Results
Summary statistics of diel
activity
We continuously recorded ~35,000 dives from 54 U.
aalge individuals and ~8,000 dives from 16 A.
torda individuals, ~36,000 prey depths and
bins of 10° azimuth. Both predators showed higher foraging efficiency
during twilight, with a higher peak during dusk compared to dawn (+28%
and +17% for A. torda and U. aalge respectively; Fig 1A),
but for A. torda the elevated efficiency also lasted much longer
during afternoon/dusk (110° azimuth) than in the morning (60° azimuth).
Both predators concentrated depth of their dives to the depth of the
largest abundances of prey (median of prey depth = 16m, A. torda depth = 12m and U. aalge = 15m; Fig 1B). Still, U. aalge ,
but not A. torda , also used deeper depths (~20%
dives at >50m) but only during daylight hours, while prey
depth ranged from 4 to 96m both during the day or under low light
levels. Both predators had a higher number of dives in light conditions
with high foraging efficiency (Fig 1C), however U. aalge utilized
a much wider range of light levels, both with the highest effort
performed in lower light periods and with a higher proportion of dives
throughout the day as compared to the A. torda . A. torda instead concentrated its effort during peak of twilight, with much more
dives during dusk than dawn, and a more rapid decline in effort after
the dawn peak. The median ° azimuth for dives were 73.7° for A.
torda and 68.8° for U. aalge before noon (i.e. 0-180° azimuth),
and 289° versus 290° after noon (180-360° azimuth), respectively. In
summary, both predators foraged more in shallower waters, and were more
efficient in twilight hours, most marked at dusk compared to dawn, and
for Alca T. also more throughout the afternoon. Numbers of prey
aggregations were highest around dawn and dusk, lowest during the night
and at intermediate levels during daylight periods (Fig 1C). There was a
distinct difference in prey abundance in the upper 50m of the water
column between low-light and daylight periods (Table A3) as a result of
vertical fish migration. This led to 22% increase in abundance in the
upper 50 m of the water column, and an overall increase of 20% in
detected abundance, during low light periods versus daylight (Carlsen et
al., 2024).
Light-related activity and distribution
patterns
Effort (number of dives) typically started increasing before the
efficiency (BDCr) increased in the morning (0-50 versus 30-70 azimuth),
and decreased only after the efficiency started decreasing (Fig 2A).
Dive depth and bout lengths had opposite relations with solar position
for U. aalge , where deeper dives coincided with shorter bouts in
high light conditions and vice versa in the dark (Fig 2B). For A.
torda bout lengths were best described as linearly increasing with
azimuth. While the raw data previously showed an equal median dive depth
across A. torda , the models depict a depth trend reflecting the
vertical distribution of prey by azimuth similar to the trend inU. aalge . The U. aalge dives were markedly deeper during
the day than night, and with higher peaks during vertical migrations,
slightly more so during dawn than dusk (Fig 2B).
When investigating the trends in diel PDPs, the depth distribution of
prey was symmetrical around noon/midnight (Fig 2C), with inflection
points at 73° and 286°, and an estimated average depth change of
but was not symmetrical around noon/midnight (Fig 2C). The number of
aggregations increased before the migration started in the early
mornings (0-100° Azimuth), but already by 160° aggregations started
dissolving and were reduced by ~25% by early afternoon
(Fig 2C) and kept decreasing towards the evening. Notably, while the
majority of aggregations, especially in high number, were observed at
Predator activity as a function of prey
distribution