MESOSCALE VARIATION IN GROWTH AND EARLY DEVELOPMENT OF EUPHAUSIA-SUPERBA DANA IN THE WESTERN BRANSFIELD STRAIT REGION

Development and growth of larval Euphausia superba were measured in field and laboratory populations in the western Bransfield Strait during the period from December 1986 to March 1987. Three principal biogeographic zones were identified: northern Gerlache Strait, Bransfield Strait and Drake Passage; these were recognizable by consistent patterns of physical circulation. Two cohorts of E. superba larvae were produced. The first cohort appeared as C1 larvae in mid-January, and the second appeared in mid-February; the second cohort did not occur in Gerlache Strait waters. Larvae in Gerlache Strait were more abundant, larger in size and weight, better fed, and developed and grew more rapidly than larvae of the same stages in both Bransfield Strait and Drake Passage. In Gerlache Strait abundance reached 12,000 individuals m-2, intermolt periods were < 8 days for early calyptopis stages, and growth rates were higher than any previously reported. Growth of early stages was exponential during late January, but slowed markedly by late March as the population entered late furcilia stages. Calptopis stages were estimated to ingest up to 19.7% of their body carbon per day, and 14.5% of body nitrogen per day. The daily ration of late furcilia stages was reduced to 4.4% body carbon per day, and 1.9% body nitrogen per day. Gross growth efficiencies ranged, in terms of carbon, from 13.2% for furcilia to 52.5% for calyptopis stages; in terms of nitrogen they ranged from 29.9 to 65.5%. It is proposed that the northern Gerlache Strait is a primary nursery area for E. superba, and that local circulation promotes high primary productivity and prolongs the residence time of larvae in a nearshore environment which is favorable to development and growth. Coastal environments such as Gerlache Strait may make a disproportionately large contribution to annual krill recruitment.