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

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.

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

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

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

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

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

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

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

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

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

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

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

Variability depending on F and Selectivity CV Stock CV Yield

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)

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” ??