Water quality - Sampling of fish with multi-mesh gillnets

This European Standard describes a standardised method for sampling fish in lakes, using multi-mesh gillnets. The method provides a whole-lake estimate for species occurrence, quantitative relative fish abundance and biomass expressed as Catch Per Unit Effort (CPUE), and size structure of fish assemblages in temperate lakes. It also provides estimates, which are comparable over time within a lake and between lakes. This European Standard provides information on sampling routines, data handling and reporting, sampling of fish for age and growth analyses as well as applications and further treatment of data. Selected references in support of this European Standard are given in the Bibliography.

Wasserbeschaffenheit - Probenahme von Fisch mittels Multi-Maschen-Kiemennetzen

Diese Europäische Norm beschreibt ein genormtes Verfahren zur Probenahme von Fischen in Seen mit Multi-Maschen-Kiemennetzen. Dieses Verfahren liefert Daten zur Abschätzung des Artenspektrums im gesamten See, eine quantitative Abschätzung der relativen Artenhäufigkeit sowie der Biomasse, ausgedrückt als Einheitsfang (Catch Per Unit Effort, CPUE) und der Größenverteilung von Fischbeständen in Seen gemäßigter Zonen. Weiterhin liefert sie Abschätzungen, die für zeitliche Vergleiche innerhalb eines Sees und zwischen verschiedenen Seen geeignet sind. Diese Europäische Norm stellt Informationen für routinemäßige Probenahmen, den Umgang mit Daten und deren Protokollierung, die Probenahme von Fischen zur Alters- und Wachstumsbestimmung sowie Anwendungsmöglichkeiten und weitere Datenbehandlung zur Verfügung. Im Literaturverzeichnis sind ausgewählte Arbeiten angegeben, die direkten Bezug zur Anwendung dieser Europäischen Norm haben.

Qualité de l'eau - Echantillonnage des poissons à l'aide de filets maillants

La présente Norme européenne décrit une méthode normalisée pour l'échantillonnage des poissons dans les lacs, à l'aide de filets maillants multimailles. La méthode fournit une estimation, à l'échelle d'un lac, de l'occurrence des différentes espèces, de leur abondance relative, numérique et pondérale, exprimée en capture par unité d'effort (CPUE), et de la structure en taille des peuplements de poissons dans les lacs tempérés. Elle fournit également des estimations comparables dans le temps au sein d'un même lac ou entre plusieurs lacs. Elle donne aussi des informations concernant les routines d'échantillonnage, le traitement et la communication des données, l'échantillonnage des poissons en vue de l'analyse de l'âge et de la croissance, ainsi que les applications et le traitement ultérieur des données. La bibliographie présente une sélection d'ouvrages se rapportant à cette norme.

Kakovost vode – Vzorčenje rib s pomočjo zabodnih mrež (gillnet)

General Information

Status
Withdrawn
Publication Date
16-Aug-2005
Withdrawal Date
26-May-2015
Technical Committee
Drafting Committee
Current Stage
9960 - Withdrawal effective - Withdrawal
Completion Date
27-May-2015

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SLOVENSKI STANDARD
SIST EN 14757:2005
01-december-2005
.DNRYRVWYRGH±9]RUþHQMHULEVSRPRþMR]DERGQLKPUHå JLOOQHW
Water quality - Sampling of fish with multi-mesh gillnets
Wasserbeschaffenheit - Probenahme von Fisch mittels Multi-Maschen-Kiemennetzen
Qualité de l'eau - Echantillonnage des poissons a l'aide de filets maillants
Ta slovenski standard je istoveten z: EN 14757:2005
ICS:
13.060.70 Preiskava bioloških lastnosti Examination of biological
vode properties of water
65.150 Ribolov in ribogojstvo Fishing and fish breeding
SIST EN 14757:2005 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 14757:2005
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SIST EN 14757:2005
EUROPEAN STANDARD
EN 14757
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2005
ICS 13.060.70; 65.150
English Version
Water quality - Sampling of fish with multi-mesh gillnets

Qualité de l'eau - Echantillonnage des poissons à l'aide de Wasserbeschaffenheit - Probenahme von Fisch mittels

filets maillants Multi-Maschen-Kiemennetzen
This European Standard was approved by CEN on 27 June 2005.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European

Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national

standards may be obtained on application to the Central Secretariat or to any CEN member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation

under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official

versions.

CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,

Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,

Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels

© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 14757:2005: E

worldwide for CEN national Members.
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SIST EN 14757:2005
EN 14757:2005 (E)
Contents page

Foreword ..........................................................................................................................................................3

Introduction......................................................................................................................................................4

1 Scope ...................................................................................................................................................5

2 Normative references .........................................................................................................................5

3 Terms and definitions.........................................................................................................................5

4 Principle...............................................................................................................................................5

5 Sampling design and equipment .......................................................................................................5

6 Time series sampling..........................................................................................................................7

7 Inventory sampling ...........................................................................................................................10

8 Sampling routine...............................................................................................................................11

9 Data handling and reporting ............................................................................................................12

10 Corrections for gillnet selectivity for six fish species....................................................................15

11 Estimate of sampling variance.........................................................................................................17

12 Sampling fish for age- and growth analysis ...................................................................................18

13 Applications and further analyses...................................................................................................20

14 Limitations and supplementary sampling.......................................................................................21

Annex A (informative) Distribution of benthic multi-mesh gillnets at different depth strata in

lakes with different area and maximum depth................................................................................22

Annex B (informative) Example of forms for registration of fish and supplementary data ....................24

Bibliography...................................................................................................................................................27

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EN 14757:2005 (E)
Foreword

This European Standard (EN 14757:2005) has been prepared by Technical Committee CEN/TC 230 “Water

analysis”, the secretariat of which is held by DIN.

This European Standard shall be given the status of a national standard, either by publication of an identical

text or by endorsement, at the latest by February 2006, and conflicting national standards shall be withdrawn

at the latest by February 2006.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following

countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic,

Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland

and United Kingdom.
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EN 14757:2005 (E)
Introduction

This is the second of several European Standards developed for evaluation of the species composition,

abundance and age structure of fish in rivers, lakes and transitional waters. Other standards describe

“Sampling of fish with electricity” (EN 14011) and “Guidance on the scope and selection of fish sampling

methods” (prEN 14962).

In most countries the use of the method specified in this European Standard requires permits from landowners

and national or regional authorities. In many countries permits are also required from authorities for animal

rights and animal welfare demands. Both fish diseases and diseases specific for other organisms, such as

freshwater crayfish, may be spread by placing equipment contaminated with pathogens or parasites in the

lake. The user of this method should check which national legislation is applicable.

WARNING — Persons using this European Standard should be familiar with normal laboratory and

fieldwork practice. This European Standard does not purport to address all of the safety problems, if

any, associated with its use. It is the responsibility of the user to establish appropriate safety and

health practices and to ensure compliance with any national regulatory conditions.

IMPORTANT — It is absolutely essential that tests conducted according to this standard be carried

out by suitably trained staff.
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EN 14757:2005 (E)
1 Scope

This European Standard describes a standardised method for sampling fish in lakes, using multi-mesh gillnets.

The method provides a whole-lake estimate for species occurrence, quantitative relative fish abundance and

biomass expressed as Catch Per Unit Effort (CPUE), and size structure of fish assemblages in temperate

lakes. It also provides estimates, which are comparable over time within a lake and between lakes. This

European Standard provides information on sampling routines, data handling and reporting, sampling of fish

for age and growth analyses as well as applications and further treatment of data. Selected references in

support of this European Standard are given in the Bibliography.
2 Normative references

The following referenced documents are indispensable for the application of this European Standard. For

dated references, only the edition cited applies. For undated references, the latest edition of the referenced

document (including any amendments) applies.

prEN 14962:2004, Water quality — Guidance on the scope and selection of fish sampling methods

3 Terms and definitions

For the purposes of this European Standard, the terms and definitions given in prEN 14962:2004 apply.

4 Principle

The sampling procedure is based on stratified random sampling. The sampled lake is divided in depth strata

and random sampling is performed within each depth stratum. Sampling of benthic fish is performed with

specially designed multi-mesh gillnets which are 30 m long and 1,5 m deep. The gillnets are composed of 12

different mesh-sizes ranging between 5 mm to 55 mm knot to knot following a geometric series. Gillnets used

for sampling pelagic fish are 27,5 m long and 6 m deep, with the smallest mesh-size being 6,25 mm. The

sampling effort needed to allow detection of 50 % changes in relative abundance between sampling occasions,

range between 8 gillnets per night (efforts) for small, shallow lakes, up to 64 efforts for lakes of about 5 000 ha.

When less accurate estimates of abundance are needed, an inventory sampling procedure may be used,

thereby reducing the number of efforts needed.
5 Sampling design and equipment
5.1 Sampling design

Fish are not randomly distributed over a lake. Depth distribution varies between fish species and with the

ontogeny of the fish. The horizontal distribution may also be influenced by habitat heterogeneity. Neither is the

distribution constant over the year, but will vary with temperature and season.

To cope with this uneven distribution a stratified random sampling design is used. The lake is stratified in

depth strata and a random sampling is performed within each depth stratum. Each gillnet is placed to

represent an independent sample of the fish assemblage. By randomising the location of each gillnet within

each depth stratum and by randomising the angle of the gillnet in relation to shoreline, an independent sample

of the fish in each stratum will be achieved. Randomisation is performed prior to fishing by the aid of depth

maps and a co-ordinate grid. If needed, the angle of the gillnet in relation to the shoreline shall be adjusted so

that the gillnet is within the corrected depth stratum.
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5.2 Benthic gillnets

The multi-mesh gillnets have been designed for catching all types of freshwater fish species. Each gillnet is

composed of 12 different mesh-sizes ranging from 5 mm to 55 mm (knot to knot). The mesh-sizes follow a

geometric series, with a ratio between mesh-sizes of about 1,25. All gillnets have the same order of mesh

panels (see Table 1).

If experience has shown that large fish of certain species (e.g. bream, pike, tench) are difficult to catch with

the mesh sizes shown in Table 1, then these may be modified as required. However, a note on such

modification shall be given in the report (fishing protocol).

Table 1 — Mesh-size distribution (knot to knot) and thread diameter in the multi-mesh benthic gillnets

Mesh no Mesh size Thread diameter
mm mm
1 43 0,20
2 19,5 0,15
3 6,25 0,10
4 10 0,12
5 55 0,25
6 8 0,10
7 12,5 0,12
8 24 0,17
9 15,5 0,15
10 5 0,10
11 35 0,20
12 29 0,17

The gillnets shall be made out of homogeneous, uncoloured nylon. Each gillnet shall be 30 m long and 1,5 m

deep. Each mesh panel shall be 2,5 m long and mounted on a 30 m long buoyancy line (with a recommended

linear density in water of 6 g/m), and a 33 m long lead line (recommended linear density in air 22 g/m and in

water 9,9 g/m) made out of plastic in light grey colour. The diameter of the thread varies between 0,10 mm for

the 5 mm mesh, to 0,25 mm for the 55 mm mesh (Table 1). All mesh panels are commercially available. The

hanging ratio is 0,5 for all mesh sizes.
5.3 Pelagic gillnets

Each pelagic gillnet is 27,5 m long and 6 m deep. Gillnets used for sampling pelagic habitats are similar to the

benthic gillnets with the following exception. The smallest mesh (5 mm) has been excluded, because it has

not been possible to manufacture 5 mm panels mesh as deep as 6 m. The buoyancy line is 30 m, and the

lead line is 45 m with a hanging ratio of 0,5. The weight of the lines may be different from that of the benthic

nets. The pelagic nets are divided in half at 3 m depth by inserting a darkish colour (e.g. made of spun nylon)

in order to separate the catches below and above 3 m depth.
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5.4 Time for sampling

The result of fish sampling using passive gears is highly influenced by water temperature, life history and time

for spawning of specific fish species. Therefore, the sampling period has to be chosen in such a way that each

single species is neither over nor under-represented in the catch. This means that the optimal sampling period

may differ between countries and regions. To minimise between-year variation, due to differences in activity

between species, the sampling period should be defined for each lake or region to be sampled in order to

make sampling data between different lakes and years comparable.

For instance, in northern Europe the sampling should normally take place between July 15 and August 31.

During this period most freshwater fish species living in lakes do not spawn, and the epilimnion temperature

usually exceeds 15 °C in most non-alpine areas. Due to decreasing epilimnion water temperature in

September it is not recommended to prolong the sampling period as the catch may decline substantially when

epilimnion temperature drops below 15 °C. Some species, and especially cyprinids, may also change their

behaviour during autumn, thereby affecting the representativity of the sampling. When it is known that the

catch is good for the species present even at temperatures down to 10 °C, then the sampling season may be

extended until September 15.
5.5 Sampling period

The setting time for the gillnets should ensure that the activity peaks of each fish species will be included. It

should also be so short that the fish does not degrade nor will be damaged by predatory fish while being

caught in the gillnet. This usually means that the gillnets should be set before dusk and lifted after dawn. To

avoid calculating abundance relative to hours of setting time, a standard fishing period of 12 h is

recommended. This is accomplished by setting the gillnets between 6 p.m. and 8 p.m. and lifting the nets

between 6 a.m. and 8 a.m.

In highly productive lakes with abundant fish populations, it may be necessary to shorten the setting time.

Otherwise the gillnets (or at least some mesh-panels in the gillnets) may be saturated with fish, thereby

affecting the outcome of the sampling. Saturation might bias the catch when more than 0,12 kg fish per m in

a 19 mm mesh, or 0,34 kg per m in a 70 mm mesh, are caught. Assuming a random distribution of fish over

all mesh-sizes, this means that saturation in a multi-mesh gillnet may start to affect the outcome when about a

6 kg fish is caught. In such cases, it is recommended to calculate the catch per unit effort (CPUE) relative to

hours of fishing time.
5.6 Gillnet selectivity

Correction factors for gillnet selectivity of the multi-mesh gillnets have been estimated for six fish species. The

sampling method provides abundance estimates only for fish larger than about 50 mm total length of fish

species catchable in gillnets. Abundance estimates of some less catchable species, such as eel (Anguilla

anguilla), burbot (Lota lota), bullhead (Cottus sp.) and pike (Esox lucius), as well as small Y-O-Y (young of the

year) individuals, may be underestimated.
6 Time series sampling
6.1 Sampling effort (gillnet-nights)

When the sampling aims at (1) quantifying relative abundance or biomass of different fish species, and (2)

comparing differences over time and between lakes, the variance of the estimate of the mean has to be

quantified. All fish should have the same probability of getting caught in a gillnet, and, therefore, a

representative sampling in a lake shall be performed. The number of gillnets used at each sampling occasion

is determined both by the minimum number of efforts needed to catch all catchable fish species and by the

required precision of the mean value. Usually the number of efforts needed to catch all catchable fish species

is lower than the number of efforts (net-nights) required to provide an acceptable precision of the estimate.

A commonly used minimum requirement for time series sampling has been to detect a 50 % difference

between sampling occasions in relative abundance of the most abundant fish. The amount of gillnet-nights

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needed is determined by the precision, the lake area and the maximum depth of the lake. The higher the

desired precision is, and the larger and the deeper the lake is, the higher is the number of gillnet-nights. The

number of gillnets required to achieve a precision, which makes it possible statistically to determine a 50 %

difference between sampling occasions, is given in Table 2. By convenience the lakes have been divided into

six size classes (≤ 20, 21 ha to 50 ha, 51 ha to 100 ha, 101 ha to 250 ha, 251 ha to 1 000 ha, 1 001 ha to

5 000 ha), and the number of efforts based on multiples of 8, which is a normal workload for a one night

sampling made by two persons.

Table 2 — Number of efforts with benthic gillnets required to allow the detection of 50% changes

between sampling occasions in relation to lake area and maximum depth
Depth (m) Lake area (ha)
≤ 20 21 to 50 51 to 100 101 to 250 251 to 1 000 1 001 to 5 000
0 to 5,9 8 8 16 16 24 24
6 to 11,9 8 16 24 24 32 32
12 to 19,9 16 16 24 32 40 40
20 to 34,9 16 24 32 40 48 56
35 to 49,9 16 32 32 40 48 56
50 to 74,9 40 40 56 64
≥75 56 64

For small (<10 ha) and shallow lakes even 8 nets could overexploit the fish community, and especially deplete

the reproducing stock of certain species too much. The sampling effort should, however, never be less than 4

gillnets (see also 7.4).

Whole-lake estimates of the relative fish abundance in lakes larger than 5 000 ha usually require such a large

effort that it is practically impossible to use the recommended technique. In cases when larger lakes shall be

sampled, it is recommended that the lake is divided into separate basins, and that each basin is treated as a

separate lake. In large lakes, where whole-lake estimates of the fish fauna are not of main priority, sampling

can be performed at specific stations.

Stratification of gillnets is basically related to depth. The principles for depth stratification are given below. In

lakes with vegetation cover and in large shallow lakes, other stratification principles shall be considered.

However, it should be considered that depth is less variable over time than vegetation, and, therefore,

stratification related to vegetation shall be reconsidered at each successive sampling in a particular lake.

Reservoirs with steep banks may also be subjected to a modified stratification of gillnets.

6.2 Depth stratification of benthic gillnets

The depth zones are determined in relation to the volume of each stratum in such a way that each depth

stratum approximately equalises the same volume of water. Even if lake morphometry may vary considerably

between lakes, it is convenient to use a standardised scheme for stratification. For most lakes an

approximation of the depth strata can be based on morphometric lake data. Each lake is then divided in

approximately equal water volumes resulting in the following depth strata: 0 m to 2,9 m, 3 m to 5,9 m, 6 m to

11,9 m, 12 m to 19,9 m, 20 m to 34,9 m, 35 m to 49,9 m, 50 m to 74,9 m, > 75 m. Lakes deeper than 75 m are

rarely subjected to fish sampling using this type of benthic gillnets (see prEN 14962). The number of benthic

gillnets recommended in each depth stratum is given in Annex A. The table in Annex A includes optional

benthic gillnets at depth > 75 m in the largest lakes (251 m to 5 000 ha). Experience has shown that fish can

be caught in these nets, e.g. smelt, arctic char and bullhead. The information obtained from this effort should

be determined on a case-to-case basis.

To achieve a better estimate of the total fish abundance in lakes with extreme morphometry, the volume of

each depth stratum should be calculated, and the number of gillnets used at each stratum should be

distributed in relation to the volume of each stratum. Whenever the deepest stratum is too small to be used for

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EN 14757:2005 (E)

setting benthic gillnets which are independent of each other, it should be excluded in calculations of the total

number of gillnets used. When distributing gillnets over the lake, this depth stratum is treated as a part of the

stratum just above it.
6.3 Location of benthic gillnets

The location of each gillnet in the lake is determined in such way that the total catch should constitute an

unbiased sample of the catchable part of the fish assemblage in the lake. "Catchable" fish means fish species

which are usually caught in gillnets. Some predatory species with a typical ambush behaviour, such as

northern pike (Esox lucius), and some benthic species living very close to the bottom substrate, such as eel

(Anguilla anguilla), burbot (Lota lota) and bullhead (Cottus sp.), are often underrepresented in the gillnet catch.

Within the different depth strata, gillnets are set randomly over the whole lake. This could be performed by the

use of a pre-prepared co-ordinate grid placed over the depth map of the lake. By a randomisation procedure

each sampling location is located in each depth stratum, respectively (Figure 1). Gillnets are set in straight

lines and at random angles to the shoreline.

As the catch in each gillnet should be treated as an independent sample for that particular depth zone, no

gillnets shall be attached to each other.
6.4 Depth stratification of pelagic gillnets

To include samples also from the pelagic habitat, sampling with benthic gillnets should be supplemented by

sampling with pelagic gillnets in lakes with maximum depth greater than about 10 m. Even if there are no

apparent pelagic species in the lake, several fish species have a pelagic preference during part of their life

history. In contrast to sampling with benthic gillnets, the pelagic sampling does not provide an estimate over

the total water volume. Instead, pelagic sampling is performed as a depth profile over the deepest part of the

lake. The number of pelagic gillnets to be used is determined by the maximum depth of the lake. In more

shallow lakes, the benthic gillnets will provide a sufficient estimate of the pelagic fish in most cases.

Figure 1 — Morphometric map of a hypothetical 40 ha lake with 12 m maximum depth. Co-ordinate

grid, depth contours at 3 m, 6 m and 9 m, location of benthic gillnets (small marks) and pelagic gillnet

(large mark) are shown
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7 Inventory sampling
7.1 General

The inventory sampling is a simplified method for fish sampling which will provide a rough estimate of the

occurrence and abundance of dominating fish species in the lake. This type of sampling may be used in

studies aimed at describing the distribution of species and in inventory studies, when the precision of the fish

abundance is of less importance.
7.2 Depth stratification

The depth stratification varies between species and may also vary between size classes within the same

species. Therefore, it is important that both the epi- and hypo-limnion in thermally stratified lakes are included

in the sampling effort, and that all depths of the lake are sampled, also when there is no clear thermal

stratification.
7.3 Location of gillnets

The benthic gillnets are distributed in the lake in such way that all types of habitats are sampled. Gillnets are

randomly set a) over the depth zone which covers the epi- and metalimnion, and b) in the hypo-limnion. Within

these two depth zones, the gillnets are set randomly over the whole lake. In the absence of a marked thermal

stratification, the same effort is used as if the lake has a meta-limnion. Each single gillnet is loosely set in a

straight line, at random angles from the shoreline.

The catch from each single gillnet shall comprise an independent sample. Therefore, the gillnets should not be

connected to each other.
7.4 Sampling effort

The number of efforts used is dependent on the number of gillnets needed to catch all catchable species in a

lake. Therefore, the lake area determines the size of the effort. In general, less than 4 gillnets should never be

used in a lake, independent of its size. However, in small unproductive small lakes even 4 gillnets may be

unacceptable to owners of the fishing rights. The lakes are divided into four size classes (< 50 ha, 51 ha to

300 ha, 301 ha to 2,000 ha, > 2,000 ha) to determine the sampling effort. In lakes larger than 5,000 ha an

inventory sampling has to be accomplished by other sampling methods. The lowest number of gillnet-nights

which should be used and the distribution of gillnets within the lake are given in Table 3. The effort may be

increased in order to increase the probability to catch all catchable fish species.

Table 3 — Minimum effort benthic gillnets (# of gillnet-nights) used in an inventory sampling in

relation to lake area
Lake area (ha) Number of gillnet-nights
Total In epi/metalimnion In hypolimnion
< 50 4 2 2
51 to 300 8 4 4
301 to 2 000 16 8 8
> 2000 24 12 12
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8 Sampling routine
8.1 Pre-sampling

In order to maximise the output of the sampling effort a thorough planning shall precede all fish sampling.

When a lake has been selected for sampling, permission from the fishing right owner(s) has to be obtained. If

the responsible persons are informed about the aim and magnitude of the fishing activities, and the results are

communicated to responsible persons afterwards, this should normally not be an obstacle. To mitigate the

spreading of diseases due to fishing activities, a risk assessment for dispersion of pathogens has to be made.

Both fish diseases and diseases specific for other organisms, such as freshwater crayfish, may be spread by

placing equipment contaminated with pathogens or parasites in the lake.

If there is already a map of the lake with depth contours, this could be used to determine the total number of

efforts needed, and to determine if pelagic gillnets should be used. The map with depth contours is used to

divide the lake into appropriate depth strata and to determine the number of efforts that should be used at

each stratum. If the lake is being sampled for the first time, a randomisation of the gillnet locations should be

performed in advance. If the lake has been sampled earlier, the locations of the gillnets should as much as

possible resemble the earlier distribution in the lake. If data on depth of the lake is lacking, the sampling has to

be preceded by a sounding. This could be performed using a simple echo sounder and by running the boat in

predetermined transects over the lake before gillnets are set for the first time.

Supplementary information about the lake and the surroundings should be collected before sampling if

possible. All types of geographical and water chemical information should be collected. Especially information

about the fishery in the lake and on introduced fish species should be collected.

8.2 Sampling

All gillnets should be set between 6 p.m. to 8 p.m. Benthic gillnets are set randomly relative to the shore line at

the predetermined locations, and the depth of the most shallow and deepest points of the net are recorded

(Figure 1). The distribution of gillnets on each fishing night should be such that all depth strata are included, in

order to avoid bias due to differences in weather conditions between nights. A GPS instrument is

recommended to locate and record gillnet positions. Pelagic gillnets are set over the deepest part of the lake.

During the first night, gillnets are placed at depth 0 m to 6 m. The second night they are lowered to 6 m to

12 m and so on until the whole water column has been sampled according to Figure 2. Usually it is possible

for two experienced fishermen to fish with eight benthic gillnets and two pelagic gillnets per night in oligo- to

mesotrophic lakes. In eutrophic, highly productive lakes, the number of efforts per night has to be reduced

...

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