Swarm characteristics of Antarctic krill Euphausia superba relative to the proximity of land during summer in the Scotia Sea

Abstract

We studied the relationship between the proximity of land and the distribution and swarming characteristics of Antarctic krill across the Scotia Sea in January and February 2003. Krill swarms identified with a Simrad EK60 (38 kHz, 120 kHz) echosounder were grouped into 4 categories according to distance from shoreline: 0 to 50 km, 50 to 100 km, 100 to 200 km and > 200 km. Cross-sectional areas of swarms were significantly larger inshore, with a mean value of 120 m 2 in the 0 to 50 km zone compared to < 80 m 2 further offshore. The packing concentration of krill within inshore swarms was also significantly greater, with an average density of 95 ind. m -3 compared to between 24 and 31 ind. m -3 elsewhere. A large proportion of the biomass was concentrated into a small number of large, dense swarms throughout the survey area, and this trend increased with decreasing distance from shore. The highest median number of swarms per km and krill acoustic biomass per km was found in the 50 to 100 km zone. However, a significantly greater number of large, biomass-rich swarms occurred in the 0 to 50 km zone compared to all other zones. Swarms in the 0 to 50 km zone were also significantly further apart. The majority of swarms were located in the upper 50 m during the daytime although they were marginally deeper in the night in offshore regions. Krill are likely to move between inshore and offshore environments continuously over their lifetimes. The change in krill behaviour between environments could be a response to local predatory threats over short spatial and temporal scales.

School characteristics of mesopelagic fish at South Georgia

“…In our study, schools appeared to position themselves deeper in the water column and become more compact, or denser which could reduce the chances of being encountered and eaten by diving predators. A similar trend was observed for Antarctic krill across the Scotia Sea indicating that predation pressure is a key driver of the distribution and behaviour of schools of nektonic organisms in the region (Cresswell et al, 2007;Klevjer et al, 2010). Other than a response to predation pressure, the observed trends in depth distribution and echo strength of the schools could also have resulted from shifts in myctophid species or lifecycle stages between zones, with each different species/stage having a different depth distribution and swimbladder morphology in each zone.…”

“…Data were then collated according to estimated distance to shoreline into the following zones to investigate relationships between school morphology and proximity to land: 0-30, 30-60, 60-90 and 90-120 km. These zones broadly match those of Cresswell et al (2007) and Klevjer et al (2010) where aggregation and vertical migration behaviour of pelagic nekton (Antarctic krill) is predicted to change over this cline as a function of different levels of land-based predation and food availability. Data were first compiled for each survey to evaluate the possibility of inter-annual variations in school morphology along this cline.…”

“…These values were selected partially to match the resolution of our acoustic data and to faciliate comparisons with other studies in the region Klevjer et al, 2010;Tarling et al, 2009). A sensitivity analysis was conducted to examine the impacts of variations in these parameters on the number of schools detected with the SHAPES algorithm following Cox et al (2011).…”

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

“…The daytime vertical distribution observed was also very variable despite the relatively small area studied and short investigation period. These patterns could be related to variable local environmental conditions such as bottom topography, currents and predator abundance within the region the vessel operated (Ritz 2000;Klevjer et al 2010). The effect of the two latter factors could unfortunately not be assessed and averaged bottom topography showed no apparent trend or relationship to krill vertical distribution pattern.…”

“…Krill diel vertical migration is variable, most certainly depending on a range of factors including latitudinal light conditions and shelf areas of the Southern Atlantic typically have marked day night differences, normally linked to anti-predatory behavior (Hamner and Hamner 2000;Klevjer et al 2010). Deep waters provide shelter from visual predators diving from the surface, hence could explain deep daytime distributions.…”