About a new model of fish size spectra, and fishing impacts

1. About a new model of fish size spectra, and fishing impacts Eric Benoît, Université de La Rochelle Marie-Joëlle Rochet, IFREMER Nantes FRANCE

2. Pope and Knights, 1982

3. Qualitative prediction: fishing changes the slope of size spectra

4. r Log(w) Continuous time-dependent models • Why should size spectra be linear? • needs allometric processes r=awb • How should fishing impact linear size spectra?

5. Allometric losses and generalised growthPlatt and Denmann, 1978

6. Predation, growth and mortalitySilvert and Platt (1980), Beyer (1989)

7. Predation, growth and mortalityDistributed prey size

8. Balance Mortality Growth The equations x=ln(w)

9. Linear spectrum • There is a unique real l such thatis a solution of the model (linear spectrum) • l is not sensitive to the parameters’ values and is consistent with observed values

10. Simulating size spectra • Plankton input • Baseline « reasonable parameters » u Log(w)

11. How do perturbations affect the size spectrum? • Time step = 2 days • 3 years simulations • Oscillatory initial condition • Oscillatory plankton input • Stabilizing processes: fixing large predators / biodiversity / non-predation mortality a b

12. How does fishing impact the size spectrum? a high fishing mortality

13. How does fishing affect the size spectrum? • realistic fishing mortality • time step = 3 days • 10 years simulation • years 5-10 shown f

14. The impact of fishing on size spectra • If biomass flux driven by size-dependent predation: • Fishing decreases linearity • Slope • has less meaning • cannot be well estimated • is not a good indicator.