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Evaluation of Antarctic krill biomass and distribution off the South Orkney Islands 2011-2015

Skaret, Georg; Krafft, Bjørn Arne; Calise, Lucio; Watkins, J; Pedersen, Ronald; Godø, Olav Rune
Research report
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URI
http://hdl.handle.net/11250/2398441
Date
2015-06-22
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Abstract
Annual acoustic trawl surveys for krill monitoring have been carried out by the Institute of

Marine research, Norway near the South Orkney Islands since 2011. The survey has been

conducted early in the fishing season (January/February), using two different krill fishing

vessels as platforms. The vessels were equipped with similar Simrad echo sounder systems

suitable for quantitative assessments, but the frequencies operated varied between vessels and

years. In addition, the survey coverage has varied between years in particular due to ice. In

order to allow for comparison of distribution and abundance of krill near the South Orkneys

among years, we here attempt to generate a coherent series of krill density estimates from the

5 years of surveys. We follow the CCAMLR protocol for biomass estimation as far as

practically possible, given that we work with unconventional sets of frequencies for target

strength estimation and target identification. In order to avoid variability due to differences in

coverage, we also define a stratum within the survey area on the north side of the islands with

full coverage in all years except 2013. The results show that except from the year 2015, krill

densities were high, in the range 100-300 g/m2

based on the 120 kHz recordings, and a total

estimated biomass of ca. 8 million tons of krill within the stratum in the peak year 2014.

There were also consistently higher values of acoustic backscatter on the north side of the

islands where the fisheries occur, than on the south side. Values were particularly high in the

north-west shelf area associated with underwater canyons. In 2012, and particularly in 2015,

low proportions of the acoustic backscatter were allocated to krill. The low proportions are

probably caused by shortcomings in the krill identification techniques when other pairs of

frequencies than combinations of 38, 120 and 200 kHz are used
Description
-
Publisher
CCAMLR
Series
;WG-EMM-15/54

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