The spatio-temporal dynamics of eel populations

Transcription

1 The spatio-temporal dynamics of eel populations M. Gatto, P. Melià Politecnico di Milano G. De Leo, D. Bevacqua Università di Parma A. Crivelli Station Biologique de la Tour du Valat

2 Outline Background on eel biology The decline of eels Modelling the continental phase Two case studies: Comacchio and Camargue Exploring optimal management Preliminary results on the oceanic phase 6/24/2008 M. Gatto - Val Savarenche

3 The life cycle of European eel (catadromous fish) 6/24/2008 M. Gatto - Val Savarenche

4 Oceanic Phase Eel egg Leptocephalus Battista Grassi 6/24/2008 M. Gatto - Val Savarenche

5 Oceanic and Continental Phase From leptocephalus to glass eel/elver 6/24/2008 M. Gatto - Val Savarenche

6 European and American eel Johannes Schmidt 6/24/2008 M. Gatto - Val Savarenche

7 The decline of the European eel (Anguilla anguilla) Landings (million tons) Source: FAOSTAT Recruitment (% with respect to average ) Source: ICES (2006) 6/24/2008 M. Gatto - Val Savarenche

8 6/24/2008 M. Gatto - Val Savarenche

9 Possible causes of eel decline changes in oceanic circulation and food availability infection by viruses and parasites (Anguillicola) overfishing habitat disruption chemical contamination (highly polluted eels show lower genetic variability; relationship betw. genetic variability and body growth rate)

10 Possible causes of eel decline Source: Bonhommeau et al. (2008) Fish. Oceanogr. 17: 32-44

11 Possible causes of eel decline Source: Jansen et al. (2007) ICES J. Mar. Sci. 64: Source: Friedland et al. (2007) ICES J. Mar. Sci. 64:

12 Possible causes of eel decline Source: Winter et al. (2007) ICES J. Mar. Sci. 64:

13 Glass eel Elver Nat. Rec. stocking Yellows Ym Yf Life Cycle of Anguilla anguilla (continental phase) males lagoons females Sm Sf Silver Ocean Harvest 6/24/2008 M. Gatto - Val Savarenche

14 Life span quite variable

15 SLIMES project Study areas The Comacchio lagoon La Grande Camargue Etang de Vaccarès Etang de Malagroy 6/24/2008 M. Gatto - Val Savarenche

16 Valli di Comacchio: the lavorieri Harvest : ONLY SILVER EELS 6/24/2008 M. Gatto - Val Savarenche

17 Commercial harvest in the Valli di Comacchio (silver eels) 250 Catch (kg x 1000) /24/2008 M. Gatto - Val Savarenche

18 Commercial harvest in the Camargue Remontée des nasses composant la capetchade Nasses à anguilles La Pointe Courte Sète Both yellow and silver eel fishing Nasses composant la capetchade pour la pêche à l'anguille 6/24/2008 M. Gatto - Val Savarenche

19 The goals To design and calibrate realistic eel models based on an age, sex and size-classification of eels and including environmental variability (stochasticity) To analyze the biological and economic performance of different management strategies 6/24/2008 M. Gatto - Val Savarenche

20 Demographic features of European eel High variability in body growth Sexual dimorphism Skewed sex ratio (many more females in the Comacchio lagoon) Sexual maturation mainly function of body size Survivorship mainly a function of??? 6/24/2008 M. Gatto - Val Savarenche

21 Structure of the demographic model fishing body growth metamorphosis sex differentation Y F S F females R U yellows silver recruitment recruitment undifferentiated Y M S M males 6/24/2008 M. Gatto - Val Savarenche

22 Age Size structure in a cohort Age 11 Expected growth curve Age 5 Age 1 L0 Length 6/24/2008 M. Gatto - Val Savarenche Loo

23 We can simulate the size structure of eels in a given year Age 1 P.D.F. Age 2 Age 3 Age 4 Length 6/24/2008 M. Gatto - Val Savarenche

24 Comacchio: Model calibration and validation Available data for 1989 and 1990 Silver eels from commercial catch at lavorieri ; yellow eels caught in fyke nets We assume constant recruitment to lagoon 1990 data used for validation 6/24/2008 M. Gatto - Val Savarenche

25 Annual survival [%] Frequency (%) Age [years] (a) Results of calibration Fraction metamorphosing Parameter uncertainty: BOOTSTRAP Males Length [cm] Females Eel Recruitment [# ha ] 6/24/2008 M. Gatto - Val Savarenche

26 Fishing eels in the Grande Camargue Both yellow and silver eels are caught Recruitment is quite variable Good data (Tour du Valat) are available Glass eels ( ): length, weight, CPUE Adult eels: length, weight, CPUE ( ), age and sexual maturation (subsample) 6/24/2008 M. Gatto - Val Savarenche

27 Model calibration and validation No constant recruitment assumed Density-dependent elver survival Greater temporal resolution (months vs. years) More accurate description of body growth Females Total data used for validation 6/24/2008 M. Gatto - Val Savarenche

28 The resulting physiology and demography September males females Fraction metamorphosing October November Length (mm) 6/24/2008 M. Gatto - Val Savarenche

29 Density-dependent elver survival 6/24/2008 M. Gatto - Val Savarenche

30 Multi-objective assessment of conservation measures in the Camargue lagoons The European Commission proposed a regulation (COM 2005/472 final) to establish measures for the recovery of the eel stock, with the aim to achieve the escapement to the sea of a 40% of adult eel biomass (with respect to undisturbed conditions) from each river basin. The proposed regulation imposes the closure of fishing activities for 15 days each month until implementation of an approved eel management plan. Our goal: evaluate effectiveness of the measures to be implemented according to at least two (potentially contrasting) criteria: (1) preservation of a sufficient spawner escapement, (2) maintenance of an acceptable harvest by fishermen. 6/24/2008 M. Gatto - Val Savarenche

31 Exploring management alternatives Six effort-limitation rules: (a) keep the present fishing effort (baseline scenario); (b) complete closure of the fishery (an approximation of undisturbed conditions); (c) halve the present fishing effort by imposing a 15-day closure each month (EU regulation in the absence of approved management plans); (d) summer closure (in accordance with fishermen s practice of reducing the fishing effort in this season); (e) autumn closure (to facilitate downstream migration of adult eels); (f) winter closure (in accordance with fishermen s practice of reducing the fishing effort in this season). 6/24/2008 M. Gatto - Val Savarenche

32 Using the model We run the model from the 2003 state of the stock onwards constant eel input ( elvers per year), corresponding to the median recruitment observed between over a 7-year period (i.e., from 2003 until 2010), to achieve stable equilibrium, as the maximum residence time of an eel in the Camargue lagoons is about 6 years with estimated net selectivity N.B. nets currently used in Camargue have a 6 mm mesh in derogation to national law that enforces a 10 mm mesh size for eel fisheries in the rest of France 6/24/2008 M. Gatto - Val Savarenche

33 Performances of different policies no fishing Two objectives 1. maximize the spawner output from the lagoons 2. maximize the harvest by Camargue fishermen current fishery Dash: present fishing effort Bold: EU regulation (15-day closure) a: autumn closure s: summer closure w: winter closure Green dots: 40% adult output 6/24/2008 M. Gatto - Val Savarenche

34 Pareto analysis Filled symbols: Pareto policies Triangles: EU policy Circles: current effort Green shading: policies dominating current fishery Hatched area: nonfeasible policies (40% escapement violated) It is possible to increase the efficiency of the Camargue eel fishery 6/24/2008 M. Gatto - Val Savarenche

35 The oceanic phase J. Schmidt Philosophical Transactions of the Royal Society, Series B. 211: Glass eel age: more than 2 years Based on ocean circulation A.J. Kettle, K. Haines Can. J. Fish. Aquat. Sci. 63: (2006) 6/24/2008 M. Gatto - Val Savarenche

36 Estimating the glass eel age from otoliths Mean age of glass eels: days R. Lecomte-Finiger The early life of the European eel. Nature 370: /24/2008 M. Gatto - Val Savarenche

37 The oceanic model (passive transport ) Start movie

38 Where do successful eels come from? Years explored: no weakening of recruitment

39 The Eeliad/Galathea 3 Project 22 silver eels tagged with archival tags that detach and transmit the data to satellites, 15 tags recovered Eel larvae 6/24/2008 M. Gatto - Val Savarenche

40 Are eels really panmictic? The genetic evidence T. Wirth, L. Bernatchez Nature 409: The time delay theory The multiple areas theory The assortative mating theory 6/24/2008 M. Gatto - Val Savarenche

41 The dream for the future The global model that combines continental and oceanic phases (round trip), genetics and demography, parasitology and toxicology 6/24/2008 M. Gatto - Val Savarenche