Earth & Sky

The following individual(s) were interviewed for today’s show. Our thanks to:

Dawn Kirk, Forest Fisheries Biologist
George Washington & Jefferson National Forest
Natural Bridge Station, VA

Dr. Andy Dolloff
Project Leader,
U.S. Forest Service Southern Research Station Coldwater Fisheries Research Unit
Department of Fisheries and Wildlife Sciences
Virginia Tech
Blacksburg, VA

Chris Scharpf
North American Native Fishes Association

The following books, articles and web sites were used in preparing this script:

Distribution, Abundance, and Status of American Eels in Three Headwater
Streams on the George Washington-Jefferson National Forest by Craig N.
Roghair, J. Keith Whalen, John D. Moran, and C. Andrew Dolloff,
of the U. S. Forest Service’s Center for Aquatic Technology Transfer at the
Virginia Polytechnic Institute and State University.

Report is available here.

2) The American Eel, Anguilla rostrata, in Nature and in the Aquarium by
Christopher Scharpf
Tropical Fish Hobbyist, March 2001

Author’s Notes:
Transcript of Interview with Ms. Dawn Kirk, Forest Fisheries Biologist,
George Washington & Jefferson National Forests. Additional comments by Dr.
Andy Dolloff, project leader for the eel survey, are included in {}.

E&S: The eels that we find in freshwater are primarily female, right?

DK: They seem to think that … most eels that head very far inland, they
think, are females. {Studies have shown that the farther up a watershed you
sample, the more likely you are to capture female eels.} The males stay
more towards the bays and estuaries, or closer to the ocean. It’s hard to
sex an eel until they’re sexually mature but that’s what they seem to think.

E&S: Can you describe what happens to the eels physiologically during
hibernation, does their temperature go down and all that stuff?

DK: Hibernation is not something that we’re looking at, however, their
temperature does go down {Eels are cold-blooded, so their body temperature
tracks ambient water temperature very closely} and they are not active. I’m
not suggesting that it’s true hibernation like a bear goes through. They’ll
probably just go and bury themselves under the rocks or in the substrate
and wait for the water to warm up, and when the food is more abundant. From
what we’ve seen, they’ve gone under the rocks and they don’t seem to move
around very much. They don’t seem to be found in the stream actively
feeding. So I’m not saying it’s a true hibernation where they fall asleep.
As fish, eels are cold-blooded and they can go for a long period of time
without eating if they’re not active, and if their bodies turn cool enough,
{metabolic rate slows but doesn’t cease}.

We though it was unusual to go into these small streams and find such a
large density of eels of all different sizes, from the size of a pencil to
a couple of feet long. Then we went back in the fall, and could not find
them. They must have all gone down to the larger rivers to overwinter in
the mud. So we started this project to put radio transmitters on the eels,
and found that they didn’t leave the small stream, that they buried
themselves under rocks. The transmitter was telling you where the eel is
and you look down, and it’s nothing but a rocky stream bottom.

E&S: Do you know how deep they go below the rock?

DK: No. That’s something we hope to look at more, the microhabitat of eels
that winter in the streams.

E&S: Would you try to dig them up?

AD: {If things go according to plan, we do plan to excavate a few next
winter so we can characterize their habitat.}

DK: So that was interesting and had implications for management.

E&S: When you talk about them living in the substrate, how long do they
stay down there?

DK: They seem to disappear in September and October.

E&S: While it’s still warm?

DK: Yes, that’s kind of odd. They started moving more in March, so that
would be about 4 months.

E&S: It gets pretty warm in September and October so you’d think …

DK: The water is still low and warm, so we were surprised that they were
not out and about.

E&S: When you talk about implications for management, what do you mean by that?

DK: If the animals are using spaces in the stream bed and along the stream
banks, along the stream edge where the water probably goes further back
away from the stream, where they use spaces, we need to make sure we’re not
compacting those spaces. We need to manage not only for spaces we can see,
but also for spaces we can’t see … and also, knowing that we’re going to
go out and do a survey, we wouldn’t want to go in October, November,
January, or February because we cannot accurately count them and we would
be under-representing them.

E&S: Is this a pebble or gravel stream substrate?

DK: It’s more cobble or boulder. Certainly in other places, they tend to be
more in the mud or leaf packs within the stream. But this stream is
actually rocks … normally when you hear about eels over-wintering, they
overwinter in the mud. And that’s why we thought they were not in the
stream, they had left the stream to overwinter. There weren’t any deep
pools with lots of mud to overwinter in, so that’s why we assumed they had
left. We were surprised to find that they didn’t leave.

E&S: And the study was done at one particular stream?

DK: There were three streams, and they all drain into the Tye River.

E&S: And they all have the same cobble/boulder substrate?

DK: Yes. Another interesting thing is that these streams all have very good
native brook trout populations. Eels are big predators of trout and there
may be some connection between the eels actually helping the trout because
the trout are not becoming over-abundant – the eels prey on the smaller
trout so there are more food resources for the trout to get larger. The
stream also has lots of insects and crayfish. Trout and eels like to eat
crayfish and there are lots of crayfish. It certainly opens the door for a
lot of research – what is the interaction? In the same streams where we
have lots of eels, we have lots of trout. Another management implication,
if eels need special spaces or spaces in-between the rocks, we certainly
don’t want to do anything that would fill those spaces in, silt or other
things like that. So we just need to be aware of that.

E&S: That could result from logging operations …

DK: Yes, or {especially} road construction. Or when you get out of the
forest service [land] housing development and agriculture.

E&S: Can you describe how radio tagging is done?

DK: The eels are anesthesized. The radio transmitter is inserted into the
body cavity of the eel, and they’re sewn back up. It’s internal, in the
body cavity of the eel.

E&S: And what is the lifetime of the transmitter?

DK: You can set them to turn on and off, and they can be on for a couple of
days and be off for a couple of days to increase the life of it. They
should last, I think, for a year.

E&S: How large are they?

DK: They are about, maybe 3 inches long, and about 3/4 inches, probably
less than an inch in width. {About 1 inch long and 5 grams in weight not
including antenna.}

E&S: What are the [tagging] size limits for the eels?

AD: {For radios, minumum of about 500 mm long or 19-20 inches.}

DK: The eels that we tag are all larger eels, ones that we feel comfortable
about doing this to. And we clip their fins so we know we have captured
them before. They will grow their fins back, but for the short term, we’ll
know if it’s an eel that we caught previously. We have also been giving
them a PIT – Passive {Integrated} Transponder – tag. We have been placing
them in each of those animals. {The tag applicator} looks like a
heavy-gauge needle, and the PIT tag {itself} is the size of a large
long-grain rice. {Each tag has a unique 11 digit code.} You run this over a
reader that scans it, almost like at the grocery store. Every eel we catch
gets a PIT tag. So we double mark some – those that have radio
transmitters also have PIT tags, so if we do have a transmitter we know
which animal it is.

E&S: This is in addition to clipping the fins?

DK: Yes. We sample a couple of hundred meters stretch of stream, and catch
all the eels and tag them in order to get a population estimate. Then we go
back in the stream and do a recapture survey, so we can look at the animals
quickly to see if their fins are clipped, so we know if they were the ones
caught the day before.

E&S: How deep is the water in these streams?

DK: It varies by season. There are some deep pools that are probably about
5 to 6 feet deep. But mostly it’s only about a foot deep, it’s not real
deep.

E&S: To clarify … in the past, people thought that eels went into larger
rivers to burrow in mud, but you now have evidence this is not the case in
smaller streams, that they live under the rocks along the stream bed?

DK: Right.

E&S: What are future plans for the study?

DK: We would like to look at the microhabitat, what habitat they’re using,
where they move in the stream, if they move out of the stream. So we’d like
to see exactly what they’re using. Also the relationship between the trout
and the eels … there are some other streams within the watershed that
just don’t have eel populations, and we need to look at the differences
between these streams where they’re found and where they’re not found.
Maybe there’s some microhabitat difference or something.

E&S: When you talk about microhabitat, you’re talking about substrate? DK:
Yes, substrate. Flow relative to substrate, things like that.

- end of interview –

Note: this study was led by Dr. Andy Dolloff of the U. S. Forest Service,
Southern Research Station’s Coldwater Fisheries Research Unit.