Key words: amphipods, Baikal, daily vertical migrations, artificial illumination, video observations.
Daily vertical migrations (DVMs) of pelagic and
even many benthic organisms are characteristic of both
marine and continental aquatic ecosystems. The causes
of such migrations and factors relevant to them are of
great interest of the involved factors is highly interest-ing. For several years, we investigated DVMs of Baikal
amphipods (Crustacea, Amphipoda), namely, the noc-turnal migration of many shallow-water benthic species
to the pelagic zone. We identified the main life forms of
amphipods involved in DVMs, dominant species of the
nocturnal migratory complex in several areas of Baikal,
and rejected the defense–feeding hypothesis of DVMs
traditionally used to explain the vertical migrations of
plankton (Takhteev et al., 2000; Mekhanikova and
Takhteev, 2001).The prevalence of immature juveniles
in the nocturnal migratory aggregations also led to a
conclusion that DVMs in most species are not related to
reproduction and mating (Govorukhina, 2001). We pro-posed that daily migrations provide for the accumula-tion of a certain sum of temperatures (if a vertical tem-perature gradient is present) or for activation of metab-olism via motion proper in order to complete
maturation in due time under conditions of a cold-water
lake (Mekhanikova and Takhteev, 2001).
The intensity of DVMs depends on meteorological
conditions (first of all, wave height) and illumination on
a certain night. The level of illumination is determina-tive in the vertical migrations of Baikal zooplankton
(Mogilev, 1955). Previously (Bessolitsyna, 2000), it
was noted that the intensity of DVMs of benthic amphi-pods decreased on moonlit nights. In an experiment
(Bessolitsyna and Stom, 2001), amphipods making
night migrations actively avoided both bright daylight
(300–400 lx) and weak artificial light (35–40 lx). On
the other hand, we repeatedly observed that a weak arti-ficial illumination (e.g., with a flashlight) attracted
amphipods swimming in open water. This fact was used
to increase sample size in qualitative collections. At the
Baikal Biological Station of the Irkutsk State Univer-sity in Bol’shie Koty, a searchlight installed on the pier
had been used for several years to collect amphipods
and juvenile sculpin at night. However, it remained
unclear if the searchlight attracted nocturnal migrants
or just aided in collecting them.
The purpose of this study was to elucidate the effect
of weak illumination (in this case, artificial) on the
intensity of DVMs. Technical devices for underwater
video observations provide new opportunities for inves-tigating the Baikal ecosystem. We have gained the first
experience in applying video equipment to studies on
DVMs of benthic organisms in Lake Baikal.
Observations were made in the course of expedition
aboard the research vessel Professor Treskov
(June 2002) at two sites of northern Baikal: in the
Solontsovaya Bay on the side of Cape Sagan-Maryan,
at the western coast (the area of the Baikal–Lena State
Nature Reserve), and in the Peshcherka Bay, on the
eastern side of the Bol’shoi Ushkanii Island (figure).
The work was done from the vessel anchored in a shoal
above a platform.
A Sony TR8000E video camera with an accessory
wide-angle lens (to expand the field of view in the
aquatic environment) was placed in a sealed box with
two 35-V halogen lamps. The signal from the box was
transmitted through a cable on deck, where it was
recorded by a Hitachi VM-8480LE video camera, with
the image being controlled on its color display. Record-ing was made in two modes: under artificial light from
the lamps installed on the box and in a night-vision
mode, using a built-in infrared emitter of the camera. In
both cases, the range of vision was 3–4 m.
Recorded images were analyzed on a wide-screen TV
set. Amphipods appearing on the screen were counted
using a freeze-frame option when necessary. As some
crustaceans could repeatedly enter the field of view, one
instance of an animal entering and leaving the frame was
recorded as one specimen. Such counting did not allow
us to determine the absolute density of crustaceans (per
unit water volume) but was appropriate for a compara-tive analysis of their migration activity.
Solontsovaya Bay(depth 8 m; June 24, 2002;
1:40−2:25 a.m.). The ground consisted of uneven-sized
boulders and pebbles of proalluvial origin, mostly
rounded and partly submerged in sand, with single
small green sponges.
When the box with lamps onsubmerged, no moving
organisms occurred in the field of view, including the
period when the bottom was already seen. Before the
box reached the bottom, five to eight migrants
appeared. Within 10–20 s after the box touched the
ground, animal movements became more active: the
number of amphipods increased from 2–4 to
10−15 specimens swimming at different distances from
the lens. After 1.5–2 min, the number of amphipods in
the field decreased again (1–5 ind. per frame).
In night-vision mode,no amphipods occurred in
the field of vision for 75 s after the box touched the
ground for the first time. The box was then transferred
to another place, where only a single small amphipod
swimming 20 cm above the ground was detected within
25 s. After moving the box along large boulders and
placing it on the bottom, amphipods (no more than two
specimens per frame) appeared only after 25 s, with the
field of view remaining empty most of the time. In a
new place, only 11 small amphipods of different spe-cies (precise identification by the recorded image was
impossible) appeared before the camera during 160 s.
After the next transfer and descent, observations con-tinued for 6 min 55 s, and amphipods (including one
egg-bearing female) entered the field of view only six
times. No migrants were detected in the course of hoist-ing the box on the deck.
Bol’shoi Ushkanii Island(Peshcherka Bay, depth
8 m; June 25, 2002; 2:30–3:10 a.m.). The ground con-sisted of boulders encrusted with sponges on sand with
Draparnaldioidesalgae.
When the box submerged with the lamps on, aggre-gations of white swimming amphipods were observed
throughout the water column, from the surface to the
bottom. They could have been attracted by lights on the
deck and on the box, but it appeared that some speci-mens had already been there before the onset of obser-vations. According to our unpublished data, this could
have been Micruropus wohliiamphipods, which form
mass nocturnal aggregations attracted by the search-lights of vessels in the area of the Ushkanii Islands. The
density of migrating amphipods reached a peak at a dis-tance of 2−5 m from the bottom and decreased at
greater depths.
Immediately after the box touched the ground and
was adjusted horizontally, 50–60 amphipods could be
detected in the field of view. They belonged to at least
two species: a white one (most probably, Micrurops
wohlii) and a dark one. The number of migrants
increased by a factor of 1.5 after 30 s and almost doubled
after 60 s, with the animals moving closer to the sources of
light. A pair of amphipods was noted near the lamps. The
density of migrants remained high for 3 min. The white
specimens were initially 1.5 times more numerous; later,
the proportion of the dark specimens increased, and the
ratio of the two forms became equal. The dark amphipods
kept closer to the bottom than the white ones: when the
lens was directed upward, the latter began to prevail, with
the number of migrants in the field of view reaching 180
specimens. When the lens was pointed to an area that had
not been illuminated, the recorded density of aggregation
proved to be 13 times lower. This fact indicated that artifi-cial light attracted the amphipods.
In the course of lifting the box, the highest density
of “light” migrants (about 60 specimens in the field of
view) was observed at depths of 0.5–3 and 5–7 m.
In the night-vision mode,virtually no migrants
were detected. In the bottom layer (when boulders
appeared), no more than 50 specimens were found in
the field of view. When the box touched the ground,
they disappeared for a minute because of roiling and
then started appearing again, in one or two specimens.
The same was observed in a different location. When
the box was lifted to the pelagic zone, only three spec-imens appeared on the screen during 20 s. After return-ing the box to the bottom for 4 min 15 s, the migrants
were detected in the field of view only twice.
Thus, observations at both sites showed that light
from the lamps of the video box obviously stimulated
the movement of amphipods in the open water of the lit-toral zone. Weak artificial light attracted these animals
and activated their migratory behavior. Therefore, the
observed decrease in the intensity of DVMs on moonlit
nights (Takhteev and Bessolitsyna, 1999; Bessolitsyna,
2000) is probably explained by some as yet unknown
factors, rather than by avoidance of moonlight. Video
recording does not allow accurate species identification
in most cases. It may well be that different species of
amphipods respond to artificial light in different ways
and the picture observed in this study is averaged. Fur-ther investigations are necessary for elucidating the
responses to artificial illumination in different species
and life forms of amphipods.
Video observations confirm our earlier conclusion
(Govorukhina, 2001) that DVMs of amphipods are not
related to mating: in this study, only one pair swimming
in the pelagic zone was noted.
