Download Population dynamics models Issues and state of art in modelling

Population dynamics models
State of art and future of
modelling fish populations
Gunnar Stefansson
Marine 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
Y/R
S/R
2
20
1,5
15
1
10
1,5
1,4
1,3
1,2
1,1
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0
0,2
0
0,1
5
0,0
0,5
F
450
400
350
300
250
R
200
150
100
50
0
0
500
1000
1500
Classical models - well known since 1954:
Density dependent growth
Cannibalism
...
Missing:
Age- and time-variable natural mortality
Food supply effect
2000
S
Conclusion: Low F in long term
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 policy
...
Considerable effects on
long-term catch predictions
Simple extensions
- forward projections
Minke
Fin
Humpback
Used for testing
harvest policies
Grey
seal
Cod
Capelin
Harbour
seal
Shrimp
M: Very easy to test various assumptions
Table 1. Decomposition of total mortality (Z) of cod into components in 1994
1
2
3
4
5 Total 1-3
Grey seal
Harbor seal
Minke
0.05
0.06
0.04
0.02
0.06
0.04
0.02
0.04
0.04
0.01
0.02
0.04
0.00
0.00
0.04
0.09
0.16
0.13
Predation M
0.15
0.13
0.09
0.08
0.05
0.37
Cannibalism
Resid. M
0.19
0.16
0.00
0.16
0.00
0.16
0.00
0.16
0.00
0.16
0.19
0.49
Total M
Fishing
0.50
0.00
0.29
0.00
0.26
0.05
0.24
0.24
0.22
0.45
1.06
0.05
Total Z
0.50
0.29
0.30
0.48
0.67
1.10
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
• statistics
Disraeli
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:
• TAC control:
– Multispecies, technical
interactions
• Understanding any
controls:
• Effort control, analysis:
– Spatial models
– Spatial models
– Multispecies, technical
interactions
– 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
• 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
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
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.