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

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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

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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

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Does intertidal mirror subtidal recruitment?

Question 3

Do seed beds survive overwinter in the Irish Sea?

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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)

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Flood Ebb High Low0

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Larval density and position varies with tidal state

Mea

n L

arva

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(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

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arva

l Den

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(m

-3 +

SD

) pe

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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.