bitte übersetzten:      1. Introduction
Antarctic krill (Euphausia superba, hereafter krill) is a highly abundant crustacean of disproportionate
importance to the functions of the Southern Ocean ecosystem [1–3]. Not only is krill a key component
of the pelagic food web, it is also the target of a growing commercial fishery [4]. The krill fishery
typically operates most of the year, excluding the late winter/early spring months of September and
October, with the main fishing grounds being located in the Atlantic sector of the Southern Ocean.
The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) was formed
in 1982 and is responsible for managing this fishery. More recently, in the face of increasing krill catches,
the resurgence of some species of large krill predators [5,6] and the increasing impacts of climate change
on key krill habitats [7–9], calls have been made to develop a more adaptive and sustainable management
strategy that protects the integrity of the Southern Ocean ecosystem and its functions [4,10–14]. Although
highly desirable, such a management plan requires solid knowledge of the ecology of krill, its population
dynamics and temporospatial distribution. Many of these aspects of krill ecology are still poorly resolved,
and in this study we aim to improve our understanding of krill vertical migration behaviour and its
underlying mechanisms.
Diel and seasonal vertical migration are an integral part of zooplankton, and especially krill, ecology
[15–17] and of great importance for biogeochemical cycles [18,19], species interactions, and spatial
distribution through advection by depth-varying ocean currents [20]. Investigating vertical migration
behaviour also yields information about the physiological functioning of the migrating organisms
[21,22] and contributes to our overall understanding of the marine pelagic ecosystem. Observations
over many years have shown that krill swarms can ascend and descend in the water column in
synchrony with the daily light cycle [23–25]. This diel vertical migration (DVM) is thought to be a
behavioural adaptation to the increased mortality risk from visual predators during the day and the
urge to feed in the productive surface layers [26,27]. As a consequence, krill swarms have been
observed to move to the surface after sunset to feed, and descend into deeper waters during the day
to avoid predation [23–25]. While DVM is commonly seen in krill behavioural data, it is not
ubiquitous and a variety of behaviours from reverse DVM to no vertical migration have been
observed and reported [23,28–31]. The mechanisms explaining this variability are not clear but it has
been suggested that reverse DVM may be caused by the presence of planktivorous fish [23,32], and
that krill swarms remain near-surface in their high-latitude habitats during summer when
illumination differences between day and night are less pronounced or even absent [29]. Less is
known about the seasonal vertical migration (SVM) of krill. SVM describes seasonal shifts in mean
residence depth and has predominantly been observed in polar zooplankton taxa [17,33,34].
Organisms performing SVM typically spend the winter months at increased depths while being
shallower in summer [15,33,34]. SVM by Antarctic krill has not been fully determined with some
observations indicating a shift to deeper waters during winter [35,36] and others reporting the
presence of krill close to the surface, especially when sea ice is present [37,38]. These differences
suggest that environmental conditions affect the seasonal depth distribution of krill.
Some of these uncertainties arise from the common limitation of many studies of krill vertical
migration in that they only provide snapshots of behavioural dynamics over a few days or weeks,
with a strong bias towards summer observations [31,39]. In some studies, data from multiple research
campaigns were aggregated but these observations were still restricted to the summer months [25] or
based on multiple shorter-period observations [40] with winter typically missing. The use of acoustic
instruments on moorings permits the study of the vertical distribution of Southern Ocean zooplankton
over longer time periods, but these data are limited to one location [33]. It can also be difficult to
identify the recorded species as validating net samples do not exist for the duration of the mooring
deployment. There is no perfect solution for these issues as research campaigns in the Southern Ocean
are expensive, and getting research time on the existing research platforms is competitive. On the
other hand, the commercial krill fishing fleet operates almost year-round, targeting krill swarms in a
royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 230520
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variety of habitats, and hence providing a potential additional source of observations and data. The
advantage of data from fishing vessels is that the vessels operate for much longer time periods than
individual field campaigns, and that they explicitly target krill swarms across seasons. Although the
vessels do not fish in fully ice-covered waters, they still operate in some of the key krill habitats
in the Atlantic sector of the Southern Ocean such as the Antarctic Peninsula, the South Orkney
Islands, the Scotia Sea and South Georgia [1,41]. Although CCAMLR deploys independent fishery
observers on board these vessels, no systematic programme exists to use the vast amount of
information that these vessels could provide on krill ecology and behaviour [4].
In this study, we demonstrate the value of data collected by krill fishery vessels to provide detailed
information on the seasonal and diel vertical distribution of krill swarms. We analyse eight months of
continuous acoustic recordings that were collected with an ES80 echosounder (Kongsberg Maritime
AS) from December 2020 to July 2021 on board the FV Antarctic Endurance, a commercial krill fishing
vessel operated by Aker BioMarine. Two previous studies used data from the Japanese krill fishing
fleet to describe dynamics of seasonal vertical krill distribution [35,36], mainly based on the seasonal
variation of net depths and demographic indices of the catch. By using acoustic data, we are able to
provide detailed analyses of krill swarm vertical distribution over the upper 250 m of the water
column in space and time. Although krill are known to occur at much greater depths [42–45], the
main vertical distribution of krill swarms is thought to be in the upper 300 m. In a review of 30
studies assessing the vertical distribution of krill swarms during summer, Schmidt et al. [44] found
that typically 80–98% of the krill biomass is found in the upper 200 m of the water column. We
analysed the vertical migration behaviour of krill from qualitative and quantitative perspectives and
considered the observed behavioural variability in relation to environmental conditions. The study
demonstrates ways in which krill fishing vessels could contribute to a better understanding of krill
ecology and consequently, improved management strategies. We conclude by providing
recommendations for additional data that could be collected with relatively little effort on board
fishing vessels to allow for even more detailed analyses.