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2. Materials and methods
2.1. Experimental area and procedure The study was carried out in April 1995 in a branch fjord in western Norway 2.5 km
long and approximately 800 m wide, Fig. 1 with depths ranging from 80 to 100 m. The muddy bottom is relatively flat, but with some stones. On each side of the fjord is a
steep rock wall, and where the fjord joins the main fjord the depth falls steeply to about 400 m. The current velocity and direction were recorded every 10 min by two current
meters SD6000, Sensordata A S, Bergen, Norway suspended one metre above the
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seabed in the experimental area. The current velocity was usually below 2 cm s .
A stationary telemetry system VRAP, Vemco Ltd., Halifax, Canada was used to track the fish, which were tagged with acoustic transmitters. The positioning system consisted
of a fixed array of three hydrophone buoys anchored to the seabed in a triangular configuration with radio transmission to a base station installed on board a research
vessel. The distances between the buoys were between 381 and 592 m. The hydrophone buoys received acoustic pulses from acoustic transmitters. Using the time delays of pulse
arrivals at the hydrophones, the base station calculated the position of the transmitters relative to the three buoys. A number of received pulse-times were aligned and an
average position was calculated. The system was capable of tracking up to ten fish tagged with transmitters that operated at different frequencies. However, the system
monitored only one frequency at a time, and the elapsed time between successive positional fixes thus varied with the number of animals being tracked. In this study each
fish was tracked approximately every three minutes except during the first days, when fewer individuals were tracked. Under optimal acoustic conditions the transmitter can be
localized to an accuracy of about one metre within the triangle of buoys. When the transmitter was outside the triangle the accuracy decreased.
The cylindrical transmitters had their electronics and batteries sealed in epoxy, and produced one pulse per second. Two types of transmitters were used; one, 65 mm long
and 16 mm in diameter weighing 10 g in water, that gave horizontal two-dimensional positional fixes x
,y coordinates and one, 80 mm long and 16 mm in diameter weighing 12 g in water, that contained a depth sensor and gave three-dimensional positional fixes
x ,y,z coordinates.
Five ling estimated length: 50–80 cm were tagged in situ by allowing them to voluntarily ingest a transmitter wrapped in bait. A high-sensitivity SIT camera Osprey
1321 mounted in a frame was used to determine the species and size of fish that ingested the transmitters. Two 150 W lamps were attached to the frame to enable
observations to be made under poor light conditions. The transmitters were wrapped in mackerel baits cut in thin slices and enclosed in bags of fine-mesh nylon fabric. One
bait-wrapped transmitter and two mackerel baits providing additional feeding attractants were attached by fine threads to a rod mounted in front of the camera. The thread
allowed the fish to tear off the bait and ingest the transmitter. The rod and the frame had marks at 10 cm intervals to allow the length of the fish ingesting the transmitter to be
estimated. The frame was placed on the seafloor, and observations of fish approaching
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Fig. 1. The experimental area in a fjord on the west coast of Norway. The position of the triangle of hydrophone buoys is indicated.
S . Løkkeborg et al. J. Exp. Mar. Biol. Ecol. 247 2000 195 –208
199 Table 1
Summary of tracking information on individual ling in a fjord in western Norway. Dark bars indicate periods with baits present in the experimental ova
and ingesting the transmitter were videotaped. A more detailed description of the
˚ positioning system and tagging procedure has been provided by Engas et al., 1996.
During the first four days of the experiment five ling were tagged, one of them with a depth transmitter. Their movements and basic behaviour were tracked continuously for
2–6 days. Responses to food odour were then studied by setting a mackerel-baited longline 415 hooks, 1.3 m hook spacing inside the triangle of hydrophone buoys each
night between 18:45 and 20:15 for five days and retrieved the following morning between 08:00 and 09:00. In addition, on two of the days, five mackerel-baited pots
about 100 g bait in each pot in a chain and a single pot were set Table 1. A transmitter was attached to the baited gear to determine their positions and provide a
fixed reference point. In the period with no gear in the area, a transmitter was placed on the seabed as a reference point in order to indicate the accuracy of the positional fixes.
2.2. Data analysis Spurious positional fixes due to background noise or signals reflected from rocks were
manually removed before data analysis. A positional fix was regarded as spurious if it was far from both its previous and successive fixes whereas these were close to each
other. Speed, distance and direction of movement between positional fixes were calculated for all fixes where the time since the last positional fix did not exceed 10 min.
These data were used to calculate mean swimming speed and longest rectilinear distance between two fixes LRD per hour. As the real swimming distance between two fixes
may be longer than the straight-line distance, actual swimming speed may be under- estimated. LRD is the diameter of the circle within which the animal has moved, and it
provides a rough measure of the area the animal has covered during the specified time. The size of a core area was also calculated. A core area is defined as the part of a home
range that is used more intensively than the rest Burt, 1943, and was defined here as the area rectangle within which ling remained in an inactive state i.e. inactive for more
than 3 h. The criterion used to define whether a ling was inactive was that the distance both to the previous fix and to the one four fixes earlier i.e. at least 10 min previously,
should be less than 10 m, taking into account the inaccuracy of the positional fixes. This ensured that the ling had not moved far since the previous fix, and it excluded those fixes
where the ling moved slowly for a few minutes during an active period. Only one core
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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
