Université de La Rochelle 3ème année de Licence générale Biologie et Ecologie marine

Promotion 2010-2011

Internship, from April 17th to June 24th 2011

Predating and interspecific competition on Mytilus edulis in

the Dutch Wadden sea

By Aaron HARTNELL

M A R I N

E E C O L O G Y Directed by Andreas Waser

[andreas.waser@nioz.nl]

At the NIOZ institute *Landsdiep 4, 1791 ‘t Horntje (Texel), The Netherlands+

Abstract

Français

Les moulières de la mer de Wadden constituent une ressource en coquillages

consommables importante avec la présence de la moule Mytilus Edulis. En début des années

1990, la quasi-totalité des moulières avaient disparues dû à une pêche intensive et une

mauvaise période de ponte. Cette baisse a provoqué des conséquences sur les populations de

leurs prédateurs et sur d’autres bivalves à propriétés invasives. Depuis, les moulières sont

contrôlées et retrouvent leur densité connue auparavant. Ce rapport démontre quelles sont les

procédures employées pour essayer de comprendre l’intensité de la predation ainsi que de la

competition interspécifique pour l’espace et la nourriture sur les moules Mytilus edulis.

English

The Wadden sea mussel beds are an important resource in sea food with the presence

of the mussel Mytilus edulis. In the early nineties, almost all mussel beds have been

disappeared due to an intensive fishing and a bad spawning year. The decrease occasioned big

consequences on their predators’ populations and on invasive bivalve species. Since then,

mussel beds are controlled and showing a high increase in their populations’ density. This

report shows the methods applied to try to understand and measure the predation intensity and

the interspecific competition for food and space on the mussels Mytilus edulis.

Glossary

AFDW : Ash free dry weight

NIOZ : The Netherlands institute for sea research

Filter feeders : Species capable of feeding on suspended organic material by filtering the

water column

Acknowledgments

First of all, I want to thank Prof. Dr. Jaap van der Meer, who offered me the opportunity to

realize this internship at the NIOZ institute.

I’m also grateful to M. Andreas Waser, my internship supervisor, for his backing, his availability

and for the scientific knowledge he taught me.

Sytze Terpstra helped us for the field work. I thank him for his contribution to the project

progress.

Finally, I express my gratitude to M. Gilles Radenac for agreeing to be my personal tutor, and

more generally to the University of La Rochelle for the education received.

Summary

Context .............................................................................................................................................. 1

1. The NIOZ institute ............................................................................................................... 1

2. Project description .............................................................................................................. 2

Introduction ...................................................................................................................................... 3

Methods ............................................................................................................................................ 4

1. Studied zone ........................................................................................................................ 4

2. Main species involved ......................................................................................................... 5

2.1. Filter feeders ............................................................................................................... 5

2.2. Birds ............................................................................................................................. 6

3. Sampling preparation .......................................................................................................... 6

4. Field work ............................................................................................................................ 7

4.1. Bird counts ................................................................................................................... 7

4.2. Mussel bed sampling ................................................................................................... 7

5. Filter feeders measurement ................................................................................................ 8

Results ............................................................................................................................................... 9

1. Example of a mussel bed evolution over time: De Cocksdorp ............................................ 9

2. Intercompetition in-between filtering species .................................................................. 10

2.1. Filter feeders’ density of 4 mussel beds analyzed ..................................................... 10

2.2. Interspecific competition frequency histograms ...................................................... 11

2.3. Bird predating results ................................................................................................ 12

Discussions ...................................................................................................................................... 13

Conclusion ....................................................................................................................................... 14

Bibliography .................................................................................................................................... 15

1

Context

1. The NIOZ institute

The Royal Netherlands Institute for Sea Research, known as NIOZ, was founded in

1876.

The institute is located on the Texel island at the border between the North Sea and

the Wadden Sea, and situated about 100 km from north of Amsterdam.

The NIOZ mission is to gain and communicate scientific knowledge on seas and oceans

for the understanding and sustainability of our planet, and to facilitate and support marine

research and education in the Netherlands and Europe.

The research is organised in five multi-disciplinary themes (“Open ocean processes”; “Sea

floor dynamics”; “Wadden and shelf sea systems”; “Climate variability and the sea”;

“Biodiversity and ecosystem functioning”) carried out by five scientific departments

(“Physical Oceanography”; “Marine Geology”; “Marine Organic Biogeochemistry”;

“Biological Oceanography”; “Marine Ecology”).

Figure 1 : The NIOZ

location in the Netherlands

2

2. Project description

The work realized at NIOZ concerns the “Wadden and shelf sea systems” theme and the

“Marine Ecology” scientific department. This department aims to obtain a mechanistic

understanding of the structure and dynamical behaviour of marine macrobenthos populations.

The general approach that is followed is to try to understand the properties of populations and

communities on the basis of characteristics of individual organisms.

Within the department, several research clusters occur. This project is included in a more

general project directed by Jaap van der Meer: “Understanding population dynamics on the

basis of individual behaviour”.

The internship was supervised by Andreas Waser, a PhD student working more precisely

on the predation on littoral mussel banks. His research is included in the MOSSELWAD

project. Launched in 2008, this last aims to define the factors responsible for the lack of

stability of the Dutch Wadden sea mussel beds.

Thus, M. Waser tries to highlight the biotic factor roles on mussel beds evolution by

studying the crab and bird predation pressure. Two different monitoring programmes are

used:

- the “big brother” beds program using cameras to realize continuous bird counts

- the 20-bed-program involving 20 random mussel beds (10 west / 10 east) where birds

counts are realized 6 times a year and mussel sampling twice a year.

The internship is part of the 20-bed-program and focus only on the West Wadden sea

Dutch coast. This report will able you to understand the fieldwork and the database treatment

which were done on the Western Wadden sea coast during the internship.

3

Introduction

The common mussel Mytilus edulis is one of the main species found in the Wadden sea

(S. Munch-Petersen et al. 2001; N. Dankers et al. 1995). It’s an important sea food resource

for humans and for its predators such as crabs and birds (Cor J. Smit et al. 1998). The figure 2

shows that this last was almost instinct in the 1990’s. The reasons were mainly a bad

spawning year and fisheries pressure (Cor J. Smit et al. 1998). The high mussel density

decrease in the sea caused major fluctuations in the trophic chains related to them. A lot of

common bird species mainly feeding on mussels changed to other preys which engendered a

higher pressure on their populations (Cor J. Smit et al. 1998). Some other bird species

commonly found in the area started migrating elsewhere lowering their density on the

Wadden sea coast (Cor J. Smit et al. 1998). Furthermore, other intercompetitive filter species

such as the pacific Japanese oyster Crassostrea gigas (known as an invasive species), started

spreading out and replacing mussel beds (F. Fey et al.2009).

Since, laws have been put in place (Cor J. Smit et al. 1998) to redevelop the mussel beds

in all the Wadden sea. The figure 2 shows a high population increase since 1994. Every bed

and region has a different development explained by the variability of biotic and abiotic

factors acting on them. A major

disparity is found between East and

West Wadden sea with a higher

abundance in the East. To explain

this difference, it’s necessary to

analyze the factors influencing the

mussel beds.

Birds feeding on mussels could have a major role on the variability in between West and

East. As the West coast has less mussel beds, the predation could have a higher effect for a

same feeding intensity. To find out if this is a factor potentially acting on the beds, this report

shows the methods applied to analyze their feeding and the filter species found on each bed.

As the project is only at its first stages, only a few beds have been observed and analyzed on

the West coast.

Figure 2 : East and West mussel beds evolution in biomass over time

4

Methods

1. Study zone

In North Netherlands, a chain of island separates the Wadden sea from the North sea. The

substrate composing the country coast is heterogeneous from sandy to muddy grounds. The

Wadden sea has a high percentage in shallow muddy areas that can extend to a few kilometers

from the coast. Such a location is a good place for mussel beds development. The studied

plots were mainly realized on those off shore muddy floors shown on the figure 3.

The biotic and abiotic factors influence the mussel bed dynamics, which have become

heterogeneous throughout the coast. The sampling plots locations, showed on figure 3, were

realized to try to encompass all the different possibilities. Thus, a general overview of the

mussel bed development and their predators’ population in function of the location can be

made.

Figure 3 : Sampling plots inthe Wadden

sea

5

2. Main species involved

2.1. Filter feeders

Common or blue mussel (Mytilus edulis)

Baltic tellin (Macoma balthica)

Common cockle (Cerastoderma edule)

Habitat: wave-washed rocks,

attached by millions through their

foot and their byssus (threads)

Description: blue-black shell,

sometimes covered by others

individuals such as barnacles (Balanus)

Size: up to 10 cm [1]

Spawning: March to October

Habitat: first centimetres of the

sediment, intertidal zones

Description: various colours

(pink, yellow, white) often in

bands

Size : 1,5 to 3,5 cm [3]

Spawning: February to May

Habitat: first centimetres of the

sediment, intertidal zones

Description: white shell

sometimes slightly striate

Size: up to 5cm [3]

Spawning: March to July

Pacific oyster (Crassostrea gigas)

Slipper limpet Crepidula fornicata

Sea squirt Styela clava

Habitat: hard surfaces (rocks or

shells), intertidal zones

Description: bluish-grey shell with

deep purple patches

Size: up to 18 cm [2]

Spawning: June to September

Imported in Europe from the

Northwest Pacific (Japan, Russia…) for

commercial purposes.

Became Invasive

Habitat: hard surfaces (rocks or

shells), intertidal zones

Description: colour varies

(white to pink) with brown

blotches

Size : up to 5cm [1]

Spawning: February to March

Imported in Europe from

North America in association

with oysters (Crassostrea

virginica). Became invasive

Habitat: hard surfaces, shallow

water

Description: long club-shaped

body with a slender stalk

Size: up to 12cm [2]

Spawning : summer/autumn

Imported in Europe from

Northwest pacific on the hulls of

war ships (Korean war).

Became invasive

Figure4 : Common mussel

(Mytilus edulis) with barnacles

Figure 6 : Common cockle

(Cerastoderma edule)

Figure 7 : Baltic tellin

(Macoma balthica)

Figure 8 : Slipper limpet

(Crepidula fornicata)

Figure 9 : Sea squirt

(Styela clava)

Figure5 : Pacific oyster

(Crassostrea gigas)

6

2.2. Birds

3. Sampling preparation

To estimate the predation presence of the different predators on the different mussel beds

it’s necessary to analyze the mussel bed and bird population development every 6 months.

To begin, a first field trip on the mussel bed is necessary to delimit it. The procedure is to

walk around the bed with a GPS. This last one is set to save the latitude and longitude

coordinates of its actual position every 5 seconds. Once the mussel bed has been totally

walked around, the coordinates are transferred to a computer. The exact mussel bed

Eurasian Oystercatcher (Haematopus ostralegus)

Eider (Somateria mollissima)

Herring gulls (Larus argentatus)

Figure 4: Pied oystercatcher (Haematopus ostralegus)

Figure 5: Male Common eider

(Somateria mollissima)

12 species

The most common species :

the pied oystercatcher

(Haematopus ostralegus)

Habitat: sandy coasts or salt

marsh, sometimes inland

Description: black and white

plumage, long orange beak

Size: 40 to 45 cm [4]

Food: shells, crabs, worms -

Shell eating type: opens the shell

and eats the inside

Spawning: May or early June

3 species

The most common species:

the common eider

( Somateria mollissima)

Habitat: rocky and sandy coasts,

sometimes inland

Description: black and white

(male) or brown (female) plumage

Size: 50 to 71 cm [5]

Food: shells, crabs, sea urchins –

Shell eating type: swallows the

entire shell

Spawning: May to June

6 species

The most common species:

The European herring gull

(Larus argentatus)

Habitat: rocky and sandy coasts,

sometimes inland

Description: white and grey

plumage

Size: 55 to 67 cm [4]

Food: garbage, shells, eggs, fish…

Shell eating type: breaks the shell

or swallows all of it

Spawning: April to May

Invasive species due to an

anthropogenic adaptation

Figure 6: European herring gull

(Larus argentatus)

7

emplacement and circumference are obtained by integrating and analyzing the data with

Garmin Mapsource, R and GPS Babel software.

By using Garmin Mapsource, 50 sampling spots are spread out randomly on the

mussel bed surface. The coordinates of every spot are integrated on the GPS. This last is set to

calculate the shortest track in between the sampling points.

4. Field work

4.1. Bird counts

Before arriving on the mussel bed, the samplers stop at a distance in between 300 and

500 m and install the scope. One of the samplers counts all the birds on the mussel bed helped

with hand counters and the scope. The birds are counted and separated in function of their

species. This count is done 1 to 3 hours before low tide.

4.2. Mussel bed sampling

The mussel bed sampling program is conducted by a 3 people team. Each sampling

spot is reached when the GPS gives a sound signal (with an 8 meter precision). While walking

in between each spot, one person estimates the mussel coverage by using the boot method,

where steps on mussels and steps on mud/sand are counted using hand counters. To achieve

the most precise estimation by doing a straight line in between 2 spots, the person in charge of

the coverage estimation waits at the previous spot until the rest of the team reaches the next

one. After their arrival, he can estimate the coverage by walking straight up to them.

After arrival on each spot, the GPS holder looks if he’s standing on mussels or on

mud/sand. If there are no mussels, this is noted down and the team moves to the next spot. If

there are mussels, the team has to realize a sampling procedure by measuring the mussel

attachment strength and the filter feeders’ density.

The density sampling is done with a 15cm*15cm quadrate which is put on top of

where the GPS holder is standing. The macro-fauna existing in the quadrate is gathered and

8

cleaned by using an iron sieve with 0.5mm holes. The samples are then put in plastic bags

with an identification number.

The attachment strength is measured with a

clamp and force scale represented in figure 8. A

random mussel near the GPS holder is chosen,

grabbed with the clamp and pulled off the rest of the

bed. This last mussel is rinsed and put in a plastic

bag with an identification number. The scale needle

blocks on the maximum force exerted to detach the

mussel which is then noted down.

After sampling the 50 spots on the bed, the bags are then kept in a -18°C room.

5. Filter feeders measurement

Before measuring the filter feeders individuals, the samples are properly cleaned with

fresh water and sorted out. The filter feeders are the only macrofauna kept. The rest is thrown

back in the sea.

Every individual is identified and its length is measured with a caliper. The

measurements are written down on an Excel sheet with a 0.1mm precision. This sheet is

composed with columns indicating the location, mussel bed number, date, month, sectors

(when they are some), sampling number, sampling size, individual species, barnacle coverage

and length measurement of each individual.

The mussels counted per bed are sorted in function of their length by separating them

per 2.5mm±0.5mm. After counting all the individuals, 15 random mussels are picked from

each length group to measure their ash free biomass. The ash free biomass is measured by

putting the samples in a high temperature oven until only ash is left. This ash is then measured

with a scale. By calculating the dry biomass in grams, it is possible to determine the bivalve

shell thickness.

Figure 12 : System used to measure the mussel attachment strength

9

Figure 13 : De Cocksdorp mussel bed evolution over time

Results During the internship, the field work and measurement mainly concerned four eastern

beds (De Cocksdorp; Balgzan/Kuitje; Balgzand/Amsteldiep; Texel/Krassekeet).

1. Example of a mussel bed evolution over time: De Cocksdorp

This image shows an

evolution example of a mussel bed

over time between September 2009

and March 2011. The colored lines

are the bed contours for every

sampling date. The mussels tend to

modify their composition every time a

sampling is realized. In March 2011,

the juvenile bed near land was

existing but forgotten to be sampled.

These histograms show the Mytilus

edulis population development over time in the

De Cocksdorp mussel bed over a winter. The

population tends to decrease in numbers but

increase in size. Almost a third of the

population died after the winter.

Figure 14 : De Cocksdorp frequency histogram over the winter 2010-2011

10

2. Intercompetition in-between filtering species

2.1. Filter feeders’ density of 4 mussel beds analyzed

The density of every filter feeder species is highly different in function of the location.

The Standard deviations are important due to the high heterogeneity of the sample counts and

can’t answer the significant difference in between the beds.

The only filter feeders with high numbers are the mussels Mytilus edulis and the

oysters Crassostrea gigas. Mytilus edulis is found in high abundance on all the beds

compared to the other species. But we can still observe a certain high density of Crassostrea

gigas in Amsteldiep and Krassekeet.

Kuitje is the only location with a lower abundance in filtering species compared to the

rest of the locations.

Figure 15 : Filters' population on each bed analyzed during the internship

11

2.2. Interspecific competition frequency histograms

It’s observed that Mytilus edulis has a maximum size not extending above 65mm

compared to Crassostrea gigas that can measure up to 20cm. The 2 beds show that Mytilus

edulis has a higher number of adult individuals compared to the oysters which have a high

juvenile density.

The higher number of individuals in Amsteldiep compared to Krassekeet is due to the

higher number of samples done on the bed and its bigger surface.

Amsteldiep shows lower sizes for its different filtering species compared to

Krassekeet.

Figure 16 : Size frequency histograms of the 2 beds showing a higher interspecific competition

12

2.3. Bird predating results

Tableau 1 : Bird countings and food consumption

To estimate the bird predation pressure on each mussel bed, the total dry-biomass of

food eaten by the three main species per day has been calculated. The average food per

day eaten by each bird was determined by Laursen et al. (2010).

Tableau 2 : Mussel beds dry biomass

The total dry-biomass of the mussels on each bed has been estimated through the bed

surface, the density and the mussel average weight. S. Munch-Petersen et al. (2001) have

created a formula linking the average length (L) and the average weight (W) of mussels

thanks to the coefficient “q”:

Bed Date Species Average food per day (g) Count

estimation Total food per

day (g AFDW)

Balgzand / Amsteldiep

01/03/2011

Oystercatcher 45 18

810 Herring gull 91 0

Eider 165 0

03/05/2011

Oystercatcher 45 6

361 Herring gull 91 1

Eider 165 0

Texel / Krassekeet

21/02/2011

Oystercatcher 45 47

13217 Herring gull 91 122

Eider 165 0

08/03/2011

Oystercatcher 45 42

10626 Herring gull 91 96

Eider 165 0

18/04/2011

Oystercatcher 45 19

4791 Herring gull 91 36

Eider 165 4

Balgzand / Kuitje

16/02/2011

Oystercatcher 45 603

27135 Herring gull 91 0

Eider 165 0

04/05/2011

Oystercatcher 45 75

13385 Herring gull 91 110

Eider 165 0

Location Date Number of

samples Mytilus counts

Average length L(mm)

Coefficient q Average

weight W(g)

Density

(ind/m2)

Bed surface

(ha)

Total AFDW (g)

Balgzand / Amsteldiep

05_2011 19 810 31,72 3,73E-06 0,119 729,47 4,2 36448,0

Balgzand / Kuitje

11_2010 23 402 34,55 3,73E-06 0,154 500,27 19,9 1529389

05_2011 19 471 41,81 3,73E-06 0,272 292,16 16,9 134441,0

De Cocksdorp

08_2010 58 3542 24,90 3,73E-06 0,057 1117,18 ?? ??

01_2011 55 2850 27,85 3,73E-06 0,081 1175,06 8,9 84207,6

03_2011 45 2014 28,66 3,73E-06 0,088 1167,68 8,6 88073,6

Texel / Krassekeet

04_2011 21 458 38,60 3,73E-06 0,214 588,69 2,7 34060,2

13

W(g) = q . L(mm) with q=3,72E-06

This formula has been established through mussel samplings from the Denmark

Wadden sea coast. Consequently, it can give enough accurate results for the Netherlands

Wadden sea coast.

Discussions

Mytilus edulis has recovered its numbers since the 1990’s and is now the species found

in higher numbers on most mussel beds coming from the Wadden sea.

The figure 15 shows that the main competitor is Crassostrea gigas as the other filter

feeders have very low numbers compared to Mytilus edulis. But this oyster can still be found

in high numbers on some mussel beds. Because of its length, shown in the frequency

histograms on figure 16, it can highly minimize the food resources available for Mytilus edulis

due to a high filtering consumption. The competition for space in between these 2 species is

hard to estimate. They could have a certain competition for space as they’re found in the same

areas but some other research proves that Mytilus edulis is commencal on Crassostrea gigas.

F. Fey et al. (2009) showed that Crassostrea gigas shells can be used as a solid support for

Mytilus edulis to fix on to. Furthermore, he demonstrated that predators tend to have more

difficulties catching the mussels stuck in between the oysters. These 2 researches point that

Crassostrea gigas is used as a shelter for Mytilus edulis.

The correlation between interspecific competition for space and commencal relations

for these 2 species makes it difficult to understand if Crassostrea gigas has a negative or

positive effect on Mytilus edulis. Although, the results on figure 15 tend to show that there’s

an inhibitor effect by Crassostrea gigas on Mytilus edulis population. On the beds observed,

Mytilus edulis tends to have a lower density when the oysters have a higher one. This

tendency can show a certain trend that these species have a negative effect on each other.

The project is at its first results and it’s still hard to give any conclusions on the real

effect of bird predating on Mytilus edulis but trends can still be made. The table 2 shows that

14

the mussels have a high potential food resource for birds. The table 1 demonstrates that birds

can eat a high volume of mussels per day. It’s impossible to calculate the percentage of

mussels eaten on a bed each day as the birds don’t only feed on one unique species. But still,

the feeding intensity can have a real effect on the mussel populations and decrease their

numbers. We can see the bird feeding is really high compared to the total mussel dry biomass

of each bed. These results only prove a rough idea and are probably wrong or not precise.

The “big brother” bed program (realized in the same project) will hopefully able to create a

modeling capable to measure the precise intensity effect on mussels by bird predating. This

modeling would take in account the other food resources available for the birds, for example:

other bivalves (mainly Crassostrea gigas) and other preys as fish or crustaceans.

Beds on the East Wadden sea (not observed during our internship) are bigger in size

and in numbers compared to the West side (figure 3). The only hypothesis that could explain

the cause of this distribution is the high intensity of predation and the interspecific

competitors keep the mussel populations constant. The results coming up in the next years

with the project will probably be able to have a better view on the understanding of that

distribution.

Conclusion

The project is still at its first stages and no conclusions on the results can still be made.

This report showed trends proving a real effect of other bivalve and birds’ predation but the

intensity can’t yet be measured. The birds are one of the 2 main mussel predators with crabs.

The rest of this internship will be spent on finding a method to estimate the crab predating

pressure on mussels.

The project is having more and more data and will be able after a couple of years to

determine more precisely the causes of the mussel beds evolution and their distribution.

Models will then be set up to try to calculate future mussel bed apparitions and eventually

determine the evolution of artificial mussel beds.

15

Bibliography

N. DANKERS. 1995. The Role of the Mussel (Mytilus edulis L.) and Mussel Culture in the

Dutch Wadden Sea. Estuaries Vol. 18, No. 1A, p71-80

S. MUNCH-PETERSEN. 2001. On the dynamics of the stocks of blue mussels (Mytilus

edulis L.) in the Danish Wadden Sea – Marine biodiversity Volume 465, Numbers 1-3, 31-

43, DOI: 10.1023/A:1014539414345

COR J. SMIT et al. 1998. Birds, mussels, cockles and shellfish fishery in the Dutch Wadden

Sea: How to deal with low food stocks for eiders and oystercatchers? – Marine biodiversity

Volume 29, Numbers 1-6, 141-153, DOI: 10.1007/BF03043952

S. DIEDERICH et al. 2004. Introduced Pacific oysters (Crassostrea gigas) in the northern

Wadden Sea: invasion accelerated by warm summers

LAURSEN et al. 2010. Assessment of Blue Mussel Mytilus edulis Fisheries and Waterbird

Shellfish-predator Management in the Danish Wadden Sea

F. FEY el al. 2009. Development and distribution of the non-indigenous Pacific oyster

(Crassostrea gigas) in the Dutch Wadden Sea

[1] http://doris.ffessm.fr/

[2] http://www.marlin.ac.uk

[3] http://www.eol.org

[4] http://www.oiseaux.net

[5] http://www.seaduckjv.org