1. MSFD Descriptor 3 population age and size distribution indicative of a healthy stock
Gerjan Piet & Thomas Brunel

2. MSFD Descriptor 3
Populations of all commercially exploited fish and shellfish are within safe biological limits,
exhibiting a population age and size distribution that is indicative of a healthy stock.
Two issues related to this criterion were studied:
Is it possible to identify “a population age and size distribution that is indicative of a healthy stock” and hence determine a reference level for GES
Is it possible to manage the age-structure, independent from the regulation of stock abundance through Fishing mortality.

3. Operating model for Stock Reproductive Potential
Age structured population model parameterised for three species (i.e. North Sea plaice, herring and cod) using information from stock assessments
Fecundity of fish per unit of weight is often higher for larger/older females than for smaller/younger ones
Egg survival chances may also be age-dependent
Total (viable) Egg Production (TEP) is more sensitive to age- structure than SSB. Therefore, TEP was used to represent Stock Reproductive Potential
Create populations with different age-/size structure through manipulation of
Fishing mortality
Size selectivity

4. Age-/Size distribution depending on F
Show resulting size structure according to indicators: pold and spmeanage. Conclusion: selectivity and F determine structure based on fig 2

5. Selectivity-at-age
Contrast protect old versus protect young. Based on fig 1

6. Age-/Size distribution depending on Selectivity
Good?
health
Bad?
health
Good?
health
Show resulting size structure according to indicators: pold and spmeanage. Conclusion: selectivity and F determine structure based on fig 2
Bad?
health

7. “Healthy” ?? Resistance Not easily perturbed
Slow response to perturbation
Resilience
Ability of a system to return to normal
Speed of return to equilibrium

8. Resistance to fishing-induced perturbation Increase F from MSY to lim=
Time (yr) to reach
limit value of
Total Egg Production
Based on Total Egg Production (TEP) as best proxy for Total Reproductive Potential (TRP). Time to reach TEP lim
Size structure has no effect but selectivity does
Anthropogenic perturbation=mortality

9. Resilience after cessation perturbation Decrease F from lim to MSY=
Time (yr) to return
to initial state
Based on Total Egg Production (TEP) as best proxy for Total Reproductive Potential (TRP). Time to reach TEP lim
Size structure has no effect but selectivity does
Anthropogenic perturbation=mortality

10. Resistance to environmental perturbation 4 years of low recruitment=
Time (yr) to reach
limit value of
Total Egg Production
Based on Total Egg Production (TEP) as best proxy for Total Reproductive Potential (TRP). Time to reach TEP lim
Size structure has no effect but selectivity does
Anthropogenic perturbation=mortality

11. Summary 1st part
No consistent results of better health in terms of the response to fishing-induced perturbation but increased resistance to environmental perturbation
Selectivity has much bigger effect: Protect old results in higher resistance but slower resilience: less variability
Selectivity has more effect than fishing mortality when it comes to resistance to environmental perturbation: apply appropriate selectivity depending on situation

12. Management towards “optimal exploitation” through manipulation of Fishing mortality Size selectivity

13. Yield: MSY per selectivity
Annual yield at FMSY at current protectyoung and protectold selectivities showing highest yield for the protectyoung but consisting of relatively smaller fish which has consequences for the revenue. Variability in yield lower than current. At mortalities above MSY the variability issue becomes more pronounced

14. Variability depending on F and Selectivity
CV Stock
CV Yield

15. Summary 2nd part Selectivity has a large effect on catches at MSY
Protect
Yield
Relative to
current
MSY
Size of
fish CV
stock
yield
Old
-32%
Large
(> 4 yr)
Small
(> current)
Young
+91%
(< 4 yr)
High
(± pristine)

16. Conclusions
This study has not resolved what constitutes a “population age and size distribution that is indicative of a healthy stock”
Therefore it is not possible to suggest any target for GES
From a management perspective the selectivity pattern of the fishery would be the preferred criterion instead of the age-/size distribution
Largest effect on the resistance to environmental perturbation
Large effect on yield (overall and variability)
Easier to monitor selectivity (pressure) than age-/size structure of the stock (state).
Modify the 3rd criterion to “selectivity indicative of an optimal exploitation” ??