Transcript Restoring the Boyne Estuary mussel population Gavin Burnell and Tony Knights Aquaculture and Fisheries Development Centre, University College Cork 9th International Conference on Shellfish Restoration Image: GLOBEC/Glynn Gorick Boyne.

Restoring the Boyne
Estuary mussel
population
Gavin Burnell and Tony Knights
Aquaculture and Fisheries
Development Centre,
University College Cork
9th International Conference on Shellfish Restoration
Image: GLOBEC/Glynn Gorick
Boyne Estuary has a sustainable
artisinal fishery
Since at least 1904 the
Boyne mussel fishery
has provided a partial
livelihood of 100 – 450
tonnes per year between
15 – 25 local fishermen.
(Dougherty, 1904)
Fishermen use 5.5m
(18ft) wooden handled
rakes to gather mussels
for 2 hours either side of
low water in 1.5 – 2.0m
depth.
The fishery
Surveys in 1974 and
1988 estimated the
standing stock to be
1000 tonnes (+/- 200 t).
“There are regular
spatfalls and the
fishery is selfperpetuating”.
(Edwards, 1988)
The river channel was deepened in 1999
to allow larger vessels to access
Drogheda Port
Deepening the river
to 2.2m below Chart
Datum removed over
1 million tonnes of spoil.
Drogheda Port Company
were obliged to restore
the mussel fishery as
part of conditions of the
licence.
Mussel spat (20 – 25mm)
from the Irish Sea were
were seeded (July –
November) from 2000 – 2003.
A total of 3365 tonnes mussel spat (20 – 25mm)
were seeded over 4 years
1000
415t
830t
1765t
Tonnage
1500
Dredging
355t
2000
500
Seed was sourced
from S. Arklow to
N. Dublin banks.
0
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Year
Changes in mussel biomass during the restoration project
The mussels were sampled each
March/April
DGPS used to map
accoustic data (RoxAnn
1998 – 2002; ECHOplus
2003 – 2006) onto a
digitised 1: 10,000 marine
chart.
Accoustic data groundtruthed by divers using 2 x
0.25m² quadrats at four
chosen sites: high;
medium; low and zero
mussels.
Wet weight
Condition Index
calculated for a
sub-sample at
each site.
Mussel coverage increased from 5.7% to 57%
2000
2004
2006
High density mussels
Low density mussels
Sand, gravel, shell
Gravel, stones
Muddy sand
Mud
Hard compact sand
Compact mud, some rock
Rocks
Rocks
45
1998
40
Results of mussel surveys
35
30
25
20
Before dredging
15
•1998 size structure reflects
regular harvesting
10
5
0
45 1
6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86
• In 2004 some of the seeded
cohorts still visible
• By 2006, after 7 years of no
fishing, a large proportion
of mussels at market size.
Frequency
40
35
4 months
after last
seeding
30
25
20
15
10
5
0
45 1
6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86
40
2006
35
2 years after
last seeding
30
• Limited annual recruitment
from 2003 onwards.
2004
25
20
15
10
5
0
1
6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86
Shell length (mm)
What factors may have contributed to
the partial success of the project?
•New channel was ridged
•4 successive seed imports
•Moratorium on fishing for
duration of project
1000
830t
Dredgi
ng
1765t
•Seed laying was zoned to
reduce wash-out
1500
Tonnage
•Seed was sourced in Irish
Sea
355t
2000
415t
•Seed quality was monitored
500
0
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Year
Why did we fail to fully restore the mussel
biomass?
•Wash-out: flows up to
6,000 cubic feet /sec and
surface flows of up to 6ft/
sec (1.8m/sec).
•Predation
•Lack of suitable substrate
•Poor settlement
•Reduced productivity
•No harvesting – clump
formation
• Limited amount of seed
after 2000
Is the Boyne mussel fishery self - recruiting?
•Presence of small seed
(<25m) from 2004 onwards
indicates recent
settlement.
•Some evidence for larval
retention has been
obtained from
hydrographic modelling
data.
North
Channel
Isle of Man
Boyne River
U.K.
Dublin Bay
Ireland
St. George’s
Channel
8500
7000
5000
4000
3800
3600
3400
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
50
Vertical distribution of Irish Sea
mussel larvae was monitored in 2005
1. Site
2. Time period
0-25m
(May-June, July-August)
3. Phase (Spring/Neap)
4. State (Flood, Ebb,
HW, LW)
5. Zone (Top, Middle,
Bottom)
Knights et al. 2006. Marine Ecology Progress Series (in press)
Larval density and position
varies with tidal state
Larvae closer to
seabed during
high, low and ebb
tides
15
Mean Larval Density (m-3 +SD)
per depth zone
Mean Larval Density (m-3 +SD)
Low nos. during ebb
tides
10
5
0
Flood
Ebb
High
Low
20
Mixed during
flood tides
Top
Middle
Bottom
High nos. during
flood tides
15
10
5
0
Flood
Ebb
High
Low
Dispersal Models
Lagrangian Particle Tracking
(Coherens*)
Lagrangian SPM
Module
River & Open
Boundary Input
Initial Particle
Position
Advective
Transport
Diffusive
Transport
Currents
Turbulence
Updated
Particle
Position
* Marine Science and Technology Program
Inputs:
•local
hydrodynamics
•local meteorology
2 models:
•passive particle
• larval behaviour
•larval duration (41
days
Larval concentration – Coherens Model
Model with
behaviour
Model without
behaviour
Southern Irish Sea release (50,000particles)
Boyne
Hydrodynamic
Model
Passive particle model
shows larval
concentration near
coast after 4, 8, 16, 25,
33 and 41 days.
If larvae swim up into
water column during
flood tides, then
transport up estuary is
possible.
Boyne
Is the Boyne mussel fishery sustainable?
Sustainable
development is that
which meets the
needs of the present
without comprimising
the ability of future
generations to meet
their own needs.
(The U.N. Brundtland
Commission, 1987)
Future scenarios for the management of
the mussels
•Fishermen may be
compensated and
“bought out”.
•If no fishing, then
mussel biomass
may fluctuate more
dramatically than in
the past.
•May need to add
substrates for seed
collection and/or
stabalisation of river
bed.
Hatcheries may be
necessary if Irish Sea seed
remains scarce.
Acknowledgements
Thanks to
Captain Martin
Donnelly,
Drogheda Port
Company, for
permission to
use this data
and to my coauthor Tony
Knights for
analysis of data
and
preparation of
some figures.