Fisheries Research Fish sampling with active methods

Fisheries Research 123–124 (2012) 1–3
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Fisheries Research
jou rn al hom epage: www.elsevier.com/locate/fishres
Editorial
Fish sampling with active methods
a r t i c l e i n f o
Keywords:
Fish stock assessment
Active and passive gear
Intercalibration
Standardization
Accuracy
Ground truthing
Interpretation
Selectivity
Avoidance
a b s t r a c t
The conference ‘Fish Sampling with Active Methods’ (FSAM) was held in September 2010 in Ceske Budejovice,
Czech Republic. A total of 108 participants from 29 countries attended the meeting and 100 lectures
and posters were presented. The meeting brought together scientist working with an interesting combination
of techniques for investigation of fish populations in seas, lakes, reservoirs and rivers (mostly
hydroacoustics, trawling, electric fishing, comparison of active methods with the use of gillnets and use
of visual approaches). In contrast to passive methods, active methods can be used to measure absolute
fish abundance because the sampling volume can be defined and estimated. If fish welfare precautions
are applied, active methods can be less destructive than passive. But there is still a need for using combinations
of multiple sampling methods and for improved understanding of gear efficiency, scrutinizing
the efficiency through independent methods or by studying of variation in sampling efficiency due to the
robustness of the sampling strategy (comparing the nets of different dimensions, mesh sizes, speed etc.).
© 2011 Published by Elsevier B.V.
The international conference Fish Sampling with Active Methods
was held September 8–11, 2010 in the Biology Centre, Academy
of Sciences of the Czech Republic, České Budějovice. Over one hundred
participants from 29 countries presented 63 oral and 37 poster
communications. This conference was a follow­up of an earlier
meeting (Fish Stock Assessment Methods for Lakes and Reservoirs:
Towards the True Picture of Fish Stock) held at the same location
September 11–15, 2007 (published as a special issue of Fisheries
Research, Kubečka et al., 2009). A true picture of the fish present
in the sea, lake or river can only be obtained if we understand the
efficiency of our various methods for sampling different fish species
and size groups. Although active gear may be more difficult to operate
than passive gear, these methods often provide a “truer” picture
of species composition and size structure, and have the potential
to provide absolute abundance. Continued development of active
sampling gear is important for improving our ability to provide a
realistic picture of fish stocks. The papers in this special issue on
fish sampling with active methods are a contributing to this goal.
1. Structure of the meeting
Active sampling is used in both marine and freshwater fisheries
research and the problems associated with active sampling
are similar across systems. This meeting provided a forum for
marine and freshwater scientists to meet and discuss these common
problems, including topics such as gear avoidance, sampling
efficiency, replicability, standardization, and gear comparisons. The
meeting was divided into sessions covering sampling with electrofishing
gear, trawls, seines, larval fish nets, visual methods and
acoustics (Table 1). Many presentations compared different sampling
methods, including comparisons between active and passive
gears. In marine environments, the three main approaches investigated
were trawling, acoustics and visual methods. These methods
were also discussed in the freshwater presentations, but freshwater
methods also included electrofishing, which is much less applicable
in salt water. Acoustic applications were used in larger waters,
typically seas, lakes and reservoirs, while electrofishing most often
applied to small wadeable streams. We also noted an increased
attention to trawling in freshwater surveys. Other active methods
discussed in both marine and freshwater studies included hand and
cast nets, use of commercial fishers, explosives, and toxicants. In
addition comparisons were made with passive gear such as longlines,
gillnets and traps (Table 2).
2. Active and passive sampling gear
The distinction between active and passive fishing gear is important.
Efficiency of sampling with passive gear depends on the
activity of the fish to encounter the gear and the retention probability
once a gear has been encountered (Hamley, 1975; Rudstam
et al., 1984; He and Pol, 2010) – Fish activity most certainly varies
Table 1
The numbers of oral and poster lectures in individual sections of the conference.
Section Oral lectures Posters Total
Large rivers, estuaries 6 1 7
Streams and electrofishing 8 5 13
Trawling 14 6 20
Seining 5 4 9
Small fish sampling 7 2 9
Commercial fishing issues 2 2 4
Visual methods 5 4 9
Acoustics 12 7 19
Standardization 4 3 7
Combination of methods 3 3
Total 63 37 100
0165­7836/$ – see front matter © 2011 Published by Elsevier B.V.
doi:10.1016/j.fishres.2011.11.013

2 Editorial / Fisheries Research 123–124 (2012) 1–3
Table 2
Absolute and relative frequencies of different gear scrutinized in conference
contributions.
Gear
Marine
systems
Freshwater
systems
% of total
Acoustic surveys 23.6 22.2 22.8
Trawling 27.3 16.7 20.7
Electrofishing 0.0 21.1 13.1
Gillnets 10.9 14.4 13.1
Seining 7.3 11.1 9.7
Visual surveys 18.2 3.3 9.0
Traps 1.8 6.7 4.8
Hand and cast nets 3.6 3.3 3.4
Explosives/toxicants 1.8 1.1 1.4
Longlines 3.6 0.0 1.4
Industry information 1.8 0.0 0.7
Total number of comparisons 55 90 147
with season, time of day, species, size, sex and even the physiological
state of an individual fish. Active gear on the other hand relies on
the movement of the gear rather than the fish, although fish behavior
is still important as fish may avoid the gear or the boat, and
may escape the gear after encounter (as discussed in Winger et al.,
2010; Rakowitz et al., in this issue). Active methods are likely more
efficient when fish are inactive (often at night) whereas passive
methods are more efficient when the fish are active (often during
dusk and dawn or the spawning period). Complexity is added
to this picture by the modifying effects of diel patterns in habitat
choice by different fish species. This also determines when they are
vulnerable to a particular gear.
Another advantage of active sampling methods is that the sampling
volume can often be defined (area or volume swept by a trawl,
area covered by electrofishing, volume insonified by acoustics). For
passive gear, we are usually limited to correlations of catch per
unit effort (CPUE) with some measure of population size and such
correlations are often lake specific (e.g. Irwin et al., 2008). More
comparisons of CPUE of passive gears with absolute density or
biomass estimated using active methods would be valuable given
the increased use of standardized gill nets for fish community surveys
in both North America and Europe. When properly operated,
active and passive sampling methods represent two “true” but different
pictures of the target stock or ecosystem, like pictures of
an object from different angles. The move towards methods giving
absolute estimates depends both on appropriate spatio­temporal
coverage as well as reliable ways of combining information from
various gears. Several meeting presentations demonstrated that
a steadily improving technology prepares the ground for future
developments in this direction.
The destructiveness of the sampling gear to fish and habitat is
another cause for concern. This is gear­specific and not related to
whether the gear is passive or active. Passive traps can be very fishfriendly
while fish caught by gillnets and longlines seldom survive.
When fish welfare is receiving appropriate care, the survival from
the catch of active gear (trawls and electrofishing) can be good
(Jurvelius et al., 2000; Broadhurst et al., 2006; Gatz and Linder,
2008), although trawl catches are often lethal and bottom trawls
can damage the sampled habitat (Watling and Norse, 1998). The
conference audience clearly indicated a move from highly destructive
active approaches like the use of toxicants and explosives
towards non­intrusive methods like acoustic, visual and photographic
techniques (Table 2).
3. Perspectives
Many of the contributions confirmed that obtaining “the true
picture of the fish stock” (a notion introduced in the preceding conference
FSAMLR; Kubečka et al., 2009) is the goal of fish sampling,
although an elusive one. With the need to develop an ecosystem
approach to the fisheries management (FAO, 2003) comes a need
to obtain absolute information on fish quantity, species and age
distribution. In many cases we are not yet satisfied with the representativeness
of the information collected. Comparing several
different sampling methods is important but suffers from a lack
of ground truth information (Axenrot et al., 2010; Emmrich et al.,
2010; Draštík et al., 2010). Rarely is the true status of the fish stock
known during sampling, but see Godlewska et al. (in this issue)
for a comparison of active sampling with a complete enumeration
of fish after draining the waterbody. Another very promising
approach is to scrutinize the efficiency of sampling using independent
remote methods like optical or acoustical cameras (Winger
et al., 2010; Handegaard, 2010; Rakowitz et al., in this issue). Such
approaches can lead to an understanding of the species and size
specific catchabilities of the gear in question. Increasing efficiency
of a single sampling method and comparing the estimates obtained
with the improved and earlier methods are also valuable (Jůza et al.,
in this issue; Baldwin and Aprahamian, in this issue). When further
increase in net dimensions, towing speed, and/or change of mesh
sizes fail to increase catches, it may be assumed that the asymptotic
catch rates are obtained with 100% efficiency.
This meeting invited contribution from both marine and fresh
water ecosystems. Differences in approaches and methods were
well demonstrated, and the obvious mutual benefit from comparison
and interaction between them was enlightened. The smaller
water bodies in fresh water system permit controlled ecological
experimentation that is unrealistic in the ocean. The extensive
research in fishing and acoustic gear technologies taking in place in
the marine environment can easily be made available for freshwater
research. Both issues would benefit from extended interaction
between the two scientific communities.
Obtaining reliable data on fish stocks remains an extremely difficult
task especially in larger waters (seas, large rivers, lakes and
reservoirs). However, fisheries scientists are making progress in
this direction thanks to new developments in sampling and validation
technology. There is no method that suites all fish species
and sizes but improved understanding of the efficiency of various
methods will lead to better choices of the complement of methods
required to get a true picture of the fish stock. This meeting
represents a step forward towards this goal.
References
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Jan Kubečka ∗
Biology Centre of the Academy of Sciences of the
Czech Republic, v.v.i., Institute of Hydrobiology, Na
Sádkách 7, 370 05 České Budějovice, Czech Republic
Olav Rune Godø
Institute of Marine Research, P. O. Box 1870 Nordnes,
5817 Bergen, Norway
Phil Hickley
Environment Agency, Hoo Farm Industrial Estate,
DY11 7RA, Kidderminster, UK
Marie Prchalová
Milan Říha
Biology Centre of the Academy of Sciences of the
Czech Republic, v.v.i., Institute of Hydrobiology, Na
Sádkách 7, 370 05 České Budějovice, Czech Republic
Lars Rudstam
Cornell University, Dept Nat Resources, 900
Shackelton Point Rd, Bridgeport, NY 13030, USA
Robin Welcomme
Department of Life Sciences, Imperial College London,
Ascot, SL5 7PY, UK
∗ Corresponding author. Tel.: +420 604344267.
E­mail address: kubecka@hbu.cas.cz (J. Kubečka)