Dynamics of Antarctic krill in the Bransfield Strait during austral summer and autumn investigated using acoustic data from a fishing vessel

The Antarctic krill fisheries have evolved to operate in ever more concentrated geographical areas of predictable high krill densities, and knowledge about trends and variability in krill biomass, distribution and swarm structure on these local scales seems critical for assessing the three-way interactions between krill, krill-dependent predators and the krill fishery. Acoustic data collected from the fishing vessels might help provide such insight. In the present work, the dynamics of krill with regards to abundance and distribution as well as swarm characteristics and diel vertical migration were studied using acoustic data from the Chinese krill fishing vessel ‘Fu Rong Hai’ operating in the Bransfield Strait from late austral summer (February) to autumn (March to May), and its potential impact on the fishing efficiency were also examined. Only detected and delineated swarms were included in the integration assuming they constituted the bulk of krill biomass. Our results indicated a major shift in mid-April which included increased biomass, increased vertical distribution of swarms, a shift in diel vertical migration from upward migration during daytime in February-March to downward migration during daytime in May, and also a shift in the length distribution of krill. The results strongly support recent findings of an inshore krill migration from summer to winter, and indicates that the migration is also followed by a gradual shift in swarming behavior. Our results further showed that the fishing vessel operated in areas of very high swarm encounter rates, and that the trawling occurred on the highest krill concentrations, indicating that the fishing strategy of this vessel was to target krill swarms of maximum density. In addition, the functional relationships between the catch per unit effort (CPUE) and potential impact factors showed that the catching efficiency increased over the season and was positively related to both krill packing density and acoustic biomass, but negatively related to the center depth of gravity. The results demonstrate that new understanding of krill seasonal dynamics and its influence on fishing efficiency can be gained based on data collected from a fishing vessel under operation, and underlines the utility of a full implementation of the SG-ASAM plans for data collection from fishing vessels.