EcoTroph: modeling marine ecosystem functioning and impact of fishing

Transcription

1 EcoTroph: modeling marine ecosystem functioning and impact of fishing Didier Gascuel (1,2), Villy Christensen (2), Daniel Pauly (2) 1. Fisheries and Aquatic Sciences Center, Agrocampus Rennes, France 2. Fisheries Centre, University of British Columbia, Canada ECEM7, November 27

2 1 - EcoTroph: principles and basis equations Biomass 1,2,8 Predation Predation A continuous representation of the biomass distribution, according to trophic level τ -> the Biomass Trophic spectrum,6,4,2 Ontogeny Biomass flow 1,5 2, 2,5 3, 3,5 4, 4,5 5, Primary Production Detritus recycling Trophic level τ The ecosystem functioning: a flow of biomass trough trophic levels Don t confuse: the biomass Bτ present in the trophic class [τ, τ+ τ[ the biomass flow Φτ, passing trough the trophic class

3 1 - EcoTroph: principles and basis equations The dynamic of the trophic flow Trophic level A discrete process at the particles level and a continuous model that expresses the mean process 2 1 Time 2, 3, 4, 5, The trophic flow is characterized by: the biomass flow Φτ (in t/year), the speed of trophic flow τ/ t (in TL/year) Biomass τ = Flow τ. Speed τ Bτ = Φτ. τ/ t. τ

4 1 - EcoTroph: principles and basis equations The biomass flow model: Φ(τ + τ) = Φ(τ). e ( µτ + ϕ τ ). τ Biomass flow (t/year) 1 1 Increasing fishing efforts Natural loss. Excretion,. Non pred.mort.. Respiration e µ =Trophic efficiency Selectivity Fishing loss. Exploitation ϕ τ = mf τ. S τ,1,5 TL5=3. 2, 2,5 3, 3,5 4, 4,5 5, Trophic level (TL) 2, 2,5 3, 3,5 4, 4,5 5, Tropic level (TL)

5 1 - EcoTroph: principles and basis equations The speed of the flow model 1. Reference state (current or virgin) For case studies: field estimates For theoretical purposes: an empirical generic model ( τ / t) ref = a x τ - b = 2.2 x e.41 θ x τ (Gascuel et al., sub. Ecol.Mod) Speed of the flow P/B (TL.year-1) Ecopath models n = 1,718 groups r 2 =.54 P/B obs P/B mod 2, 3, 4, 5, Trophic level TL 2. The Top-down equation: ( t / τ) τ = (1-α). Mref τ + α. Mref τ.[ B( τ+1) ] γ Bref(τ+1) + F τ The speed of the flow depends on predators abundance α = : Bottom-up <α<1 : Top-down Higher the catch, lower the life expectancy, higher the speed of transfer.

6 2 - EcoTroph: simulating virtual ecosystems Theoretical analysis of functioning -> generic relationships between parameters Biomass (t) Increasing fishing efforts Bottom-up ecosystem direct impact of fishing indirect impact of the decrease in the preys abundance 5 2, 2,5 3, 3,5 4, 4,5 5, Trophic level (TL)

7 2 - EcoTroph: simulating virtual ecosystems Theoretical analysis of functioning -> generic relationships between parameters Biomass (t) Release of predation Increasing fishing efforts Top-down ecosystem the decrease of predators induces an increase in preys abundance 5 2, 2,5 3, 3,5 4, 4,5 5, Trophic level (TL)

8 2 - EcoTroph: simulating virtual ecosystems Theoretical analysis of functioning -> generic relationships between parameters Biomass (t) 25 Increase in predation A cascade effect Release of predation Increasing fishing efforts 5 2, 2,5 3, 3,5 4, 4,5 5, Trophic level (TL)

9 2 - EcoTroph: simulating virtual ecosystems Theoretical analysis of functioning -> generic relationships between parameters Biomass (t) 25 A resilient ecosystem 2 15 Not trophically-selective fishing efforts 1 5 Predation release 2, 2,5 3, 3,5 4, 4,5 5, Trophic level (TL)

10 2 - EcoTroph: simulating virtual ecosystems Ecosystem biomass decreases with exploitation Top-down ecosystems are more resilient Detritus-based ecosystems are less Predators are severely affected Relative biomass,5 biomass of predators total biomass Top-down ecosystem Bottom-up ecosystem Detritus-based ecosys. Relative biomass,5 1,5 2, Fishing mortality F,5 TL5=3.5 TL5=3. TL5=2.5 Impact of fishing on ecosystem biomass increases when the trophic level of first catch TL5 decreases,5 1,5 2, Fishing mortality F

11 2 - EcoTroph: simulating virtual ecosystems Catch: Y τ = ϕ* τ. Φ* τ. τ Catch (t/year) Accessible flow: Φ*(τ+ τ) = Φ*(τ). e - (µ* τ + ϕ* τ ). τ Increasing fishing efforts 2, 2,5 3, 3,5 4, 4,5 5, Trophic level (TL) With µ* τ : rate of flow loss + flow gain (from inaccessible biomass) Accessible 8 biomass (t) Increasing fishing efforts 3 Wide potential catches on low trophic levels Over fishing of high trophic levels Fishing down the marine food web 2 1 2, 2,5 3, 3,5 4, 4,5 5, Trophic level (TL)

12 2 - EcoTroph: simulating virtual ecosystems Catches (t/year) Top-down ecosystems are more productive Detritus-based are less 3 2 Ecosystem over-fishing occurs for highest fishing mortalities 1 Top-down ecosystem Bottom-up ecosystem Detritus-based ecosystem Catches (t/year) 7,5 1,5 2, Fishing mortality F TL5=2.5 TL5=3. TL5=3.5 Exploiting low trophic levels leads to higher catches and higher ecosystem FMSY 2 1,5 1,5 2, Fishing mortality F but it induces strong biomass depletion and severe over-fishing for top predators

13 3 - EcoTroph: application to a case study Application to the Guinean ecosystem (1985 and 24) Here (but it s not a requirement of EcoTroph), the Ecopath model (35 boxes) is used for estimating the EcoTroph input parameters (B, P/B, and TL) Whales Barrac.+ S.Dem.pr. Grunds+ Mullets+ L.zoopl. Dolphins Sharks+ Carang. L.pelagics Rays+ Bobo Royal.thr. L.Dem.pred Lesser.thr. Croakers Sea.Br. Cephalo. Bathy.inv S.Dem.inv. L.Dem.inv. Horse.mack. S.zoopl. Crust. Ethmalosa Sardin. Benthos Giant.thr. Bathy.pr. Sea.catf. Soles+ 1 Prim.Produc. Detritus

14 Biomass (T/Km2) Catches (T/Km2) 3 - EcoTroph: application to the Guinean ecosystem zoo G zoo G 2,5 Cephalopodes Cephalopodes 2,5 Crustacés Crustacés Bathy-dem inv Bathy-dem inv GUINEA 1985 Bathy-dém pred GUINEA 24 Bathy-dém pred Démersaux inv.p Démersaux inv.p 2, Démersaux 2, inv.gm Démersaux inv.gm Démersaux pred.mp Démersaux pred.mp Démersaux pred.g Démersaux pred.g Soles+ Soles+ Grondeurs+ 1,5 1,5 Grondeurs+ Mulets+ Mulets+ Machoirons Machoirons Dorades+ Dorades+ Capitaine royal Capitaine royal Gros capitaine Gros capitaine Petit capitaine Petit capitaine Bars+ Bars+ Bobo Bobo Sardinelles+,5 Ethmalose,5 Sardinelles+ Ethmalose Chinchards+ Chinchards+ Carangues Carangues Barracudas+ Barracudas+ Grands pélagiques Grands pélagiques Requins+ 2, 2,5 3, 3,5 4, 4,5 5,Raies+ 2, 5,5 2,5 3, 3,5 4, 4,5 5,Requins+ 5,5 Raies+ Trophic level TL Dauphins Baleines Trophic level TL Dauphins Baleines,4,3,2,1 Guinea 1985 Biomass (T/Km2) A synthetic overview A way of thinking at ecosystem scale Industrial fishery Small scale fishery Fishing mortality F Catches (T/Km2),4,3,2,1,8,6,4,2 Guinea 23 Industrial fishery Small scale fishery Fishing mortality F,8,6,4,2 2 2,5 3 3,5 4 4, ,5 3 3,5 4 4,5 5 Trophic level TL Trophic level TL

15 3 - EcoTroph: application to the Guinean ecosystem 1 Biomass Trophic Spectrum Catch Trophic Spectrum Tonnes / Km2,1 Tonnes / Km2, ,5 3 3,5 4 4,5 5 Trophic level 2 2,5 3 3,5 4 4,5 5 Trophic level B / Bref 3, 2, Relative biomass B3 B3.5 B4 B4.5 B5 B / Bref 3, 2, Relative catch Y2,5 Y3 Y4 Y4.5 Y5 mf F multiplier mf F multiplier

16 3 - EcoTroph: application to the Guinean ecosystem 1 Biomass Trophic Spectrum Catch Trophic Spectrum Tonnes / Km2,1 Tonnes / Km2, ,5 3 3,5 4 4,5 5 Trophic level 2 2,5 3 3,5 4 4,5 5 Trophic level B / Bref 3, 2, Relative biomass Biomass decreases consistent with surveys data B3 B3.5 B4 B4.5 B5 B / Bref 3, 2, Relative catch Diagnoses consistent with single-species assessments Y2,5 Y3 Y4 Y4.5 Y F multiplier mf mf F multiplier

17 Conclusion Functioning of marine ecosystems can be conceptualize as a continuous trophic flow, from low to upper trophic levels 2. The EcoTroph model is based on simple assumptions: The biomass flow decreases with trophic levels (according to the trophic efficiency) The speed of the flow is faster in low trophic levels Top-down control: flow kinetics depend on predators abundance Secondary production partly comes from biomass recycling 3. EcoTroph input parameters are calculated as functions of TL (leading to a strong decrease in the number of parameters required) The model leads to a consistent theoretical representation of almost all we already know (yield, biomass, mean TL, cascades, resilience, ) 1. It is complementary to Ecopath for case study analyses (and should be proposed as an additional routine of EwE in few months)

18 Conclusion Functioning of marine ecosystems can be conceptualize as a continuous trophic flow, from low to upper trophic levels 2. The EcoTroph model is based on simple assumptions: The biomass flow decreases with trophic levels (according to the trophic efficiency) The speed of the flow is faster in low trophic levels Top-down control: flow kinetics depend on predators abundance Secondary production partly comes from biomass recycling Grimm, 27 (yesterday before!): The more patterns a model 3. EcoTroph input parameters are calculated as functions of TL (leading reproduces simultaneously, the more to a strong decrease in the number of parameters likely is has required) captured the key elements of the system organisation The model leads to a consistent theoretical representation of almost all we already know (yield, biomass, mean TL, cascades, resilience, ) 1. It is complementary to Ecopath for case study analyses (and should be proposed as an additional routine of EwE in few months)

19 Thanks Study supported by the EU Marie Curie programme MOIF-CT