Acoustic and ecological investigations into predator-prey interactions between Antarctic krill (Euphausia superba) and seal and bird predators

• Main Author: Cox, Martin James
• Other Authors: Brierley, Andrew, Borchers, D. L., MacKenzie, Monique Lea, Reid, Keith, Watkins, Jonathan L.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of St Andrews 2008
• Subjects: Antarctic krill; Euphausia superba; Acoustic; Distance sampling; Multibeam echosounder; Predator-prey interactions; Swarm; Pelagic; QL444.M338C7; Euphausia superba > Ecology; Euphausia superba > Predators; Euphausia superba > Effect of predation on; Echo sounding; Ecological surveys; Antarctic; Austral; Antarc*; Antarctic Fur Seals; Antarctic Krill; Arctocephalus gazella
• Online Access: http://hdl.handle.net/10023/579

1. Antarctic krill (Euphausia superba) form aggregations known as swarms that vary greatly in size and density. Six acoustic surveys were conducted as part of multidisciplinary studies at two study sites, the western and eastern core boxes (WCB and ECB), during the 1997, 1998 and 1999 austral summers, at South Georgia. A quantitative, automated, image processing algorithm was used to identify swarms, and calculate swarm descriptors, or metrics. In contrast to acoustic surveys of aggregations of other pelagic species, a strong correlation (r = 0.88, p = 0.02, 95% C.I.= 0.24 to 0.99) between the number of krill swarms and the mean areal krill density [rho.hat] was found. Multivariate analysis was used to partition swarms into three types, based on contrasting morphological and internal krill density parameters. Swarm types were distributed differently between inter-surveys and between on and off-shelf regions. This swarm type variation has implications for krill predators, by causing spatial heterogeneity in swarm detectability, suggesting that for optimal foraging to occur, predators must engage in some sort of adaptive foraging strategy. 2. Krill predator-prey interactions were found to occur at multiple spatial and temporal scales, in a nested, or hierarchical structure. At the largest inter-survey scale, an index of variability, I, was developed to compare variation in survey-scale predator sightings, sea temperature and [rho.hat]. Using I and a two-way ANOVA, core box, rather than year, was found to be a more important factor in determining species distribution. The absence of Blue-petrels (Halobaena caerulea) and the elevated number of Antarctic fur seals (Arctocephalus gazella) suggest that 1998 was a characterised by colder than average water surrounding South Georgia, and a high [rho.hat] in the ECB. At the smaller, intra-survey scales (<80 km, <5 day), the characteristic scale (distances in which predator group size, or krill density were similar, L_s) were determined. For krill and predators L_s ...