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. Løkkeborg et al. J. Exp. Mar. Biol. Ecol. 247 2000 195 –208
area was defined for each individual as the fish were seldom inactive for longer than 10 min at locations outside this area only on three occasions for longer than 3 h. The
positional fixes of the core area were trimmed to compensate for drift of the hydrophone buoys and inaccuracy of the fixes the 20 most extreme values in each direction were
removed, before the size of the area was calculated. This was defined as the area of the rectangle given by the minimum and maximum X- and Y-values.
Using the tracking data of the first part of the experiment, when basic behaviour was studied, diel rhythms in activity were studied by dividing the 24-h cycle into 3-h periods.
A Friedman ANOVA was run to compare differences in swimming speed and LRD between times of day. This nonparametric test adjusts for potential individual differences
and allows for deviation from normality. It does not, however, take into account possible interaction between time and individual. The test requires that all time periods during a
day contain data and therefore only 24-h cycles fulfilling this requirement were included in the analysis. The data were log-transformed before the analysis to stabilize the
variance between groups. Diel rhythms in ‘staying within the core area’ were analysed by Friedman ANOVA. An ANOVA was run to test whether the direction of current
influenced direction of movement.
A Mann–Whitney U test was performed to compare differences in activity mean speed per hour and LRD between nights from 20:00 to 09:00 with and without baits
in the experimental area. As baits were set only early in the night, only data recorded at night were compared as the release rate of feeding attractants is highest shortly after
setting Løkkeborg, 1990.
3. Results
All ling stayed inside or close to the buoy triangle, except for one fish that swam for a few hours beyond the range of the hydrophones. The ling were most active at dawn and
dusk, with peak activity between 04:00 and 08:00, and a less clear activity peak from 20:00 to 22:00 Fig. 2. There were significant differences during the 24-h cycle in mean
swimming speed and LRD P , 0.001. The ling tagged with a depth transmitter remained near the bottom at depths ranging from 90 to 100 m during both movement
and periods of inactivity.
Usually at dawn, but sometimes at dusk, the fish swam around a large area, but with few exceptions they returned to the same area where they spent the day and night. Most
of the time when the fish were inactive was spent within this core area, which averaged 62 3 79 m Table 2. The fish spent 65 SD 5 4.9 of their time on average within the
core area. The real size of the core area was smaller than the estimate due to the drifting of the buoys, as was indicated by the observation that the static transmitter serving as a
reference point had a core area of 36 3 68 m when calculated as for the fish. Ling had thus presumably a very small core area.
There were significant differences in ‘time spent within the core area’ percentage of the positional fixes within the core area during the 24-h cycle P , 0.001, with most
S . Løkkeborg et al. J. Exp. Mar. Biol. Ecol. 247 2000 195 –208
201
Fig. 2. Longest rectilinear distance between two positions LRD and swimming speed per hour mean6S.E., n 5 5 of ling throughout the 24-h cycle. Dark and light sections indicate night and day defined by the times of
sunrise and sunset in the middle of the study period.
time being spent outside the core area in the morning from 05:00 to 08:00, Fig. 3. This coincided with the high-activity period at dawn.
The movement of one ling during a 24-h period is shown in Fig. 4. Its most wide-ranging movements were made at dawn, but it also moved from 12:30 to 14:00 and
from 18:15 to 19:45. In the active period at dawn, the fish often moved to a certain area where it swam around for a while before returning to the core area. On three occasions a
ling entered the core area of another ling when the holder was present.
For the most part, the ling seemed to move predominantly in one direction. The
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. Løkkeborg et al. J. Exp. Mar. Biol. Ecol. 247 2000 195 –208 Table 2
a
Size of and time spent within the core area Ling no.
Core area Length of sides
Time spent
2
m of core area m
within core area 1
8419 70 3 120
69 2
3835 58 3 67
72 3
2328 53 3 44
62 4
3287 49 3 67
61 5
7526 79 3 95
62 Average
5079 62 3 79
65
a
The core area is the rectangle x 3 y within which the fish stayed when it was inactive.
preferred direction varied between individuals, and there was no correlation between current direction and movement direction P . 0.05. As the seabed in the area was
relatively flat and uniform, it is unlikely that the ling moved in relation to the topography.
Three of the five tracked ling located baits; two of them made three localisations and one made two. These eight localisations occurred throughout the day, except between
00:30 and 06:30. Ling showed significantly lower swimming speed and smaller LRD during nights from 20:00 to 09:00 with baits present in the area than during nights with
no baits in the area P , 0.001; Table 3.
Fig. 3. The proportion of time i.e. percentage of the positional fixes ling spent within the core area throughout the 24-h cycle mean per hour6S.E., n 5 5. Dark and light sections indicate night and day defined
by the times of sunrise and sunset in the middle of the study period.
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203
Fig. 4. Movement of one ling throughout an entire day from midnight to midnight. Arrows indicate direction of movement. Times and alternating symbols indicate successive periods of activity and squares indicate the
position of the fish during inactive periods. A, B and C indicate the positions of the hydrophone buoys.
4. Discussion
