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Hi Darrel and Cynthia and others interested in this sort of thing:
Well, this is a surprise. I thought I carefully cut and pasted the
address of the original inquirer into my message but whoops. The events
in question happened about 30 years ago. The story is accurate, but
imprecise and I am not in a position, three decades and half a world
away, to improve on it much. That's why I only meant to respond to the
original. I suppose the "grain of logic and expectations" is formed and
constrained by personal experience and influenced by circumstances. I
just blundered into this controversy and I'll try to answer your
questions and provide you with some alternative sources if it is
something you wish to follow up.
The yellow perch (Perca flavescens i case there is any doubt) population
inhabited a shallow lake (max depth 2 m) which was subject to extremes of
temperature in the summer (28+ deg C, fluctuating several degrees in 24
hr if I remember correctly). The perch population was much larger than
expected (never did get a good fix on it) and seemed to be under stress.
At first, catches were infrequent, and recaptures rare of course, but
often they were dying. Later, when I became suspicious and began
releasing the fish at known points, I would observe moribund and dead
fish on the bottom the following day. Immediate cause of death was
bacterial followed by fungal infection (Pseudomonas liquifasciens was one
of the pathogens if I remember correctly - the chorus here) and I
injected hundreds of fish with the prescribed antibiotic, to no avail.
Other measures taken to reduce tension were: frequent net lifts- every
few hours; rapid handling - 20 seconds per fish - spaggetti tags fixed
with monofilament using surgical needles - couldn't get much faster.
Later on in the summer, when catches increased in frequency and more fish
entered a net than I could handle, I released them. That was when I
discovered that the stress of being in the net, even for only a few
hours, was the critical factor - these fish were found dead on the bottom
at point of release also (I regularly picked up the corpses and they were
concentrated at point of release).Fish I held in 100 gallon tanks for
observation would often throw fits when subjected to a sudden stimulus
such as me approaching the tank - and no wonder - it was suggestive of
Selye stress syndrome symptoms. The only fish I ever
recaptured in good condition i.e., not dying, at any time after tagging
(for the electroshocked fish this was weeks to months), were the
electroshocked fish and perhaps a few fish caught with rod and reel (I
convinced some anglers to fish for fun).
There was a winter kill that winter and what I didn't inadvertently kill,
Ma Nature took. Of the few survivors of my efforts, a few recaptures had
been electroshocked and tagged (and died after being recaptured in a trap
net) and some of the winterkill dead were also electroshocked but I won't
hazard the numbers. John Casselman was studying the pike in that lake and
was up there first in the spring so may recall first hand. If he had
similar problems with his pike it was not so obvious. The electofishing
equipment was his and he had a lot of experience using it so will know
its characteristics better than I do.
We were able, because of the shallow water, to put the electrode very
close to the fish which were sitting on the bottom at night. The shock
was light and you had to scoop the stunned fish very quickly or it would
escape. The fish were held in a laundry tub on the boat and then in a
holding tank on shore until tagged - which I think was the morning
following, (chorus). They were tagged before I was aware of the problem
so handling probably more stressful and certainly took longer (I used a
tape recorder later to record data). On the other hand, water
temperature, thought warm (don't recall details) was cooler, but this
made no difference for trapped fish caught at the same time and later, in
the Fall. After the winter kill the population was much reduced and I
used fin clips only for mark-recapture estimate and did not witness the
much mortality. The fish were eating more and larger food items in all
size cohorts than pre-winter kill and I found fish in the diet of larger
perch for the first time.
I had a unique fish population and a unique electroshocker with probably
a unique electrofishing guru. I don't know John's address now but if you
tried Harold Harvey or Brian Schuter at the Zoology Dept., U of Toronto,
you could probably raise him. The very selective manner in which we could
shock the fish is probably a critical factor and another approach
requiring a stronger shock to cover a larger area could well produce very
different results.
Regrettably, the work was not written up. I got discouraged by the
carnage and the resultant holes in my study, and opportunities abroad
beckoned.
Sincerely,
William Allison (Bill), M.Sc, M.B.A.
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