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Re: Electrofishing methods


Jim Reynolds <[log in to unmask]>


Academic forum on fisheries ecology and related topics <[log in to unmask]>


Mon, 16 Jun 1997 13:16:20 -0800





text/plain (1 lines)

This is a response to the inquiry by Hans Guttman regarding the long-term
effects of electrofishing on fish. Hans also received a response from Dr.
Snyder at Colorado State University regarding his comprehensive literature
review, so I will not repeat that information. Instead, I will summarize
what we know in a few paragraphs.

We know that salmonids are quite susceptible to pulsed direct current. The
most common form of pulsed DC presently used in North America is 50-60
hertz (pulses per second) and 25-50% duty cycle (percent on-time in each
cycle). Among rainbow trout and other salmonids (30 cm or longer), captured
with pulsed DC, it is common to see about half of them with spinal fracture
and/or muscle hemorrhage. We also know that frequency (pulse rate, hertz)
is very important and injury can be reduced if frequency is less than 30
hertz; however, capture efficiency often drops to unacceptable levels at
lower frequency. Higher frequencies (50-100 hertz and higher) are often
needed to catch smaller fish (10 cm or less), but may injure larger fish in
the process. Amplitude of pulsed DC (voltage level) also can have bad
effects. Some evidence suggests that injury increases with voltage, but
some data suggest otherwise; more needs to be done on voltage. Although
many North American biologists are concerned about electrofishing injury of
fish, the fact is that most of our concern is based on results with
salmonids; we have very little information about other species,
particularly warm water species. The general guidelines are: use no more
voltage or frequency than necessary to capture fish; remove them from the
electrical field as soon as possible; don't assume they are uninjured just
because they look OK -- fish caught with electricity are stressed at the
least and may be severely injured. One should sacrifice a few fish for
internal examination, particularly of the dorsal musculature, to verify if
injury is a problem.

What about long-term effects? The short answer is that we don't know
because the long-term effects may operate at the population level (e.g.,
reproduction). We do know that fish have an amazing capacity to survive
severe injuries. However, it seems unlikely that they will function
normally afterward. They may cease to grow, may develop infection, or, in
the case of females, cease to produce eggs. Whether or not these effects
could be seen at the population level is unknown; this would depend on the
population size and what percentage were injured during sampling. Usually,
one would expect the effect to be unmeasurable in large populations. But a
small population, in which sampling affected a large percent, could be
impacted. Other questions, related to disease resistance or genetic
mutation after electric shock, are being investigated to some degree, but
no information is yet available.

I'll finish here by re-emphasizing my point that most of our concerns are
based on results with salmonids and little is known for other species. We
should not assume that other species are similarly affected. I have started
a 3-year study to use a standardized method to evaluate
electrofishing-induced injury in a variety of non-salmonid fishes in the
continental U.S. If anyone is interested in the details of this project,
please contact me directly.

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