
'55011'
'61937'
'65041'
56140
76753
"
Year average
'18828'
'60663'
66447
•
Table 4.
Area (km2)
with musseIs
The stock in sector 1-3
in tonnes
Fished amount in tonnes:
Gross (Iogbook data)
Net (account data)
Exploitation rates %
1988
1991
1992
1993
1994
1,7*
4,1
'4,1'
4,01
'4,01'
15500·
18828
60663
66447
56857
2560
1161
6905
5539
29
6344
5040
8
5031
3267
5
6368
4635
7
15
8
•
1
•
.'
Table 5.
Bird species
Musse( meat
Mussei meat
demand (100%) demand (100%)
Annual bird days* tonnes of AFDW tonnes wet weight
Oystercalcher
7117500
270
3005
Herring gull
4901220
441
4901
Eider
5490695
681
7565
All species
17509415
1392
15471
1440
16000
10530
117000
Food (P-1994)
available
Food (8-1994)
available
•
16
Oredge sempies tokon In verious mUllel beds In the Ollnish WIldden Seoln Sept. 1994.
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11.40751914
11
31.404
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2,01
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1,11
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0,938562092
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11,050882818
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1,4114
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0,818
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9.769910504
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, ..ei .,.. • , ....
li'
b,d, In ."" lJonl.h W,dd,n S.. In 1994: ',19 km2
9593.223628
9!,9:t
Melln in tonnes
116879
MlIx,:
162773
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70986
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45894
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rio IA""",", lflBlI
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ton..,,..
.'
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.,.
36679
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Fllh..v unlnhation b...d (tn P:
Sune,io 1
111",tu"".Ii,,njnl!)~1
M Unet. pradalion'
Scena,io '1
Scena,kJ 3
tl'u'l"""''''
tt
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,,'. 11m...
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11
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89
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n
111
M (...:•. 1"'.'lthUII'
;llil!l:J
:J'F f1!1!M !r,vnU
11/09
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M (.h';1. prndll'ionl
~:fOOO
01
IV
0
Fi,h.,y •• P!o;lalion b...d on B:
Ra••'a' pi B:
Scenario 1
F
M fincl. pfad.tionl
Scenario 2
Scena'NI 3
84 V AfDW/b.d/dl" Ikn'. 10.1, 1,0 ky w" w.lyhl 01 mu...II)
Bi'd. an
''I.'"
foocf dom.nd 0' In 0YI'fWcllcher, • ho"lno CU••nd an oicfOf' per dlY.
SClnwJo 1:
50%.' M • 10208
AFDW 18%1 • 1797 ,."""" gi..... 15.4 ..,In. b'd dl,1
..._102:
50% ."+M· 14070
S••n.,lo 3,
50% .r P+M I t 1945
AFDW 18%' • 117610",.. g"''' .1 13.4.nll. b.d dl"
AFDW (8%1 • 956 I....... Oiw.. CI 11.4 mil. b.d dl"
4
96
U!i:lO
1
< 10'«UJ
93
3"
"Mt.
tI.
!ill
< 113000
2'r
.od. 11 , M
f.tlml'" fblrd daYI hing .n mu...I. MIrI.1
Ib...d Oft 11M Inn_ prodvctlon .1 mu...
llilli
r. M
<
lU1I9
11
104000
Oll
..
Figure 1
The landings, VPA and TAC of M. edulis in the
Danish Wad den Sea
140
1
Ä
120 t
I
g
100
+
VPA
i :1.
20
o
,.
Ä
!
A
/
."
~,
A .. · ,.
'A
-.-.0--
TAC
--r-
79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
_ _ _ _
-
_0 _ _ - -
•
_
21
!
iI
year
.:
,
.
Figure 2.
T•••"pi.,1 "
22