Variability and predictability of Antarctic krill swarm structure

• Published in: Deep Sea Research Part I: Oceanographic Research Papers
• Main Authors: Tarling, Geraint A., Klevjer, Thor, Fielding, Sophie, Watkins, Jon, Atkinson, Angus, Murphy, Eugene, Korb, Rebecca, Whitehouse, Mick, Leaper, Russell
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2009
• Subjects: Marine Sciences; Zoology; Ecology and Environment; Antarctic; Scotia Sea; Antarc*; Antarctic Krill; Euphausia superba
• Online Access: http://nora.nerc.ac.uk/id/eprint/11310/; http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6VGB-4WTRS81-1-R&_cdi=6034&_user=1773399&_orig=browse&_coverDate=11%2F30%2F2009&_sk=999439988&view=c&wchp=dGLbVzz-zSkWb&md5=c40f22bae01a938bb6bfbc18df328fcc&ie=/sdarticle.pdf

Swarming is a fundamental part of the life of Euphausia superba, yet we still know very little about what drives the considerable variability in swarm shape, size and biomass. We examined swarms across the Scotia Sea in January and February 2003 using a Simrad EK60 (38 and 120 kHz) echosounder, concurrent with net sampling. The acoustic data were analysed through applying a swarm-identification algorithm and then filtering out all non-krill targets. The area, length, height, depth, packing-concentration and inter-swarm distance of 4525 swarms was derived by this method. Hierarchical clustering revealed 2 principal swarm types, which differed in both their dimensions and packing-concentrations. Type 1 swarms were generally small (<50 m long) and were not very tightly packed (<10 in.m(-3)), whereas type 2 swarms were an order of magnitude larger and had packing concentrations up to 10 times greater. Further subdivisions of these types identified small and standard swarms within the type 1 group and large and superswarms within the type 2 group. A minor group (swarm type 3) was also found, containing swarms that were isolated (>100 km away from the next swarm). The distribution of swarm types over the survey grid was examined with respect to a number of potential explanatory variables describing both the environment and the internal-state of krill (namely maturity, body length, body condition). Most variables were spatially averaged over scales of similar to 100 km and so mainly had a mesoscale perspective. The exception was the level of light (photosynthetically active radiation (PAR)) for which measurements were specific to each swarm. A binary logistic model was constructed from four variables found to have significant explanatory power (P < 0.05): surface fluorescence, PAR, krill maturity and krill body length. Larger (type 2) swarms were more commonly found during nighttime or when it was overcast during the day, when surface fluorescence was low, and when the krill were small and immature. A ...