Aggregation and Vertical Migration

Watkins, J. L.

doi:10.1002/9780470999493.ch4

Cited by 25 publications

(11 citation statements)

References 63 publications

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“…In this context, it is interesting that density estimates averaged over all aggregations identified acoustically in the overall vicinity of where the net system sampled were much closer to net estimates than was the case when we examined only volume backscattering in the exact depth interval and as close as possible to the spatial area sampled by the nets. Furthermore, although the acoustic estimates of krill density exceeded those from the nets, they fell within the range of estimates made with nets elsewhere (nets 0.2-348.3 individuals m-3; Siegel, 2000), but below those estimated visually by divers (20,000-60,000 individuals m-3; Watkins, 2000).…”

Section: Acoustic Methodologiessupporting

confidence: 50%

“…Most euphausiids aggregate to some extent (Mauchline, 1980a), but the Antarctic krill (Euphausia superba) is notable for its consistent formation of strikingly cohesive aggregations, ranging in horizontal extent from meters to several kilometers, and vertically from 1 to 250 m (Watkins, 2000). Krill aggregative behavior is not restricted to adults, but is also seen in larval forms (Marr, 1962;Hamner et al, 1989).…”

Section: Acknowledgmentsmentioning

confidence: 99%

“…The accepted explanation for diel vertical migration is that krill migrate upwards in the water column during the night to feed, returning to greater depths by day to avoid visual predators (Watkins, 2000). Under low chlorophyll a conditions, krill have been observed to migrate with smaller amplitude than when food was more plentiful, arguably because the need for food outweighs the predation risks of remaining in shallow waters during the day (Godlewska, 1996).…”

Section: Diel Vertical Migrationmentioning

confidence: 99%

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Distribution, patchiness, and behavior of Antarctic zooplankton, assessed using multi-frequency acoustic techniques

Lawson¹

2006

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The physical and biological forces that drive zooplankton distribution and patchiness in an antarctic continental shelf region were examined, with particular emphasis on the Antarctic krill, Euphausia superba. This was accomplished by the application of acoustic, video, and environmental sensors during surveys of the region in and around Marguerite Bay, west of the Antarctic Peninsula, in the falls and winters of 2001 and 2002. An important component of the research involved the development and verification of methods for extracting estimates of ecologically-meaningful quantities from measurements of scattered sound. The distribution of acoustic volume backscattering at the single frequency of 120 kHz was first examined as an index of the overall biomass of zooplankton. Distinct spatial and seasonal patterns were observed that coincided with advective features. Improved parameterization was then achieved for a theoretical model of Antarctic krill target strength, the quantity necessary in scaling measurements of scattered sound to estimates of abundance, through direct measurement of all necessary model parameters for krill sampled in the study region and survey period. Methods were developed for identifying and delineating krill aggregations, allowing the distribution of krill to be distinguished from that of the overall zooplankton community. Additional methods were developed and verified for estimating the length, abundance, and biomass of krill in each acoustically-identified aggregation. These methods were applied to multifrequency acoustic survey data, demonstrating strong seasonal, inter-annual, and spatial variability in the distribution of krill biomass. Highest biomass was consistently associated with regions close to land where temperatures at depth were cool. Finally, the morphology, internal structure, and vertical position of individual krill aggregations were examined. The observed patterns of variability in aggregation characteristics between day and night, regions of high versus low food availability, and in the presence or absence of predators, together reinforced the conclusion that aggregation and diel vertical migration represent strategies to avoid visual predators, while also allowing the krill access to shallowly-distributed food resources. The various findings of this work have important implications to the fields of zooplankton acoustics and Antarctic krill ecology, especially in relation to the interactions of the krill with its predators.

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Section: Acoustic Methodologiessupporting

confidence: 50%

Section: Acknowledgmentsmentioning

confidence: 99%

Section: Diel Vertical Migrationmentioning

confidence: 99%

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Distribution, patchiness, and behavior of Antarctic zooplankton, assessed using multi-frequency acoustic techniques

Lawson¹

2006

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“…Alling et al 1991, Fiedler et al 1998, Palacios 1999, Hucke-Gaete et al 2003, Croll et al 2005, Etnoyer et al 2006, Branch et al 2007, which occur at multiple spatio-temporal scales (Watkins 2000). The use of appropriate scales is central to ecological studies (Levin 1992), and scales of oceanographic features that aggregate prey vary greatly, from the macro-scale Antarctic Circumpolar Current (Tynan 1998) to mesoscale coastal upwellings (Croll et al 2005) and more fine-scale bathymetric features such as individual banks (Stevick et al 2008).…”

Section: Introductionmentioning

confidence: 99%

Blue whale habitat selection and within-season distribution in a regional upwelling system off southern Australia

Gill¹,

Morrice²,

Page³

et al. 2011

Mar. Ecol. Prog. Ser.

105

104

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Blue whales Balaenoptera musculus aggregate to feed in a regional upwelling system during November-May between the Great Australian Bight (GAB) and Bass Strait. We analysed sightings from aerial surveys over 6 upwelling seasons (2001-02 to 2006-07) to assess within-season patterns of blue whale habitat selection, distribution, and relative abundance. Habitat variables were modelled using a general linear model (GLM) that ranked sea surface temperature (SST) and sea surface chlorophyll (SSC) of equal importance, followed by depth, distance to shore, SSC gradient, distance to shelf break, and SST gradient. Further discrimination by hierarchical partitioning indicated that SST accounted for 84.4% of variation in blue whale presence explained by the model, and that probability of sightings increased with increasing SST. The large study area was resolved into 3 zones showing diversity of habitat from the shallow narrow shelf and associated surface upwelling of the central zone, to the relatively deep upper slope waters, broad shelf and variable upwelling of the western zone, and the intermediate features of the eastern zone. Density kernel estimation showed a trend in distribution from the west during November-December, spreading south-eastward along the shelf throughout the central and eastern zones during January-April, with the central zone most consistently utilised. Encounter rates in central and eastern zones peaked in February, coinciding with peak upwelling intensity and primary productivity. Blue whales avoided inshore upwelling centres, selecting SST ~1°C cooler than remotely sensed ambient SST. Whales selected significantly higher SSC in the central and eastern zones than the western zone, where relative abundance was extremely variable. Most animals departed from the feeding ground by late April.

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“…Neither of these traits, by definition, can be exhibited by plankton, and krill, as explained previously, are automatically classed as zooplankton. There is well-documented and uncontested biological evidence of krill populations exhibiting behavior that affects their location both horizontally and vertically (see Watkins 2000) 10 but the mechanism by which they navigate is unknown. However, assuming they cannot navigate because we do not know how they might do it is an unsatisfactory argument-we know whales navigate their way around the ocean but we have no real understanding of how they do it.…”

Section: Impact Of Perceptions On Conservation Of Krillmentioning

confidence: 99%

Charismatic Krill? Size and Conservation in the Ocean

Leane

Nicol

2011

Anthrozoös

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The open ocean is an alien place for human beings and for most of history it has been studied using very indirect means. A great deal of what we know about animals that live on land, from ants to elephants, is based on centuries of direct natural history observations of animals alive and in situ.There is no such body of observation of marine animals. Most of what we know about animals in the ocean comes from studying dead animals, collected in nets towed through ocean depths that we can rarely observe. Marine biological knowledge is flavored by the material we have to work with and this, in turn, is affected by the terminology that early scientists used in an attempt to describe the strange world that their nets revealed. Despite the recent development of more sophisticated ways of investigating the ocean, many marine scientists continue to adopt nineteenth-century terminology and approaches (such as the use of the term zooplankton and the use of plankton nets), and this colors interpretation of the results of oceanographic studies. Using Antarctic krill Euphausia superba as an example, we suggest that the perception amongst both scientists and the general public of krill's small size, passivity, and lack of individuality has led to their being treated less like animals themselves than as background habitat for other animals. Because krill have such a central role in the Antarctic ecosystem and are the subject of a large fishery, these assumptions may have serious consequences for krill themselves and the conservation of the Antarctic region as a whole.When tourists departing for the Antarctic are asked to rank the attractions of the region, vertebrate wildlife tops their lists. Penguins are the favorite, followed by whales, seals, and birds (Bauer 2001, p. 56). These findings are unsurprising. Research has repeatedly shown that human aesthetic response to animals favors what Stephen Kellert terms "charismatic megavertebrates"-"larger, more

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Aggregation and Vertical Migration

Cited by 25 publications

References 63 publications

Distribution, patchiness, and behavior of Antarctic zooplankton, assessed using multi-frequency acoustic techniques

Distribution, patchiness, and behavior of Antarctic zooplankton, assessed using multi-frequency acoustic techniques

Blue whale habitat selection and within-season distribution in a regional upwelling system off southern Australia

Charismatic Krill? Size and Conservation in the Ocean

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