An Antarctic krill (Euphausia superba) hotspot: population characteristics, abundance and vertical structure explored from a krill fishing vessel

Abstract

Antarctic krill (Euphausia superba) aggregate in various ways depending on a range of biological and physical factors. In some areas, typically associated with bathymetric features such as shelf edges and canyons, they may aggregate densely to form hotspots. Despite the importance of such hotspots, their development over time in demographic composition and spatial distribution is not well understood. A fishing vessel during regular operation was used for collection of krill demographic and acoustic data on the shelf northwest of South Orkney Islands. Results show a decrease in the proportion of subadult males, partly reflected in an increase in mature adult males. Concurrently, there was a change in the proportion of males in the sampled population from 0.8 to 0.3, indicating immigration or emigration of krill through the hotspot. A clear trend was observed in the diurnal vertical distribution with deeper and more vertically compact swarms during the day. However, some days displayed very small differences between the day and night distribution and considerable variability in the daytime depth distribution. It was noted that although fishing was carried out during the entire period of the study, there was no obvious trend in the acoustic backscatter, suggesting that the overall krill density was not changing during this period. Using a fishing vessel as a research platform has advantages for understanding the dynamics of the fishery and in quantifying biological and physical processes during actual exploitation of these resources.

Detection of growth zones in the eyestalk of the Antarctic krill Euphausia superba (Dana, 1852) (Euphausiacea)

“…Future work on the molting of the Antarctic krill, and verification of increment rates are prerequisite before the observed growth zones can finally be used as an age indicator. This method has the potential of providing agebased monitoring and assessment useful for management, recognizing differences in growth rates between seasons and areas in the Southern Ocean involving different environmental conditions (Siegel et al, 1990(Siegel et al, , 2002Krafft et al, 2010Krafft et al, , 2012Krafft et al, , 2015.…”

“…These consist of chemicals incorporated into the growing structures and can subsequently be used to estimate growth from time of exposure. Designing a study for verification that combines marking techniques and laboratory experiments keeping conditions similar to the krill's natural environment, as in Krafft and Krag (2015), could be a possible approach.…”

“…Collections were made using a 7 mm mesh (stretched) survey trawl ("Macroplankton trawl"; Krafft et al, 2010) hauled from FV Juvel (Olympic AS), a commercial Norwegian ramp trawler, from a depth of 200 m to the surface at stations evenly distributed over the study area. Individual Antarctic krill were sampled from the catch to represent the various developmental stages present.…”

“…Individual Antarctic krill were sampled from the catch to represent the various developmental stages present. Samples were preserved on 70% ethanol and detailed measurements were made of sex and maturity stages using the classification methods outlined by Makarov and Denys (1981) and described in Krafft et al (2015). Males were separated into three sub adult stages: MIIA1 (petasma vesicles are not divided, but appear as a small "bump" or "bubble" at the root), MIIA2 (petasma has developed the "bubble" to a split with one or two "fingers"), and MIIA3 (petasma root with two short "fingers" and an incipient formation of "wings" on the opposite hold), and two adult stages: MIIIA (petasma fully developed, with swollen "fingers" and with a "wing" overlap, ductus ejaculatori are also visible ventrally, but these are sealed and spermatophores cannot be squeezed out), and MIIIB (petasma as for MIIIA, ductus ejaculatori has spermatophores that can be pressed out, or with the duct passage open where spermatophores are already deposited).…”

Quantifying the Escape Mortality of Trawl Caught Antarctic Krill (Euphausia superba)

“…This study was carried out on commercial fishing grounds off the coast of the South Orkney Islands (60°35′S, 45°30′W) [ 28 ] during February 2015. The vessel used was the FV Juvel (Olympic AS) a Norwegian, 99.5 m, 6000 kw/8158 hp (main engine) commercial ramp trawler.…”