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Response summary to electrofishing effort


Bruce Thacker <[log in to unmask]>


Scientific forum on fish and fisheries <[log in to unmask]>


Wed, 14 Jan 2004 20:09:28 +0100





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Fish folk -

Following is a summary of the responses received to my posting regarding
measuring electrofishing effort. Thanks to all who contributed.

Cheers - Bruce

Electrofishing Effort Responses posted to the Fish_Sci List, Dec. 03

Original posting:

We are attempting to create a standard for electrofishing effort. The
fishing takes place with a Smith-Root Model 15D generator powered unit on
small (>2 m wide, > 1.5 m depth) Canadian Shield streams (low conductivity,
>60 ms/c) with a target species of brook trout. Assuming we get the power
right how should we go about measuring effort? The idea is to create a
standard to allow different crews to put the same amount of effort into
electrofishing similar sites for the same target species.

Should the standard be the number of seconds that the anode is held over a
square metre of substrate? This method might under-sample complex habitats
(root wads) or over-sample others (bedrock bottom) if a standard of, say, 8
sec/m-2 was used.

Would it be better to use the length of time it takes to fish the station
(often a 40 m length of stream)? Could another electrofisher operator
sample the station in another season using a simple overall time figure?

Your thoughts/recommendations much appreciated. I will post a summary of
responses at a later date if anyone is interested.

Bruce Thacker
Senior Research Technician
Centre for Northern Forest Ecosystem Reseach
Ministry of Natural Resources
955 Oliver Road
Thunder Bay, Ontario P7B 5E1

Tel.: 807-343-4015
Fax: 807-343-4001


In British Columbia we have been standardizing electrofishing effort for
depletion surveys in similar streams.

The concept now favoured is to keep the number of seconds of electrofishing
(power on) the same for each pass of the sample reach. Thus there is no
attempt to keep the effort constant on each square meter of habitat, which
as you indicate would be counterproductive. It is hard to get
electrofishing crews to stick to equal fishing effort on repeated passes,
as when the fish really are depleted, it means more and more minutes of not
catching anything.

I feel that it is not of overwhelming importance to get exactly replicated
fishing times as other (environmental) factors affect your results more.
These include:
Was the site disturbed by technicians before the fishing started? Walking
through the sample reach drives most of the fish into refuges where they
may well not emerge when electrofished. Of course the first electrofishing
pass greatly disturbs the fish and subsequent passes will recover fewer
The complexity of the habitat. Fish hiding in root wads and under
boulders may be zapped in place or even killed without emerging.
Leaking stop nets. I haven't yet seen a stream with significant flow that
is completely stopped off with nets. Typically the last fish caught in
depletion surveys are near the stop nets, and likely are new immigrants to
the reach.

Allen S. Gottesfeld
Head Scientist
Gitxsan and Wet'suwet'en Watershed Authorities
PO Box 229,
(1650 Omineca Street)
Hazelton, B.C. V0J 1Y0
[log in to unmask]
Phone: 250 849-5649
250 842-6780 local 328
Fax: 250 849-5648
250 842-6709

Bruce: I agree with the BC contingent--way too many variables to get a
statistically meaningful standard measure of abundance in these north shore
stream types. Especially with the two pass technique, disturbance in such
small streams is inevitably going to influence the results. And the year
to year variability (low flow years or seasons) in this part of the country
makes it very difficult to standardize the assessment protocol--do we
sample on a particular date each year, a rainfall event, a critical
flow/temperature parameter, a particular month et al??

For the past ten years Grand Portage, along with the USFWS, has sampled 3
Lake Superior tributaries for bkt (coasters) and other salmonids. We have
set 2 to 3 standard stations (300' or ~90 meters) for each stream below
barriers. We do record the amount of seconds spent on each station but do
not use it to evaluate abundance. We use it to generally correlate effort
but it appears to have little meaning for the integrity of the stream
and/or fish abundance. We merely sample each station length to see if we
have a bimodal length distribution of YOY--perhaps indicating stocked vs.
natural recruitment (Although, we often find tri- and quad- modal length
distributions of rbt YOY, which are not stocked, only natural.)--and record
the presence of pre-cocials and adults.

To simulate a standardized approach, our sampling efforts for these
stations takes place in late July - early August (low flow when YOY have
reached max. ? growth). We graphically represent our data as strict
abundance year to year. It seems to give us a local index of population
(?) integrity.

We also sample in late October-early November (fall rains w/ substantial
rise in creek flow) but merely for presence absence of spawning male and
females. This last year, for example, was dry (flow rates were low/water
temps were high) and the fall rains (inches) were limited. We did not
document a spawning run of bkt. We assume that they then spawn in the
Lake. Based upon a twice per month sampling effort after ice out until ice
formation, we also notice a significant emptying of YOY in the streams. On
a monthly basis, overall abundance appears to be equal during the ice-free

We use a back-pack shocker made at the University of Wisconsin-Madison
Engineering Department. It is set-up with Quadra pulse: we seldom use it,
but probably should as it is supposed prevent tetany in these smaller
salmonids. Our settings are regularly: 68-81 pps rate; 380-400 volts;
14.4% duty cycle. In relatively low conductivity waters, >200microSiemens,
our depth and width parameters are similar to yours.

Hope these thoughts help...I would be interested to see what you come up
with; perhaps there is a statistically meaningful protocol to sample these
streams. Let me know... Ben

Benjamin Whiting, Fish and Wildlife Biologist
Grand Portage Band of Chippewa
PO Box 428
27 Store Road
Grand Portage, MN 55605
[log in to unmask]
218-475-2415 ext. 22
-2615fax -2695home

We have run into similar problems, in establishing index sites or doing
comparisons between systems or between years. The standard would be to
enclose sites (stop nets at both ends) and do a three pass removal method
or mark recapture. Both have flaws.

The three pass method requires equal effort (seconds on each pass) however,
if the fish populations are high, considerably more time is
spent "catching" fish on the first pass and more time is spent looking for
fish on the last pass. This could result in a small error in the estimated
population. (I do not place much emphasis on the time factor). It is much
more important to capture a high percentage of the fish. Using the mark
recapture, (which if done properly should be more accurate than the removal
method) you should give the marked fish a fairly significant amount of time
to re distribute. In many instances this is not practical.

By using time as a basis for CPUE a number of fairly significant variables
are introduced, as you have mentioned which will make doing comparisons
difficult. Some operators are better than others, some are faster, some
are slower, the assistants that are helping net the fish play a significant
role and can dramatically change the time required to cover an area and
capture efficiency. I prefer to make comparison on a per unit area basis,
which should eliminate a lot of these variables. If you do a three pass
removal method using stop nets (you need to spend some time on these,
particularly the downstream net). The estimated populations should be
consistent. More efficient crews should have better confidence intervals
in the population estimates but at least there is a basis for comparisons.

One factor that I have found very important is site selection. Try to
avoid sites that are difficult to electrofish. Very deep pools, large root
wads and deeply undercut banks make life much more difficult and results
more variable. Try to find areas that are representative of the carrying
capacity of the reach (based on professional judgement) but that can be
electrofished effectively. A site with a large number of "small cover
components" can be sampled much more effectively and consistently than a
site with one large piece of cover.

 Another option would be to sample habitat units ie riffles vs riffles and
pools vs pools. The only problem with this method is that in small streams
habitat units can be short and you could scare fish out of the area while
putting in the stop nets. (Bottom net should go in first- my experience is
that fish that are disturbed while putting in the net will only go a short
distance upstream and will still remain in the site but downstream travel
could take them out of the site.) Or you could sample a sequence of
habitat units ie your sites should contain a pool riffle glide sequence.
The interface between habitat units can be key fish holding areas depending
on the species.

Placement of stop nets is important. Wide, shallow, low velocity and small
substrate areas are preferred. If you put stop nets in that fall down or
have significant "leaks" you are only fooling yourself. Even the most
carefully placed nets are still "leaky" but if you do a good job you can
reduce the problems. Try not to make the downstream net a nice place to
run and hide. (lean the top of the net downstream so that it does not
provide shade and overhead cover and so fish will accumulate away from the
spot where the net hits the stream bottom i.e.,. use a deep net).

Some sort of consistency in site selection is probably more important than
the electrofishing method.

I have found also that most species of fry tend to concentrate in shallow
low velocity shoreline areas. So in wider sections of stream the density
estimates will decrease despite the fact that the number of fry per length
of stream remains the same and probably the density in the suitable habitat
is consistent as well. The same is not necessarily true for older or
larger age classes. So it depends on what you are trying to measure. It
would be preferable to document fish numbers on the basis of estimated
number per unit area and per unit length just to be on the safe site.

There has also been some work done looking at macro habitats ie small
habitat specific sites, with the same depth velocity and cover components
however I have little experience with those methods.

If you are not using stop nets, then it becomes much more difficult to get
consistency. You go from herding to hunting and differences in crew
efficiency would probably be magnified. Although with good crews I would
still prefer number of fish captured per unit area or length rather than
time. Record all three and you can compare to see which gives you the most
consistent results.

B.G. Blackman R.P. Bio
Senior Fish Biologist
Peace/Williston Fish & Wildlife Compensation Program
1011 Fourth Avenue, 3rd. Floor
Prince George, BC V2L 3H9
Phone: (250) 565 - 6413 Fax: (250) 565 - 6629
Email: [log in to unmask]
Website: <>

I agree with Allen Gottesfeld, with backpack shockers, it is better to use
a fixed time as your measure of effort (one pass), rather than try to
standardize for each square meter. Another variation of standard effort
with backpack shockers is to cover a fixed distance for each pass. Time as
an effort unit has the advantage of being easy to measure; distance has the
advantage of allowing for increased catches if more fish are present.
Either way, the crew knows how much sampling is required before stopping.
Also, you made an important point in your original inquiry: "Assuming we
get the power right..."; this is a key aspect of standardizing
electrofishing effort and it is worth the preliminary work to set up a
standardized power table for your operation so that you can adjust for the
effects of water conductivity, a key factor that varies even in Canadian
Shield streams. Water conductivity is the single most important habitat
factor affecting electrofishing efficiency.

Jim Reynolds, Peace Corps Fiji
Institute of Applied Sciences
University of the South Pacific
Suva, Fiji Islands


FYI. Below are some comments on electrofishing effort from Scott Stranko,
who does a large amount of backpack electrofishing in Maryland streams.

Erik Zlokovitz
Maryland Dept. of Natural Resources-Fisheries Service
Tawes State Office Building, C-2
580 Taylor Ave.
Annapolis, MD 21401
[log in to unmask]

-----Original Message-----
From: Stranko, Scott
Sent: Tuesday, December 23, 2003 2:12 PM
To: Zlokovitz, Erik
Cc: Millard, Chris
Subject: RE: electrofishing effort

Thanks Erik.

Catch per unit of effort in time is a commonly used measure of effort for
electrofishing. An example is # of fish/second. Distance is also used.
As you know we use a standard length of stream with the MBSS (75 m).
However, we use one anode for every 2 - 3 m of stream width to be sure we
are covering the habitat effectively. We also keep track of the seconds
shocked by each anode so that we can generate a catch per unit of effort in
seconds for our electrofishing surveys. So we have a standard length of
stream plus total seconds shocked by anodes so we can standardize time

Smith Root electrofishers use something called P.O.W. (programmable,
output, waveforms). POW uses direct current output that can be set to
pulse at whatever interval you may want. I think the most recent version
of the model 15 electrofisher is capable of about 250 different possible
pulse output settings. These settings allow effective electrofishing in a
wide range of conductivity conditions and can be set to cause the least
possible injury to the fish. It is important to record the POW setting,
voltage, and conductivity of the water while electrofishing so that your
effectiveness is documented.

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