c. pilotage, bordeaux, 15 décembre 2005 indicang – interreg iii « yellow eel surveys » pascal...

C. Pilotage, Bordeaux, 15 décembre 2005

Indicang – Interreg III

« Yellow eel surveys »

Pascal LAFFAILLE Thomas CHANGEUXChristian RIGAUD

CST, Bordeaux, 14 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 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)


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.


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.


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


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

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

Yellow Eel abundances

Example :

Yellow eel presence index in 2000 within the Loire catchment area

> 10 ind / 100 m2 (5%)

2.5 à 10 ind / 100 m2 (9%)

Laffaille, Lasne, Baisez, Steinbach, Vigneron, 2004. Comité Scientifique COGEPOMI Loire

< 0.1 ind / 100 m2 (51%)

0.1 à 2.5 ind / 100 m2 (35%)


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)


Yellow Eel abundances

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?)


Nantes

Limoges

Poitiers

Angers

Laval

Le Mans

Saint-Etienne

Le Puy

Bourges

Besancon

Chateauroux

Tours

Orleans

Nancy

Clermont-Ferrand

*Limites arbitraires

Recrutement *

Recrutement *

Recrutement *

Bassin versant (115 000 km²)

Zones inaccessibles (19000 km²)

= Surface colonisable(96000 km²)

-

+

-

Evolution survey of the eel state within a catchment area

Evolution trend surveys


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estuaire aval amont

continent

Colonization index


-20 30 80 130

Distance en km à la limite tidale

0.0

0.2

0.4

0.6

0.8

1.0D

ensi

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D_PLUS450D_450D_300D_150

Densités relatives (ind/EPA) en Loire



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


Example : Loire catchment area. Evolution trends (all sizes comprised between 1995 and 2003).

Comparison with the Maximal observed abundance

Augmentation (15%)

Stabilisation (19%)

Tendances 1995-2003 :


Absence (40%)

Diminution (28%)


Use as a local index

Observed densities increase after an eel ladder setting



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

…


1/ 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

ÊÚ

Garonne

Dordogne

Bec d'Ambès

Distance au Bec d'Ambès (en km)0- 5051 - 100101 - 150151 - 200201 - 250251 - 300301 - 350351 - 400401 - 450451 - 500501 - 550551 - 600601 - 650

0 100 200 Kilomètres

N

EW

S

Carte des classes de distance par rapport au Bec d'Ambèsdes cours d'eau du bassin versant Garonne-Dordogne

Carte6: Distance par rapport au Bec d'Ambès du réseau hydrographique simplifié du bassin versant Garonne-Dordogne.Source: Agence de l'eau

Analyse Cemagref


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.


Indicang – Interreg III

« Yellow eel surveys »

P. LAFFAILLE Th. CHANGEUXCh. RIGAUD

Thanks for y

our

attentio

n

c. pilotage, bordeaux, 15 décembre 2005 indicang – interreg iii « yellow eel surveys » pascal...

Documents

eel presence

catchment area favourables

given catchment area

tidal limit slide

fixed colonized area

observed sizes active

yellow local stock

yellow stage