Structures of Swarming and Dispersed Populations of Krill (Euphausia Superba) in Scotia Sea and South Shetland Waters During January-march 1981, Determined by Bongo Nets

Abstract

ABSTRACT Through much of the Scotia Sea and near the South Shetland Islands during 24 January–-23 March 1981, the RV Melville made extensive bongo net collections of zooplankton, including krill, Euphausia superba. Length-frequency distributions of juvenile and adult krill are studied here. An eastern population in the Scotia Sea was distinguishable by a 35 mm, nonreproductive mode, and a western population by a 35 mm and 55 mm reproductive bimodal distribution. One swarm was encountered in each subregion. Most catches indicated relatively dispersed krill. Elsewhere, in the Bransfield Strait and near Elephant Island, population length-frequency structures were more complex, indicating mixtures of age groups of different origins. A midMarch swarm located acoustically in Bransfield Strait was dominated by Year-2 krill of 30–-45 mm, with a unique minor mode at 14-22 mm and a secondary mode centered upon 50–-53 mm adults. Around Elephant Island, two kinds of size distributions, largely separate in space, characterized generally abundant krill: (1) a 50-55 mm mode wherein adult males predominated, and (2) a modeless 30-50 mm range wherein females predominated. During 7-8 March, a dense swarm occupying 150 km2 consisted of two partially overlapping segments, each having one of the two length-frequency structures. Biomass of the swarm was estimated at 47,200 metric tons, based on bongo net samples. During 20-23 March, dominance by 50-55 mm males prevailed; swarms were then particularly associated with the 150-160 m bathymetric interval. Circumstantial evidence of selective effects of intensive trawling by an industrial fleet is seen in relatively low densities of Year-2 krill in places where trawling had taken place, resulting in domination by Year-3 krill. The possibility that the population in which 50-55 mm adults dominated is a natural result of advection is also considered, but not resolved.