1 Null Hypothesis Alternative Hypotheses AH I: the different - - PDF document

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Jo King: Jo King:

The scientific implications of the EU Project WESTHER (Q5RS - 2002 – 01056) to the assessment and management of the herring stocks to the west of the British Isles.

Emma M.C. Hatfield & the WESTHER consortium Introduction

the WESTHER objectives the WESTHER null and alternative

hypotheses for the first three objectives

results of the first three objectives interpretation of results and implications for

assessment

recommendations from WESTHER to HAWG next steps

WESTHER Objectives

estimation of genetic and phenotypic

differentiation between spawning aggregations

determination of stock origins and life

history of juveniles

determination of composition of adult

feeding aggregations

improved guidelines for the conservation

and management of biodiversity and stock preservation Techniques used 1) body morphometry 2) otolith morphometry 3) meristic analysis (pyloric caeca counts) 4) microsatellite DNA analysis of tissue 5) fish parasite assemblages 6) parasite genetics 7) otolith microstructure 8) otolith core microchemistry and 9) multi-disciplinary, multivariate combination Partners & sub-contractors

University of Aberdeen

• sub- DARDNI

FRS Marine Lab. Aberdeen Institut für Seefischerei, Hamburg Marine Institute, Galway, Ireland National University of Ireland, Cork University of Liverpool

• sub- IMR & University of Bergen

Current assessment and management

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Null Hypothesis

there is only one herring population to

the west of the British Isles, with no detectable differences between any of the geographically and temporally separated spawning components Alternative Hypotheses

AH I: the different spawning aggregations

sampled are discrete at spawning time and are, therefore, separate components

AH II: there is clear distinction of juveniles

sampled in different nursery grounds

AH III: fish from each spawning aggregation

remain discrete on their feeding grounds Alternative Hypothesis I - spawners

the different spawning aggregations

sampled are discrete at spawning time and are, therefore, separate components Spawner samples collected 2003 - 2005

• 14
• 12
• 10
• 8
• 6
• 4
• 2

50 52 54 56 58 60

Period 1 Period 2

Outgroup samples collected 2003 - 2005

• 10
• 5

5 10 15 20 25 30 50 55 60 65 70 75

2003 2004 2005

Methods used to test spawning groups

• 1. body morphometry
• 2. otolith morphometry
• 3. microsatellite DNA analysis of tissue
• 4. fish parasite assemblages
• 5. otolith core microchemistry and
• 6. multi-disciplinary, multivariate

combination

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Results of testing spawning groups All methods, except the genetics (microsatellite DNA), showed strong evidence for a clear distinction between all

• f the spawning groups, except Donegal and

Rosamhil samples Alternative Hypothesis I - spawners

accept Alternative Hypothesis I the different spawning aggregations

sampled are discrete at spawning time and are, therefore, separate components Alternative Hypothesis II - juveniles

there is clear distinction of juveniles

sampled in different nursery grounds Juvenile samples collected 2003 - 2005

• 14
• 12
• 10
• 8
• 6
• 4
• 2

50 52 54 56 58 60

2003 2004 2005

the WPs on parasites and otolith

microchemistry act as tags for the juvenile stages of herring and therefore provide the best place to search for evidence of juvenile separation Analysis of juveniles Parasites as tags: tag prevalence

Stanton Bank is a nursery ground

where juveniles originating from spawning grounds from the north-west

• f Ireland and western Scotland mix.

the Irish Sea contains juveniles arising

from spawning in both the Irish Sea and the Celtic Sea.

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Otolith microchemistry: nursery areas

the Irish Sea contains a mixture of juveniles

spawned in the Irish Sea, the Celtic Sea and probably the Clyde too

Stanton Bank and the Minches likely act as

nursery areas for fish spawned to the northwest of Ireland and around Skye (western Scotland).

there is a lesser signal of mixing of

juveniles in the Scottish west coast sea lochs of fish spawned in the Clyde, and both spawning samples off the Scottish coast (Skye and Cape Wrath). Alternative Hypothesis II - juveniles

accept Alternative Hypothesis II there is clear distinction of juveniles

sampled in different nursery grounds

there is, however, strong evidence that

juveniles from different spawning grounds mix in some of the nursery grounds sampled Alternative Hypothesis III – feeding adults

fish from each spawning aggregation

remain discrete on their feeding grounds Adult “mixed” samples collected 2003 - 2005

• 14
• 12
• 10
• 8
• 6
• 4
• 2

50 52 54 56 58 60

2003 2004 2005

• 1. body morphometry
• 2. fish parasite assemblages
• 3. otolith core microchemistry and
• 4. multi-disciplinary, multivariate

combination Methods used to test for possible mixtures The results suggested:

the “mixed” aggregations collected in VIaS consisted

mainly of NW Ireland spawners with possibly a small component of Irish Sea spawners

the “mixed” aggregations collected in VIaN consisted

mainly of Cape Wrath spawners, with a component of

• ther spawners recruited from Scottish coastal nurseries

and/or NW Ireland. Additionally, there was evidence for the presence of Irish Sea adults and possibly even Clyde adults within the VIa North mixed samples

most of the Irish Sea “mixed” aggregations comprised

Irish Sea spawners, with a small component of Celtic Sea spawners

most of the Celtic Sea “mixed” aggregations comprised

Dingle Bay spawners, with no indication of any component from the Irish Sea

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Alternative Hypothesis III

the science, therefore, suggests links between

the areas, with fish spawning in different areas mixing, to varying extents, on feeding grounds

there is evidence to suggest mixing of adults

from separate spawning components, especially in VIa North

reject Alternative Hypothesis III there is evidence to suggest that the Celtic Sea

and VIIj spawning ground fish do not mix with the more northerly spawners to the same extent that the more northerly spawners mix with each other

Hypothetical movements of juveniles and adults to feeding and spawning grounds based on historical evidence and WESTHER results

So what does this mean in terms of the current assessment and management of the western herring stocks? Assumptions of stock assessment

the stock is a closed unit data used in assessments are

representative of the entire stock This implies:

stock gain only through birth or

mortality, not immigration or emigration

F distributed homogenously over the

stock

Data from surveys should be a relative

measure of the entire stock and its geographical distribution Are we violating these assumptions for the western stocks?

WESTHER results suggest that both basic

assumptions are violated:

for VIaN the acoustic tuning index is derived

from a survey in VIaN, an area shown to contain a mixture of adults from different spawning components

the bulk of the VIaN fishery is currently in Q3 in

the north of VIaN – not homogenously distributed at all

in VIaS the 2004 acoustic survey was shown to

comprise spawners quite different to those sampled from spawner aggregations taken from the fishery Recommendations:

1) assess the herring to the west of the British Isles as two stock units - Malin Shelf (including the current ICES stocks VIa

North, VIaS and VIIb, c, Clyde and Irish Sea (VIIaN)) and Celtic Sea (the current Celtic Sea and VIIj stock). In the area studied in WESTHER we can hypothesise that there are two stock units within which data can be pooled for assessment. N.B. the boundary at the northern edge is unclear and there is no evidence presented in the report which separates autumn spawners in the north of Scotland west of 4 degrees W from autumn spawning fish east of 4 degrees W (the North Sea stock). The boundary is here for convenience.

2) survey effort should be increased or diverted to a combined survey on non-spawner distributions mixing

• n the Malin Shelf

3) the current monitoring of the spawning components should be maintained, but not to the detriment of a wider scale Malin Shelf survey. Spawning ground surveys might

provide data on the dynamics of individual stock components, which are thought to be useful for the development of a fleet-based advice

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HOWEVER,

1) management plans should be fleet/area based, aiming at preventing the local depletion of any

population unit in the area, and should make adaptive changes if current fishing practices change

2) management plans should recognise the importance of the populations in the north of area VIa as a potential source of herring to spawning grounds to the south 3) management plans should recognise that there are potentially two separate stock units

• n the west coast of the British Isles, these

constitute a population in the Celtic Sea and VIIj and a metapopulation centred on area VIa

Next steps…

recommendation from ICES Herring

Assessment Group for ICES study group to examine the feasibility of the WESTHER recommendations

to be called SGHERWAY

SGHERWAY ToRs

The Study Group on the evaluation of assessment and management strategies of the western herring stocks [SGHERWAY] (Chair: Emma Hatfield) will meet in Aberdeen, Scotland (UK) from 8-12 December 2008. Taking into account the results of WESTHER in relation to VIaN, VIaS and VIIaN stocks:

evaluate the utility of a synoptic acoustic survey in the summer for the

Hebrides, Malin and Irish shelf areas, in conjunction with PGHERS surveys of VIaN and the North Sea.

explore a combined assessment of the three stocks and

investigate its utility for advisory purposes.

evaluate, through simulation, alternative management strategies for

the metapopulation of VIaN, VIaS and VIIaN.

comment on the best way to maintain each spawning component in a

healthy state, whilst managing the fish of that component when they are in a neighbouring area.

Acknowledgements