FprEN 18161

SLOVENSKI STANDARD
01-marec-2025
Kakovost vode - Navodilo za monitoring populacije sladkovodnih školjk in
njihovega okolja
Water quality - Guidance standard on survey and monitoring freshwater mussel
populations and their environment
Wasserbeschaffenheit - Anleitung für das Monitoring und Bewertung von
Süßwassermuscheln
Ta slovenski standard je istoveten z: prEN 18161
ICS:
13.060.70 Preiskava bioloških lastnosti Examination of biological
vode properties of water
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
EUROPEAN STANDARD
prEN 18161
NORME EUROPÉENNE
EUROPÄISCHE NORM
January 2025
ICS 13.060.70
English Version
Water quality - Guidance standard on survey and
monitoring freshwater mussel populations and their
environment
Wasserbeschaffenheit - Anleitung für das Monitoring
und Bewertung von Süßwassermuscheln
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 230.
If this draft becomes a European Standard, 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.

This draft European Standard was established by CEN 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 CEN-CENELEC
Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 18161:2025 E
worldwide for CEN national Members.

prEN 18161:2025 (E)
Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Taxonomic summary of the species . 10
5 Survey and monitoring freshwater mussel populations . 11
5.1 General. 11
5.2 Monitoring methods for different attributes of freshwater mussel populations . 11
5.3 Distribution, abundance and population size of mussels . 12
5.4 Population structure: demography and recruitment . 15
5.5 Brooding levels . 16
5.6 Training and quality assurance for unionid mussel survey and assessment . 16
5.6.1 Survey training . 16
5.6.2 Training manuals . 17
5.6.3 Data entry and validation . 17
5.6.4 Licences . 17
6 Monitoring the environmental conditions needed to support freshwater mussel
populations . 18
6.1 Fish hosts . 18
6.1.1 General. 18
6.1.2 Host fish suitability . 18
6.1.3 Host fish availability . 18
6.1.4 Fisheries management . 18
6.1.5 Identification of problems . 19
6.2 Hydromorphology . 19
6.2.1 General. 19
6.2.2 Fluvial systems. 19
6.2.3 Standing water systems . 26
6.3 Water quality . 31
6.3.1 General. 31
6.3.2 Temperature. 31
6.3.3 pH, calcium, alkalinity . 32
6.3.4 Dissolved oxygen. 32
6.3.5 BOD /COD . 32
6.3.6 Electrical conductivity . 32
6.3.7 Nutrients . 32
6.3.8 Turbidity and suspended solids . 32
6.3.9 Contaminants . 33
6.4 Biotic features . 34
6.4.1 General. 34
6.4.2 Phytoplankton and phytobenthos . 34
6.4.3 Aquatic macrophytes . 34
6.4.4 Macroinvertebrates . 35
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6.4.5 Riparian vegetation . 35
6.4.6 Predators and competitors . 36
6.4.7 Parasites and diseases . 37
6.4.8 Other aspects . 37
7 Monitoring environmental pressures . 37
8 Information needed to assess plans or projects on water bodies with freshwater mussels
............................................................................................................................................................................. 38
Annex A (informative) Geographical distribution of Margaritifera species in Europe . 41
Bibliography . 42

prEN 18161:2025 (E)
European foreword
This document (prEN 18161:2025) has been prepared by Technical Committee CEN/TC 230 “Water
analysis”, the secretariat of which is held by DIN.
This document is currently submitted to the CEN Enquiry.
prEN 18161:2025 (E)
Introduction
This document provides guidance on survey and monitoring populations of unionid mussels (i.e. Order:
Unionida, also referred to as naiads) and the environmental features on which they depend. There are 24
species of native unionid bivalves in Europe other than Margaritifera margaritifera, which is covered in
a CEN standard [1] to provide guidance on survey and monitoring. None of these unionid species can be
considered to be secure throughout their range based on IUCN threat assessments. A standard approach
to collecting the information required to undertake accurate threat assessments is not available.
Unionid species are distributed according to their natural habitat requirements, the distribution of their
host fish, and the historical means by which both mussels and hosts have been distributed, and the
biogeographical barriers to their spread. The European unionids fill a wide range of habitat types, and
survey, monitoring and condition assessment must be relevant to the individual requirements of each
species. Some species are generalists and occupy a wide range of habitats, while others are more
restricted by river/ lake/ canal bed substrate, water quality and water flow, in particular lentic or lotic
conditions.
Freshwater mussels are threatened by a range of pressures, as outlined in Lopes-Lima et al. [2]:
a) habitat loss, fragmentation, and degradation;
b) overexploitation;
c) pollution and eutrophication;
d) loss of fish hosts;
e) invasive species;
f) water abstraction and climate change;
g) other threats resulting in mussel declines (e.g. diseases),
where the habitat appears intact with healthy populations of fish, insects, gastropods, and other biota.
Further research is needed to detect new and emerging pressures, requiring consistent and comparable
mussel survey and data interpretation. This document aims to assist in the conservation of unionid
mussels through facilitating a consistent approach to understanding mussel populations across the
species and the bioregions in which they occur, including undertaking environmental impact assessment,
and restoring mussel populations. The applications of the standard also include the provision of site-level
data that will contribute to reporting under Article 17 of the European Habitats Directive (https://eur-
lex.europa.eu/legal-content/EN/TXT/).
NOTE Although freshwater mussels are invertebrates, their survey is conducted in a very different way from
standard surveys for other benthic macroinvertebrates. Surveys for mussels are concentrated on the restricted
habitat of the species of mussel being surveyed, and must also consider the habitat and water quality conditions
required to complete its full life cycle. For this reason it is not possible to use EN 16859 (‘Guidance standard on
monitoring freshwater pearl mussel (Margaritifera margaritifera) populations and their environment’) for other
mussel species. The freshwater pearl mussel is unique among other European mussel species in living in fast-
flowing oligotrophic habitats which are unsuitable for other mussel species. It is not possible, either, to use
EN 16150 (‘Guidance on pro-rata multi-habitat sampling of benthic macro-invertebrates from rivers and streams’)
as this guidance is focused on finding the macroinvertebrate species present across a full representative range of
habitat types within a water body, and is thus unsuitable for a survey of mussels in their restricted habitat. The use
of EN 16150 is also confined to rivers and streams rather than the wide range of water bodies, including lakes and
ponds, that are important habitats for mussels.
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1 Scope
This document provides the information needed to assess the condition over time of a unionid
population, and the level of information for assessing whether a plan or project may be detrimental to
their future prospects. It provides guidance on methods for survey and monitoring unionid mussel
populations and the environmental characteristics important for maintaining populations in favourable
condition. The document is based on best practice developed and used by unionid mussel experts in
Europe, and describes approaches that individual countries have adopted for survey, data analysis and
condition assessment.
Standard methods for restoring populations are not within the scope of this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 16039, Water Quality — Guidance standard on assessing the hydromorphological features of lakes
EN 14011:2003, Water quality — Sampling of fish with electricity
3 Terms and definitions
For the purposes of this document, the following terms and definitions.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
3.1
acoustic doppler current profiler
ADCP
sonar device that produces a record of water current velocities for a range of depths
[SOURCE: EN 16859:2017, definition 3.1]
3.2
aquatic macrophyte
larger plant of fresh water which is easily seen with the naked eye, including all aquatic vascular plants,
bryophytes, stoneworts (Characeae) and macro-algal growths
Note 1 to entry: This definition includes plants associated with open water or wetlands with shallow water.
[SOURCE: EN 14614:2004, definition 2.1]
3.3
bankfull
maximum point on banks at which floods are held within the channel before spilling over onto the
floodplain
[SOURCE: EN 14614:2004, definition 2.5]
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3.4
bathtub ring
high-water mark, visible at low water levels around the shoreline of a lake, often resulting from the
deposition of minerals on previously submerged surfaces
3.5
bathyscope
bucket with a transparent bottom used for viewing freshwater pearl mussels on the river bed
[SOURCE: EN 16859:2017, definition 3.5]
3.6
brooding period
length of time that glochidia remain within the body of a gravid pearl mussel
[SOURCE: EN 16859:2017, definition 3.7]
3.7
chemical oxygen demand
COD
indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution,
−1
expressed as mass of oxygen consumed/volume of solution in mg L
3.8
cofferdam
temporary, watertight enclosure built within a body of water, from which water is pumped to provide a
dry working environment
3.9
colmation
blockage of stream-bed interstitial spaces by the ingress of fine sediments and organic material
[SOURCE: EN 16859:2017, definition 3.8]
3.10
culvert
arched, enclosed or piped structure constructed to carry water under roads, railways and buildings
[SOURCE: EN 15843:2010, definition 3.8]
3.11
flow duration curve
graphical representation of a ranking of all the flows in a given period, from the lowest to the highest,
where the rank is the percentage of time the flow value is equalled or exceeded
Note 1 to entry: These curves may be derived for flows in any time interval, such as daily flows, monthly flows or
annual flows
[SOURCE: EN 16859:2017, definition 3.17]
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3.12
fluvial audit
method for assessing the condition of a river and its associated human pressures, using information from
field survey, remote sensing, historical and recent maps, scientific literature and other sources
[SOURCE: EN 16859:2017, definition 3.18]
3.13
glide
moderately-flowing water with undisturbed surface other than occasional swirls or eddies, and with
constant depth across part of the channel
[SOURCE: EN 14614:2004, definition 2.17]
3.14
glochidium
mussel larva
Note 1 to entry: The plural is ‘glochidia’.
[SOURCE: EN 16859:2017, definition 3.21, modified]
3.15
hydromorphology
physical and hydrological characteristics of rivers including the underlying processes from which they
result
[SOURCE: EN 14614:2004, definition 2.18]
3.16
marsupium
section of mussel gill expanded to form a pouch to protect eggs
Note 1 to entry: The plural is ‘marsupia’.
3.17
monitoring
comparison of repeated surveys, ideally against pre-defined targets
3.18
penetrometry
method for assessing the resistance of the river-bed substrate in situ using a standard cone or disc
penetrometer
[SOURCE: EN 16859:2017, definition 3.28]
3.19
pool
habitat feature characterized by distinctly deeper parts of the channel that are usually no longer than one
to three times the channel’s bankfull width, and where the hollowed river bed profiles are sustained by
scouring
[SOURCE: EN 14614:2004, definition 2.24]
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3.20
recruitment
survival of juvenile mussels and their addition to a population
[SOURCE: EN 16859:2017, definition 3.30, modified]
3.21
revetment
retaining wall or facing of masonry or other materials supporting or protecting banks from erosion
3.22
riffle
fast-flowing shallow water with distinctly broken or disturbed surface over gravel/pebble or cobble
substrate
[SOURCE: EN 14614:2004, definition 2.28]
3.23
riparian zone
area of land adjoining a river channel (including the river bank) capable of directly influencing the
condition of the aquatic ecosystem (e.g. by shading and leaf litter input)
Note 1 to entry: In this document, the term ‘riparian zone’ does not include the wider floodplain.
[SOURCE: EN 14614:2004, definition 2.29]
3.24
salt bridge
device containing a chemically inert electrolyte which is used to increase electrical conductivity locally
[SOURCE: EN 16859:2017, definition 3.37]
3.25
shear stress
measure of the force of friction caused by water flowing around a submerged surface or object
[SOURCE: EN 16859:2017, definition 3.38]
3.26
survey
recording of qualitative or quantitative data using easily repeatable standardized techniques over a
restricted period without preconception of the results
3.27
tumidity
measurement of a mussel at its widest point
Note 1 to entry: While the length and width are measured across the longer and shorter side of a valve, the tumidity
is measured across the two valves
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3.28
turbidity
reduction of transparency of a liquid caused by the presence of undissolved matter
[SOURCE: ISO 6107-2:2006, definition 145]
3.29
umbonal sculpture
shape and arrangement of contours at the dorsal protuberance on bivalve shells that generally rises
above the hinge
3.30
woody material
material that falls into rivers and streams, ranging in size from leaf fragments (fine woody material) to
branches or whole trees (coarse woody material)
[SOURCE: EN 16859:2017, definition 3.41]
4 Taxonomic summary of the species
The European unionid mussels are divided into two families, the Margaritiferidae and the Unionidae.
In the Margaritiferidae, only two species currently occur in Europe – Margaritifera margaritifera
(Linnaeus, 1758) and Pseudunio auricularius (Spengler, 1793). In the Unionidae, 23 species are divided
into two subfamilies: the Unioninae and the Gonideinae. The Unioninae are further subdivided into tribes
Anodontini and Unionini.
The European Anodontini includes two genera – the genus Anodonta with three species: the widespread
Anodonta anatina (Linnaeus, 1758), Anodonta cygnea (Linnaeus, 1758), and the south-central European
Anodonta exulcerata Porro, 1838; and the genus Pseudanodonta with a single species, Pseudanodonta
complanata (Rossmassler, 1835).
The Unionini includes 16 valid species belonging to the genus Unio. This genus is divided into several
groups that correspond to four evolutionary monophyletic clades – the crassus, the pictorum, the gibbus
and tumidus clades, all of them with at least one species in Europe. The pictorum clade includes five valid
species: Unio delphinus Spengler, 1793; Unio elongatulus Pfeiffer, 1825; Unio mancus Lamarck, 1819; Unio
pictorum (Linnaeus, 1758); and Unio ravoisieri Deshayes, 1848. The crassus clade includes nine species:
Unio bruguierianus Bourguignat, 1853; Unio carneus Kuster, 1854; Unio crassus Philipsson in Retzius,
1788; Unio cytherea Kuster, 1833; Unio desectus Westerlund in Westerlund and Blanc, 1879; Unio
gontierii Bourguignat, 1856; Unio ionicus Drouët, 1879; Unio tumidiformis Castro, 1885; and Unio vicarius
Westerlund in Westerlund and Blanc, 1879. The gibbus clade includes the single species Unio gibbus
Spengler, 1793, only present in a single river in southern Spain. Finally, the tumidus clade only includes a
single species - the widespread Unio tumidus (Philipsson, 1788).
The second subfamily Gonideinae only includes two genera divided into three species in the south of
Europe. The genus Microcondylaea only includes a single species, Microcondylaea bonellii (Férussac,
1827), in south-central Europe. The genus Potomida includes two species in Europe, the Western
European Potomida littoralis (Cuvier, 1798) and Potomida acarnanica (Kobelt, 1879) only present in
Greece.
A list of the species covered in this document, together with details of their geographical distribution is
given in Informative Annex A.
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5 Survey and monitoring freshwater mussel populations
5.1 General
Obtaining adequate information on unionid populations in good quality habitats and with successful
recruitment of young mussels is the best way to determine the requirements needed to return declining
populations to favourable condition. Thus, the monitoring of the best populations as well as those of
concern is of high importance.
In order to determine the most appropriate methods for survey and monitoring it will first be necessary
to categorize the water body by type (large river, small river, stream, canal, large lake/reservoir, small
lake/large pond); size (length, width, depth); flow and substrate characteristics. This is key to decide
which sampling method will be required, such as wading in shallow and clean streams, snorkelling in
larger but still clean water bodies, scuba-diving in deeper rivers and lakes or with poor visibility, or using
dredges or other devices where conditions are not safe. The sampling method will also determine which
sampling design will be adequate to meet the objectives.
In general, smaller water bodies are easier to survey and monitor than larger ones, where there can be
large variation in habitat, and methods for measuring change are more difficult to design.
During the design of monitoring surveys, the decision on whether the target of the study is a single species
or the full mussel community present in the habitat will depend on the objective of the study.
5.2 Monitoring methods for different attributes of freshwater mussel populations
Unionid mussels are often present as assemblages of mixed species. Where this is the case, the mussel
faunal composition should first be established. Nevertheless, it should always be recognized that some
previously unrecorded species may be present as well. Changes in relative abundance can be monitored
over time. Mussel populations can be monitored by focusing on particular attributes according to the
information required (Table 1).
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Table 1 — Checklist of survey methods recommended for collecting data on mussel attributes
Attribute Method Output (units) Notes
Distribution Wading or snorkelling Map Once thoroughly to create a
/scuba survey counts baseline with annual checks or
with direct observation, at appropriate intervals, taking
into account the species’
fingertip searches or
biological cycle and the
dredging
monitoring objectives.
Where species’
distributions are largely
unknown,
environmental DNA
methods can be used
Population Wading or snorkelling/ Number of mussels
At appropriate intervals taking
density scuba survey counts per m into account the species’
(including transects or biological cycle and monitoring
quadrats) with direct objectives.
observation, fingertip
searches or dredging
Size and age Determination of Mussel measurement It is useful to establish a
structure of individual mussel sizes (mm). Growth curve relationship between size and
mussel per species in quadrats (mm per year) and age. If this is not possible, size
populations or plots growth parameters distribution can be used as a
(e.g. growth surrogate. Size distribution
coefficient). Time according to species and
comparisons of baseline data. Demography
population size should be assessed annually or
structures at appropriate intervals taking
into account the species’
biological cycle and the
monitoring objectives.
Measurements are usually
maximum length, height, and
tumidity.
Brooding levels Visual, sub-sample of Percentage of To be undertaken where levels
mussel adults checked surveyed mussels are low or no other evidence of
by trained expert with evidence of recruitment has been found.
brooding, based on a
representative sub-
sample
5.3 Distribution, abundance and population size of mussels
Baseline surveys should initially be undertaken with sufficient information collected as a reference point
to allow changes to be detected in subsequent monitoring surveys. Care needs to be taken to undertake
repeated monitoring surveys during appropriate seasons and conditions, as some seasonal changes can
affect the ease of finding some species more than others.
The area of a river or lake to survey will depend on its size. For medium to large rivers and lakes where
environmental conditions are suitable, sampling is best carried out from a boat/inflatable equipped with
an outboard motor so that a relatively large geographical area can be covered in a day and marginal areas
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are easily reached and sampled. A representative range of habitats should be sampled to ensure that no
species are missed.
The extent of distribution of a population may require a series of surveys from broad, wide-ranging
efforts to understand the overall extent, to more detailed investigations in sub-sections of the population.
A previously surveyed water body typically needs a different initial approach compared with already
known populations, to establish the species composition present, microhabitat distribution and basic
distribution patterns that are needed to design further monitoring programmes.
Providing reliable information on a species’ population size is wholly dependent on size/depth of the
water body, accessibility, and water clarity. Major constraints to sampling freshwater mussels include
difficult access to water bodies, poor visibility, deep or fast-flowing water and poor water quality that
may impose health risks. Common sense should always be used when planning a survey, so that
appropriate methods are used in specific locations to meet the required objectives without compromising
health and safety issues. Most unionids live in very soft sediment/mud and may be scarcely visible even
in clear water, and many of the mussels will be hidden within the substrate. Furthermore, as more than
one species may co-exist in a very small area, species abundance can only be determined by some form
of invasive sampling. The best accuracy will be achieved by hand sampling of quadrats, but this is only
really feasible in water < 0,7 m deep. In deeper water this would be possible by scuba sampling of
quadrats. Quadrats shall be set using a sampling design that takes into account previous basic knowledge
of the mussels’ distribution and microhabitats present. Usually, a stratified or adapted sampling design
[3] is necessary for reliable population estimates, rather than random or systematic schemes. The
number of quadrats to sample should be decided in accordance with the total area to survey, the
distribution pattern of mussels and the required precision of the estimates (see [3] for details). In larger,
deeper water bodies, and in those with turbid water, hand dredges can also be an effective way of
sampling and may be the only option for use in contaminated water, although they are less accurate with
respect to population size and abundance.
The efficiency of sampling mussels depends greatly on the substrate. Typically, three basic tools are used:
scratch sampler (a rake with netting) usually used in shallows and different types of bottom samplers
(Ekman’s grab, Van Veen grab, etc.) [4] in deeper parts. In many situations only scuba-diving and
sampling with quadrats give reliable results, assuming the removal of any obstacles for bottom sampling.
The efficiency of sampling may differ considerably depending on the vegetation and structure of the
substrate (e.g. shingle bars on the bottom built of cobbles or pebbles). Even with soft bottoms, sampling
may be hampered, e.g. by Nuphar luteum, which can completely cover the substrate with strong and very
thick rhizomes (up to 20 cm in diameter); however, this armoured layer decomposes in autumn exposing
soft sediments.
Standard methods developed for the country in which the survey is carried out should be used. Most
sampling strategies, including wading, snorkelling and scuba-diving are adequate for locating adult
mussels that are semi-buried in the substrate. Locating juveniles or other age classes that are fully buried
requires different strategies, including quadrat excavation or the use of dredges. Note that in some
countries, standard methods do not include investigations of buried mussels. A careful evaluation of the
objectives of the monitoring programme should be made before considering the use of these techniques,
as they can be locally destructive to microhabitats.
In all cases mussels should be identified to species level and counted. Identification is straightforward for
some species but may be more difficult for others. When necessary, morphometric or molecular
techniques should be used for identification. The same mussels should also be used for demographic
analysis (Clause 5.2). The intervals at which mussels are monitored depend on the objectives of the
monitoring programme; one-time surveys are usually only adequate for establishing a reference
characterization of the population. Annual monitoring can be useful for following trends in mussel
species, especially for populations that are at risk. Stable populations of long-lived species may be
surveyed at longer intervals for the same purposes, whereas specific objectives such as investigating the
reproductive success or impact of disturbance events (droughts, construction, pollution, etc.) may
require monitoring at monthly intervals. Monitoring methods are shown in Table 2. The timing of
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sampling itself should match ideal water levels, using opportunities such as when reservoirs and canals
dry up for maintenance or natural habitats dry up because of droughts. Quadrats should still be used even
if little or no water is left, as live mussels may still be present.
Table 2 — Sampling methods suitable for different water body types
Water body Criteria / Restrictions Methods
Rivers and Streams If mussel fauna and distribution is eDNA analysis of water samples at an
completely unknown appropriate number of locations
There may be beaches or shoals Rapid walkover survey of these areas
where dead shells are cast up, or (usually identifiable from aerial
spoil tips arising from channel photographs) to collect/record dead
dredging mussel shells
In water that is < 0,7 m deep, Wading using bathyscope if substrate is
clear or turbid stable enough, snorkelling if substrate is
very soft, by touch or using a robust pond
net if the visibility is poor. For
quantification place 0,5 m × 0,5 m
quadrats on the substrate surface or use
transects according to the most
appropriate sampling design for each
site. Hand search the quadrat or transect
and place all mussels into a keep net.
Ensure all buried mussels are collected if
appropriate. Ideally at least 10 quadrats
at each ‘site’ should be analysed.
Water > 0,7 m deep but with good Scuba/snorkelling survey.
visibility
Water > 0,7 m deep but with poor Small hand dredge deployed from the
visibility bank or from a small boat. For large, deep
rivers an air-lifter /suction dredge can be
deployed from an appropriately sized
vessel. Scuba survey may be appropriate
for specific objectives.
Canals In general, canals tend to have Small hand dredge deployed from bank
turbid water and an even bed or from a small boat. Wading may be
profile (with no shallower possible when water levels are low due
margins), with a muddy to canal maintenance.
substrate, and are usually
dredged for maintenance. Water
likely to be > 0,5 to 0,7 m deep.
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Water body Criteria / Restrictions Methods
Lakes/reservoirs/ For substrates comprising deep, Margins may be sampled by a robust
large ponds and soft mud ‘pond net’ or by a small hand dredge. A
lowland rivers wider area and lakes with dense fringing
vegetation are best sampled from a
shallow-draft boat using either a glass-
bottomed bucket if water clarity is good,
fingertip searching if shallow enough, or
a hand dredge.
For more stable substrates and Wading using a bathyscope, or fingertip
sparsely vegetated margins searching if water is turbid; snorkelling,
scuba or hand dredging in deeper water
5.4 Population structure: demography and recruitment
The same rationale for determining population structure and recruitment applies to all types of water
bodies.
Population structure is determined by measuring all mussels collected from samples taken for estimating
population size (Clause 5.3). Detailed measurements should be made to the nearest millimetre, and then
size distributions compared at regular intervals with baseline data. Mussels smaller than 2 cm to 3 cm in
length are likely to be missed by hand or visual searches, especially in turbid water. Where permitted and
if relevant to determine recent juvenile recruitment, all surface substrate within quadrats needs to be
collected and analysed. All sediment should be passed through a sieve (ca. 5 mm is likely to be adequate),
and all visible small unionids removed, identified and measured. Small unionids (<5 mm) are difficult to
distinguish when there are also large numbers of pea bivalves present and the samples should then be
analysed in the laboratory using a microscope. Very small unionids are also potentially difficult to identify
to species in mixed assemblages and for some species attention should be taken to the umbonal sculpture.
If the water body can only be sampled by using hand dredges (or suction lifters) then a quantity of
sediment collected in the dredge should be sieved and analysed as described above. If the mesh on the
dredge net is too coarse then mussels will slip through. Mesh sizes should be chosen appropriately in
order to capture all sizes of unionids.
Monitoring should follow a sampling design and methodology as recommended in Table 2, to establish a
baseline. Once a baseline is established, the frequency of monitoring should be based on an assessment
of risk to the population, with some aspects needing to be carried out at a higher frequency than others.
Where adverse pressures are apparent, investigative monitoring may be required to determine their
cause.
Monitoring sites should be selected in accordance with the objectives of the monitoring programme
(mussel distribution, abundance, age structure, active recruitment, growth conditions). Ideally transects
or quadrats should be placed at the same site on successive monitoring occasions, but when disturbance
events eliminate or greatly reduce mussel beds this might not be possible. In order to assess the impact
of disturbance at a population level the baseline data should include a minimum of five replicates for each
microhabitat present. If disturbance is likely to affect whole sites, several sites should be selected based
on their similarity and in relatively uniform areas or stretches of river. This will allow some losses to be
accommodated without significantly affecting the monitoring results.
Age can be estimated by counting growth rings on the shells, usually on thin sections observed under the
microscope after applying an appropriate dye. Care should be taken when doing so as growth rings might
not always be annual and false rings are common. A growth curve can be calculated according to Von
Bertalanffy, L. [4] or other suitable model, and used to compare different populations, such as in Müller
et al. [5]. Calibration of the relationship between size and age is needed as it is often lake-, river- or even
prEN 18161:2025 (E)
site-specific. for medium- to long-term monitoring some mussels should be individually tagged, and real
growth can be registered from recapture data and used for calibration and constructing growth models.
5.5 Brooding levels
A subset of adult mussels can be checked for brooding levels of glochidia if sampling is undertaken at the
appropriate time. Mussels can be checked for the presence of glochidia without harm by gently opening
the valves to allow the inspection of the marsupia. The stage of development of glochidia can be checked
by collecting a sample with a pipette and observing them under a microscope. Mature glochidia are
readily identified by the absence of a capsule and active reaction to the presence of salt (NaCl). The
presence and proportion of mussels with glochidia, and the ability to produce mature glochidia, are
important indicators of the reproductive output of a population and may help to identify reasons for
recruitment failure. In order to accurately determine glochidial production, sampling should be repeated
at regular intervals within the glochidial production period of each species. The timing and duration of
brooding periods vary according to climatic conditions. Multiple broods during the appropriate period
are also common in many species. Fertility estimates usually imply sacrificing specimens but may be
necessary for investigative monitoring.
5.6 Training and quality assurance for unionid mussel survey and assessment
5.6.1 Survey training
Surveyor training is essential to ensure consistency, accuracy and precision. Surveyors need to
understand sufficiently the biology of the various species under study to appreciate the reasons for the
methods used and the need for care in their application to avoid damage to mussels.
Training should be structured to cover the level of survey required, from minimally invasive counts of
adults to specialist demographic quadrat analysis. Where relevant, a qualification in snorkelling / scuba-
diving should precede survey training. The level of training and experience needed for mussel handling
depends on the ease with which that particular species can be found, and the level of its endangerment.
However, even common and widespread species require survey training to ensure the safety of the
surveyor, lack of damage to the mussels, and to ensure that errors between surveyors are minimized.
The content of training should include:
a) health and safety education relevant to mussel survey;
b) identifying mussels to the lowest possible taxonomic level;
c) surveying and monitoring mussels with minimal disturbance, including avoidance of trampling on
mussel beds, keeping mussels out of the water for as little time as necessary, taking precautions for
safely handling mussels (e.g. avoiding extreme heat, direct sun exposure, desiccation; safely opening
valves to inspect for glochidia);
d) taking precautions to avoid contamination with parasites or infectious diseases and to prevent alien
invasive species introduction, including the use of gloves, disinfection of all tools used to manipulate
mus
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