Population dynamics modelsState of art and future of

modelling fish populations

Gunnar StefanssonMarine Research Institute/Univ. Iceland

Fisheries management• Advice

– (On annual quotas)– On long-term utilisation– On control systems

• Implementation– Short-term (tactics)– Systems (strategy)

• Interaction between system and advice

Single species/stock

Conclusion: Low F in long term

0

0,5

1

1,5

2

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

1,2

1,3

1,4

1,5

0

5

10

15

20

Y/R S/R

F

050

100150200250300350400450

0 500 1000 1500 2000

S

R

Classical models - well known since 1954:

Density dependent growth

Cannibalism

...

Missing:

Age- and time-variable natural mortality

Food supply effect

Assumptions - examples of testing

Density dependent growth

Cannibalism

Time- and age-dependent natural mortality

Effects of food supply

Effects of uncertain assessments

Environmental variability

... Minor effects on policyConsiderable effects on long-term catch predictions

Capelin

Cod

Minke Fin Humpback

Simple extensions- forward projections

Shrimp

Greyseal

Harbour seal

Used for testing harvest policies

Table 1. Decomposition of total mortality (Z) of cod into components in 1994

1 2 3 4 5 Total 1-3

Grey seal 0.05 0.02 0.02 0.01 0.00 0.09Harbor seal 0.06 0.06 0.04 0.02 0.00 0.16Minke 0.04 0.04 0.04 0.04 0.04 0.13

Predation M 0.15 0.13 0.09 0.08 0.05 0.37

Cannibalism 0.19 0.00 0.00 0.00 0.00 0.19Resid. M 0.16 0.16 0.16 0.16 0.16 0.49

Total M 0.50 0.29 0.26 0.24 0.22 1.06Fishing 0.00 0.00 0.05 0.24 0.45 0.05

Total Z 0.50 0.29 0.30 0.48 0.67 1.10

M: Very easy to test various assumptions

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 spatially explicit models

Motto of the day

There are three kinds of lies:

• lies,

• damned lies and

• statisticsDisraeli

Models - statistics

• Natural variation

• Measurement errors

• Nontrivial effects of incorrect methods...

• Estimation of unknowns

• Prediction of effects with uncertainty

Conclusion: Lower F

Models - current status

• Greater uncertainty than earlier thought

• Multispecies concerns are important

• Statistical techniques essential

• Need holistic models for understanding

Control mechanisms

• Closed areas

• TAC

• Effort regulation

• Mesh sizes (fishing gear limitations)

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

Models and systems

• 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

Results from current models

• 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

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

Solutions?

Extreme measures?

or

Combined systems?

or

?

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

Current theme

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

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.