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Multi-Species and Ecosystem Models

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Multi-Species and Ecosystem Models Powered By Docstoc
					Multispecies models
Issues and state of art in modelling Issues in interpretation and implementation

Gunnar Stefansson
Marine Research Institute/Univ. Iceland
GS 1

Models
• • • • • • •
GS

Single species Statistics Biological interactions Technical interactions Spatial effect Temporal variation Economics
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Models - single species
Y/R 2 1.5 1 0.5 0
0.0 0.1 0.2 0.3 0.4 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

S/R 20

400 300
15 10 5 0

R

200 100 0 0 500 1000 S 1500 2000

Conclusion: Low F
GS 3

Models - more
• • • • • •
GS

Effect of reduced fishing on predator? Effect of increased harvest of prey? Effect of fishing in spawning area? Effect on bycatch species? Uncertainty in estimates? Predictive capability?
4

Need statistical multispecies explicit spatial models

Models - statistics
• Natural variation • Measurement errors • Nontrivial effects of incorrect methods...
• Estimation of unknowns • Prediction of effects with uncertainty
GS

Conclusion: Lower F

5

Models - biological interactions
Processes to be included in holistic models

Need to (1) predict Data? (2) understand Others: Hydrography, larval drift, spawning location, GS 6 maternal effects, ...

• • • •

Predation Growth Maturation Migration

• Spawning • Fishing mortality • Natural mortality

Models - technical interactions
• Species mix in catches • Different mix in different gears • Prediction of effects of gear regulation

GS

7

Models - spatial effects
• • • • • Nursery areas and spawning areas Migration Spawning fishery Species overlap is spatially variable Spatial variation in growth Need to
(1) predict (2) understand
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Data?
GS

Models - economics
• Different species: Different prices • Costs of effort by fleet

GS

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Models - temporal variation
Single species, annual assessments • Annual data and analyses Multispecies interactions and migrations: • Finer temporal scales

GS

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Models - current status
• • • • Greater uncertainty than earlier thought Multispecies concerns are important Statistical techniques essential Need holistic models for understanding

GS

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Precautionary approach
• Reference points
– F or B for single species – F mainly for multispecies?

• Uncertainty
– Increases with recent models – Requires lower F

• Multispecies reference points? • Precautionary harvesting of prey species??
GS 12

Biological interactions (growth)
Scenario 1 Prey Predator Initial stock size 1000 100 Fraction harvested 0.5 0.2 Proportion in diet 0.5 Weight conversion 0.1 Stock for consumption 500 Consumption and growth 250 25 Yield from prey or growth 500 5 Price 5 500 Income 2500 2500
GS

Scenario 2 Prey Predator 1000 100 0 0.2 0.5 0.1 1000 500 50 0 10 5 500 0 5000
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Is it in accordance with the PA to harvest the prey?

Control mechanisms
• • • • Closed areas TAC Effort regulation Mesh sizes (fishing gear limitations)

GS

14

Overcapacity
• Introduces problems in all control systems • Reduces likelihood of efficiency in any control measure • Increases political pressure and likelihood of deviations from earlier policy • Needed: Models of these effects
GS 15

Closed areas
Virtue • Guarenteed protection within the area Problems? • Choice of size and location of area • Effect of migration and fishing outside area Major issue • Enforcement
GS 16

Closed areas
No (measurable) effect on mortality F Likely effect on mortality but not economical F

E
I
GS

E
I Extreme, 90%: Likely 17

20%: Unlikely to work

Closed areas: Overcapacity
• What is the upper bound on fishing mortality outside the closed area? • Historical low catch outside does not mean catches cannot be taken outside!
• Small closed area+overcapacity: No effect on F
GS 18

TAC limitations
Virtue • Direct limitation of fishing for given species Problems? Major problem • Uncertainty in estimating correct TAC • Species mis-match Major problem • Discards Major issue • Enforcement
F at 75% of Fcrash: Unlikely Extreme F at 10% of Fcrash: Likely
GS 19

TAC limitations: Overcapacity
Dormant capacity (Latent effort) • Can always catch quota if overestimated • Will introduce discards when TAC underestimated (multispecies effect) • Effect on bycatch species often unknown • Get biassed effect (overshoot F on average) due to the system itself
NeedGS spatial and multispecies In toto: Major problems models for understanding
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Effort regulation
Virtue • Direct limitation of all effect on the ecosystem Problems? • Effort reallocation species/components Major problem • Efficiency/catchability trends Major problem • Only guaranteed to work if very low effort allowed • Vessel classes? Major issue • Enforcement?
GS 21

E at 80% of Ecurrent: Unlikely Extreme: E at 40% of Ecurrent: Likel

Effort regulation: Overcapacity
• Effort and capacity: Very similar effects Overcapacity: • Political pressures • Exchange of effort-days to more efficient vessels
Need spatial and multispecies models for understanding
GS

In toto: Major problems
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Mesh sizes (gear limitations)
Virtue • Direct limitation of fishing for juveniles or... • May make fishery sustainable • Pacifier Problems? • Slippage mortality? Major issue • Often negligible effect GS • Enforcement

23

Mesh sizes: Overcapacity

Can often negate effect

In general it is not clear that mesh changes or common gear regulations have any effect at all
GS 24

Back to models: Model requirements
• Uncertainty:
– Better statistical models

• TAC control:
– Multispecies, technical interactions

• Areal closures:

• Understanding any controls: • Effort control, analysis:
– Spatial models – Spatial models – Multispecies, technical interactions
GS

– Need to estimate effect of major change in predator on prey abundance and vice versa – Multispecies, biological 25 interactions

Effects of current model output
• Uncertainty output:
– Need lower F

• Almost all analyses:
– Need lower F

• Multispecies output:
– Need lower F on prey

• Areal closures: Large areas (or more controls) • Effort control:Lower effort+annual reductions+TAC • TAC control: Lower TAC+effort/fleet reductions
GS 26

Limitation summary
• • • • TAC: Species allocation mismatch+uncertainty Closed area: Migration/fishing outside+uncertainty Effort control: Effort reallocation+catchability Fleet reduction alone: Like effort

Common effects of levels of measures: 10% reductions: No effects

50% reduction: Some effect likely but can be negated
GS 27 90% reductions: Almost sure effects but may lose catches

Source of problems
TAC:
– – – – Correct TAC uncertain Fleet too large Dormant effort TAC too close to target

Effort control:
– – – – Correct effort uncertain Lacks species catch limits Lacks annual reductions Effort too close to target

Closed area:
– – – –
GS

Correct area uncertain Fleet too large Dormant effort Area too small
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Solutions?
Extreme measures? or Combined systems? or ?

29 NoGS single system, set at its target will suffice in general!

Combinations
• TAC + Effort control! • TAC + Closed area? • Effort control + closed area ?
• + Fleet reduction!!

GS

30

Current theme
Marine resources can be harvested using the maximum fleet size economically possible up to that maximim level of fishing mortality which does not demonstrably lead to stock collapse.

GS

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A new tenet
Marine resources should be harvested using the minimum fleet size possible and at that minimum level of fishing mortality which does not demonstrably lead to a serious long-term loss of catch.

GS

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