Fisheries Research Fish sampling with active methods

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:


Fish sampling with active methods

a r t i c l e i n f o


Fish stock assessment

Active and passive gear




Ground truthing




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.


2 Editorial / Fisheries Research 123–124 (2012) 1–3

Table 2

Absolute and relative frequencies of different gear scrutinized in conference







% 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.


Axenrot, T., Sandström, A., Asp, A., Setzer, M., 2010. Can gill­netting data improve

accuracy and precision in hydroacoustic estimates of fish abundance? In:

Kubečka, J., Hohausová, E., Soukalová, K. (Eds.), Fish Sampling with Active Methods,

Book of abstracts. Biology Centre AS CR, 3.

Baldwin, L., Aprahamian, M. An evaluation of electric fishing for stock assessment

of resident eel in rivers. Fisheries Research, in this issue.

Broadhurst, M.K., Suuronen, P., Hulme, A., 2006. Estimating collateral mortality

from towed fishing gear. Fish and Fisheries 7, 180–218, doi:10.1111/j.1467­


Draštík, V., Kubečka, J., Čech, M., Frouzová, J., Říha, M., Jůza, T., Tušer, M., Muška,

M., Prchalová, M., Peterka, J., Vašek, M., Kratochvíl, M., 2010. Intercalibration

between hydroacoustics and gillnet sampling in temperate reservoirs. In:

Kubečka, J., Hohausová, E., Soukalová, K. (Eds.), Fish Sampling with Active Methods,

Book of abstracts. Biology Centre AS CR, 21.

Emmrich, M., Helland, I.P., Busch, S., Schiller, S., Mehner, T., 2010. Hydroacoustic

estimates of fish densities in comparison with stratified pelagic trawl sampling

in two deep, coregonid­dominated lakes. Fisheries Research 105, 178–186,


FAO, 2003. The ecosystem approach to fisheries. FAO Technical Guidelines for

Responsible Fisheries. No. 4, Suppl. 2. Rome: FAO. 2003. 112 pp.

Gatz, A.J., Linder, R.S., 2008. Effects of repeated electroshocking on condition, growth,

and movement of selected warmwater stream fishes. North American Journal

of Fisheries Management 28, 792–798, doi:10.1577/M07­031.1.

Godlewska, M., Frouzova, J., Kubecka, J., Wiśniewolski, W., Szlakowski, J. Comparison

of hydroacoustic estimates with fish census in shallow Malta reservoir – which

TS/L regression to use in horizontal beam applications? Fisheries Research, in

this issue.

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Hamley, J.M., 1975. Review of gillnet selectivity. Journal of the Fisheries Research

Board of Canada 32, 1943–1969.

Handegaard, N.O., 2010. Fitting observed fish trajectories to catchability models.

In: Kubečka, J., Hohausová, E., Soukalová, K. (Eds.), Fish Sampling with Active

Methods, Book of abstracts. Biology Centre AS CR, 28.

He, P., Pol, M., 2010. Fish behaviour near gillnets: capture processes and influencing

factors. In: He, P. (Ed.), Behaviour of Marine Fishes. Wiley­Blackwell, Ames, Iowa,

USA, pp. 205–236.

Irwin, B.J., Treska, T.J., Rudstam, L.G., Sullivan, P.J., Jackson, J.R., VanDeValk, A.J.,

Forney, J.L., 2008. Estimating walleye (Sander vitreus) density, gear catchability,

and mortality using three fishery­independent data sets for Oneida Lake,

New York. Canadian Journal of Fisheries and Aquatic Sciences 65, 1366–1378,


Jurvelius, J., Riikonen, R., Marjomaki, T.J., Lilja, J., 2000. Mortality of pike­perch (Stizostedion

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salar m. sebago) caught as by­catch in pelagic trawling in a Finnish lake. Fisheries

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Jůza,T. Čech, M., Kubečka, J., Vašek, M., Peterka, J., Kratochvíl, M., Frouzová, J., Matěna,

J. The influence of the trawl mouth opening size and net colour on catch efficiency

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(Sander lucioperca) in the bathypelagic layer of a canyon­shaped reservoir. Fisheries

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Matěna, J., Peterka, J., Suuronen, P., Tereschenko, V., Welcomme, R., Winfield, I.J.,

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imaging sonar to observe fish behaviour with respect to an active surface trawl.

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Winger, P.D., Eayrs, S., Glass, C.W., 2010. Fish behaviour near bottom trawls. In:

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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: (J. Kubečka)

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