GS 1

Multispecies models

Issues and state of art in modellingIssues in interpretation and implementation

Gunnar StefanssonMarine Research Institute/Univ. Iceland

GS 2

Models

• Single species

• Statistics

• Biological interactions

• Technical interactions

• Spatial effect

• Temporal variation

• Economics

GS 3

Models - single species

Conclusion: Low F

0

0.5

1

1.5

2

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

0

5

10

15

20

Y/R S/R

0

100

200

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0 500 1000 1500 2000

S

R

GS 4

Models - more

• 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?

Need statistical multispecies explicit spatial models

GS 5

Models - statistics

• Natural variation

• Measurement errors

• Nontrivial effects of incorrect methods...

• Estimation of unknowns

• Prediction of effects with uncertainty

Conclusion: Lower F

GS 6

Models - biological interactions

• Predation

• Growth

• Maturation

• Migration

• Spawning

• Fishing mortality

• Natural mortality

Processes to be included in holistic models

Others: Hydrography, larval drift, spawning location, maternal effects, ...

Need to (1) predict (2) understandData?

GS 7

Models - technical interactions

• Species mix in catches

• Different mix in different gears

• Prediction of effects of gear regulation

GS 8

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) understandData?

GS 9

Models - economics

• Different species: Different prices

• Costs of effort by fleet

GS 10

Models - temporal variation

Single species, annual assessments

• Annual data and analyses

Multispecies interactions and migrations:

• Finer temporal scales

GS 11

Models - current status

• Greater uncertainty than earlier thought

• Multispecies concerns are important

• Statistical techniques essential

• Need holistic models for understanding

GS 12

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 13

Biological interactions (growth)

Prey Predator Prey PredatorInitial stock size 1000 100 1000 100Fraction harvested 0.5 0.2 0 0.2Proportion in diet 0.5 0.5Weight conversion 0.1 0.1Stock for consumption 500 1000Consumption and growth 250 25 500 50Yield from prey or growth 500 5 0 10Price 5 500 5 500Income 2500 2500 0 5000

Scenario 1 Scenario 2

Is it in accordance with the PA to harvest the prey?

GS 14

Control mechanisms

• Closed areas

• TAC

• Effort regulation

• Mesh sizes (fishing gear limitations)

GS 15

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 16

Closed areas

Virtue

• Guarenteed protection within the area

Problems?

• Choice of size and location of area

• Effect of migration and fishing outside area

• Enforcement Major issue

GS 17

Closed areas

F

E

I

F

E

I

No (measurable) effecton mortality

Likely effect on mortalitybut not economical

20%: Unlikely to work Extreme, 90%: Likely

GS 18

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 19

TAC limitations

Virtue

• Direct limitation of fishing for given species

Problems?

• Uncertainty in estimating correct TAC

• Species mis-match

• Discards

• Enforcement

F at 75% of Fcrash: Unlikely Extreme F at 10% of Fcrash: Likely

Major problem

Major problem

Major issue

GS 20

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

In toto: Major problemsNeed spatial and multispecies models for understanding

GS 21

Effort regulationVirtue• Direct limitation of all effect on the ecosystem

Problems?• Effort reallocation species/components• Efficiency/catchability trends• Only guaranteed to work if very low effort allowed• Vessel classes?• Enforcement?

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

Major problem

Major problem

Major issue

GS 22

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

In toto: Major problems

GS 23

Mesh sizes (gear limitations)

Virtue

• Direct limitation of fishing for juveniles or...

• May make fishery sustainable

• Pacifier

Problems?

• Slippage mortality?

• Often negligible effect

• Enforcement

Major issue

GS 24

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 25

Back to models:Model requirements

• Uncertainty: – Better statistical models

• Areal closures: – Spatial models

• Effort control, analysis: – Spatial models– Multispecies, technical

interactions

• TAC control: – Multispecies, technical

interactions

• Understanding any controls: – Need to estimate effect of

major change in predator on prey abundance and vice versa

– Multispecies, biological interactions

GS 26

Effects of current model output

• Uncertainty output: – 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

• Almost all analyses:– Need lower F

GS 27

Limitation summary

• TAC: Species allocation mismatch+uncertainty

• Closed area: Migration/fishing outside+uncertainty

• Effort control: Effort reallocation+catchability

• Fleet reduction alone: Like effortCommon effects of levels of measures:

10% reductions: No effects

50% reduction: Some effect likely but can be negated

90% reductions: Almost sure effects but may lose catches

GS 28

Source of problemsEffort control:

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

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

Closed area: – Correct area uncertain– Fleet too large– Dormant effort– Area too small

GS 29

Solutions?

Extreme measures?

or

Combined systems?

or

?

No single system, set at its target will suffice in general!

GS 30

Combinations

• TAC + Effort control!

• TAC + Closed area?

• Effort control + closed area ?

• + Fleet reduction!!

GS 31

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 32

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.