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Managing mussel seed Managing mussel seed in the Irish Sea – the in the Irish Sea – the
biological issuesbiological issues
Tony Knights and Gavin Tony Knights and Gavin BurnellBurnell
University College CorkUniversity College Cork
Imag
e: G
LOB
EC
/Gly
nn G
oric
k
9th International Conference on Shellfish Restoration
Irish Sea benthic mussel seed is an important
resource
It needs to be exploited sustainably
The Irish Sea mussel seed fishery(tonnes)
2003 – 35,2362003 – 35,236
2004 – 24,1472004 – 24,147
2005 - 14,8612005 - 14,861
(source – DCMNR, BIM, DARD)(source – DCMNR, BIM, DARD)
Project Objectives
to understand the biology and ecology of mussel seed beds in the Irish Sea in order to assess their abundance and sustainability
to suggest management strategies for the sustainable exploitation of the mussel resource in the Irish Sea
Project advisors – steering group
Partners from UCC, UCD and QUBPartners from UCC, UCD and QUB Dr Carter Newell – USADr Carter Newell – USA Dr Aad Smaal – RIVO, NetherlandsDr Aad Smaal – RIVO, Netherlands Prof Ray Seed – School of Ocean Studies,UKProf Ray Seed – School of Ocean Studies,UK Mary Ferns – ISA and SESCOMary Ferns – ISA and SESCO Dr Terrence O’Carroll - BIMDr Terrence O’Carroll - BIM Micheal O’Cinneide – Marine InstituteMicheal O’Cinneide – Marine Institute Dr Francis O’Beirn – Marine InstituteDr Francis O’Beirn – Marine Institute
Project work packagesProject work packages
Reproduction – UCC (coordination)Reproduction – UCC (coordination)
Recruitment – UCDRecruitment – UCD
Hydrodynamic modelling – Aquafact and Hydrodynamic modelling – Aquafact and Seabed Surveys Seabed Surveys
Alternative sources of seed – QUBAlternative sources of seed – QUB
Management strategies - ALLManagement strategies - ALL
Question 1
Where does the seed come from?Where does the seed come from?
Work package A - Reproduction
broodstock habitats:• estuarine • intertidal • subtidal
the spawning season:spring / summer / autumn?
Sample sites
intertidal
estuarine
subtidal
Results - Spawning periods
Intertidal Estuarine & SubtidalIntertidal Estuarine & Subtidal
May – JulyMay – July May – June May – June Oct – DecOct – Dec
0
50
100
150
200
250
300
J A S O N D J F M A M J J A S O N D J F M A M J
0
50
100
150
200
250
300
J A S O N D J F M A M J J A S O N D J F M A M J
Gonad Index
Question 2
When does the seed settle?When does the seed settle?
Work package B - Recruitment
Recruits present every month sampled
Patterns varied over spatial and temporal scales
Clogherhead (a): 2004 - Peak recruitment in July2005 - Peak recruitment in June, July and September
Dalkey Island (b): - No peak in recruitment
0
500
1000
1500
2000
2500
3000
M03
J03
J03
A03
S03
O03
N03
D03
J04
F04
M04
A04
M04
J04
J04
A04
S04
O04
0
20
40
60
80
100
M03
J03
J03
A03
S0
3
O03
N0
3
D03
J04
F04
M04
A04
M04
J04
J04
A04
S0
4
O04
Mus
sel R
ecru
item
nt p
er 9
cm2 p
ad (
± S
D)
Does intertidal mirror subtidal recruitment?
Question 3
Do seed beds survive overwinter in the Irish Sea?
0
250
500
750
FEB APR JUN
Density of mussel seed from the Blackwater bed in 2005
• Some seed beds survive from autumn recruitment to early summer
• Mussel size varied depending on position within the bed
Mytilus
den
sity
(62
50cm
3 )-1
Edge (west)
Middle
Edge (east)
Position within the bed
Question 4
Can we model the behaviour of mussel larvae?
6 model release sites
Historical map of seed collection areas and model simulation sites
Seed area
Larval patterns of distribution
1. Site
2. Time period
(May-June, July-August)
3. Phase (Spring/Neap)
4. State (Flood, Ebb, HW, LW)
5. Zone (Top, Middle, Bottom)
Factors:
1l samples (n=120)
0-25m
0
5
10
15
Flood Ebb High Low0
5
10
15
20
Larval density and position varies with tidal state
Mea
n L
arva
l Den
sity
(m
-3 +
SD
)
High nos. during flood tides
Low nos. during ebb tides
Flood Ebb High LowTop
Larvae closer to seabed during high, low and ebb tides
Mixed during flood tides
Mid
dle
Bott
om
Mea
n L
arva
l Den
sity
(m
-3 +
SD
) pe
r de
pth
zone
Dispersal ModelsDispersal ModelsLagrangian Particle Tracking (Coherens*) Lagrangian Particle Tracking (Coherens*)
Lagrangian SPM Module
Initial Particle
Position
River & Open Boundary Input
Advective Transport
Diffusive Transport
Currents Turbulence
Updated Particle Position
* Marine Science and Technology Program
Inputs:
Local hydrodynamics
Local meteorology
Assumptions:
Passive particle vs. active advection
Larval duration (28d)
Larval concentration – Coherens Model
A).
B).
Figure 7
100 hrs 200 hrs 400 hrs 600 hrs 800 hrs 1000 hrs
A).
B).
Figure 7
100 hrs 200 hrs 400 hrs 600 hrs 800 hrs 1000 hrs
Model w
ithout
beh
avio
ur
Model w
ith
behavio
ur
Southern Irish Sea release (50,000particles)
Results:
Larvae are retained within estuarine and coastal regions by a combination of their behaviour and tidal currents
Work package D – Alternative sources of seed
Hatchery
Spat settlement on longline collectors
wild seed from other areas
Are there alternatives to bottom dredged seed?
Stand alone mussel hatcheries are not currently economic
Relaying of rope seed on bottom is being evaluated by BIM / Industry
Q5: How can we use our knowledge to sustain the mussel seed fishery?
• Key results
• even small seed are reproducing
• there are 2 main spawning seasons for the subtidal mussel seed – early summer and late autumn
• modelling shows that these larvae are largely retained near the area of release
• some seed overwinters in the Irish Sea
Work package E – recommendations for management strategy
How can we put all this information together to support a sustainable
bottom mussel industry?
Possible management options for Irish Sea mussels
•carry out annual survey with agreed protocols to map the resource
• identify stable overwintering beds and conserve a portion of them as broodstock areas
•protect subtidal beds until after summer spawning (open from mid July)
Conclusions
There is a need for ongoing scientific management to ensure a sustainable future for the Irish Sea bottom mussel
industry
Acknowledgements
Thanks to all the partners and advisors for
allowing me to borrow their
tables, figures and pictures.