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C. Pilotage, Bordeaux, 15 décembre 2005 Indicang – Interreg III « Yellow eel surveys » Pascal LAFFAILLEThomas CHANGEUX Christian RIGAUD CST, Bordeaux,

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Presentation on theme: "C. Pilotage, Bordeaux, 15 décembre 2005 Indicang – Interreg III « Yellow eel surveys » Pascal LAFFAILLEThomas CHANGEUX Christian RIGAUD CST, Bordeaux,"— Presentation transcript:

1 C. Pilotage, Bordeaux, 15 décembre 2005 Indicang – Interreg III « Yellow eel surveys » Pascal LAFFAILLEThomas CHANGEUX Christian RIGAUD CST, Bordeaux, 14 décembre 2005

2 C. Pilotage, Bordeaux, 15 décembre 2005 The yellow stage : a long and diversified growth phase between 2 short migratory phases The yellow local« stock » : links with the local colonization level and the further downstream migration Glass eels Silver eel 6 months 3 to 20 years (or more) Some recalls… Within a catchment area, two compartments with very different characteristics and functionings : - the estuary (below the tidal limit ) - the river compartment (above the tidal limit)

3 C. Pilotage, Bordeaux, 15 décembre 2005 To take into account economical considerations and to set up durable surveys and analysis, some basic rules : maximal valorization of the available informations from qualitative or semi-quantitative surveys already performed even if they are not specific To perform complementary operations only if we judge that these new collected data are absolutely needful. To try quantify only within the catchments or areas where it’s technically possible.

4 C. Pilotage, Bordeaux, 15 décembre 2005 It seems important, to federate our thoughts, that we identify the two main questions (targets) the indicators we ‘ll elaborate, have to contribute to answer: Q1 : What are the characteristics of the eel presence within a given catchment area ? Q2 : Are the characteristics of this catchment area favourables for the eel ? With two main targets - to contribute to the initial diagnosis and then - to contribute to the evaluation of the impacts of the management actions decided and performed to improve the initial situation.

5 C. Pilotage, Bordeaux, 15 décembre 2005 Using the available data, some preliminary identifications - Colonizable area : defined by upper limits pragmatically fixed - Colonized area : with eel presence (whatever the observed sizes) - Active area : with presence of young stages (<15 or<30 cm) Diagnosis of the eel state within a catchment area Example : Gironde catchment Data from the French multispecific survey network (RHP) from CSP

6 C. Pilotage, Bordeaux, 15 décembre 2005 Sampling sites  Sampling within shallow sites (on the main axis or within tributaries) with a good fishing efficacy, accessible and well used by the small eels if they are present at this part of the catchment Different items  Abundance index  Biomass index If necessary to specify the situation along some axis, to perform complementary sampling operations

7 C. Pilotage, Bordeaux, 15 décembre 2005 Diagnosis of the eel state within a catchment area Yellow Eel abundances

8 C. Pilotage, Bordeaux, 15 décembre 2005 Within the deep compartments (estuary, main rivers) If presence of a fishery, different useful data may be collected : catch per unit of effort for each type of gear regular observations of the biological characteristics of the captures (size, ocular diameters, weigth) tests for mortality rates estimation (size structure analysis) Diagnosis of the eel state within a catchment area Yellow Eel abundances

9 C. Pilotage, Bordeaux, 15 décembre 2005 Different targets concerning the further yellow eel surveys  Evolution trends : Ok if positive and regular trends are observed for all the life stages (glass eels, then yellow and silver eels) Asked Questions : Amelioration rate (slope) ? / How to fix the good limits to be attained ? (theorical limits, historical limits ?……)  Silver escapment estimation : Trends are not sufficient, management has to obtain the escapment of 40 % of the silver eels that would be produced by the watershed only with natural mortality (without fishery, turbins, pollutions,…..). Observations : - Natural mortality is not known for each life stage in the different environments - The amounts for the fisheries may be announced in « equivalent glass-eels » or in « equivalent silver eels »  Biomass targets : Management has to obtain the escapment of a given biomass of spawners, estimated from the characteristics of the watershed (1, 5, 10, 20, ….50 Kg/ha/an?)

10 C. Pilotage, Bordeaux, 15 décembre 2005 Evolution survey of the eel state within a catchment area Evolution trend surveys

11 C. Pilotage, Bordeaux, 15 décembre 2005 fort moyen faible Recrutement fluvial et stock d’anguille jaune en place estuaireaval amont continent Colonization index

12 C. Pilotage, Bordeaux, 15 décembre Distance en km à la limite tidale Densités relatives d’anguille D_PLUS450 D_450 D_300 D_150 Densités relatives (ind/EPA) en Loire Diagnosis of the eel state within a catchment area

13 C. Pilotage, Bordeaux, 15 décembre 2005  Evaluation of the restoration level of the yellow eel « stock » in a given area (catchment, subcatchment,…) by survey of the evolution trend or by comparison with historical or theoretical levels Identification on each site of the maximal observed abundance (MOA = level 100 %). Similar analysis with the median level or the quantile 90  Transformation on each site of the annual observed abundances into % of the MOA or of the median level  Non linear analysis of these indexes. Analysis of the mean trend (all sizes included or by size class) all over the catchment or by groups of sites Preliminary constats : very difficult comparisons between stations (type, localization), operations (used gear, period, strategy),… So, intra-station analysis of the observed evolution trend by size group

14 C. Pilotage, Bordeaux, 15 décembre 2005 Example : Loire catchment area. Evolution trends (all sizes comprised between 1995 and 2003). Comparison with the Maximal observed abundance Augmentation (15%) Stabilisation (19%) Tendances : Laffaille, Lasne, Baisez, Steinbach, Vigneron, Comité Scientifique COGEPOMI Loire Absence (40%) Diminution (28%)

15 C. Pilotage, Bordeaux, 15 décembre 2005 Use as a local index Observed densities increase after an eel ladder setting Laffaille, Lasne, Baisez, Steinbach, Vigneron, Comité Scientifique COGEPOMI Loire

16 C. Pilotage, Bordeaux, 15 décembre 2005 Some very important complementary informations to perform the data analysis To know the local evolution of the number of glass-eels entering into the catchmant area : N total at the estuary mouth, Fishery surveys Specific operations Extrapolation from neighbouring catchments N fluvial Specific surveys Eel ladders …

17 C. Pilotage, Bordeaux, 15 décembre / Basic characteristics : watershed area, river length, estuary area, slope, mean flow, importance of wetlands Different data are very important to characterize a given watershed as an production area of eel breeders Diagnosis of each watershed characteristics Analyse Cemagref

18 C. Pilotage, Bordeaux, 15 décembre 2005 Growth context : Combination between « Water quality (especially heavy metals, PCB, brominated flame retardants, available habitats, …..) » Colonization context (physical and fishing constraints): Combination between « % river length without dams, Nb of dams/km, Nb eel ladders /km, fishing pressure,… » Downstream migration context : Combination between « % river length without turbins, Nb of turbins/km, Types of turbins,…… » Moreover, several indicators have to be elaborated These indicators may be useful to synthetically describe a catchment (or sub-catchment) area for comparisons between systems, diagnosis or evolution survey.

19 C. Pilotage, Bordeaux, 15 décembre 2005 Indicang – Interreg III « Yellow eel surveys » P. LAFFAILLETh. CHANGEUX Ch. RIGAUD Thanks for your attention


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