Distribution and population structures of immature and adult Euphausia superba in the western Bransfield Strait region during the 1986-87 summer

Abstract

Interrelationships among body size, maturity state and spatial distribution in immature and adult Euphausia superba were examined through the course of the 1986-87 reproductive season in a region centered in western Bransfield Strait and extending northward into Drake Passage and southward into Gerlache Strait. During an initial (December to January) peak in production to phytoplankton and krill larvae in Gerlache Strait and adjacent parts of Bransfield Strait, so few post-larvae and adults were caught that the parent stock of those larvae could not be identified with certainty. However, small adults of 36–42 mm body length were caught in abundance later in the summer (February to March), after larval recruitment in and near Gerlache Strait had ceased. Many of these small adults then appeared to be post-reproductive. In Bransfield Strait during late summer (late February to March) there was a nearly equal mixture of small adults (which dominated in Gerlache Strait) and large adults (>42 mm): the large adults showed most evidence of recent reproductive activity and could have been responsible for much of the late summer larval recruitment in Bransfield Strait. In Drake Passage, large adults were usually dominant; the few caught in December were undergoing maturation near the South Shetlands, and in January they were reproductive and dispersed in more offshore oceanic waters. During late February to March, krill became more available in Drake Passage with many having reassembled in aggregations near Livingston Island (South Shetlands). By then, not only small adults but also large adults appeared post-reproductive in all areas. Evidence relating to growth rate in adults was obscured by erratic variations in ratios of body-length classes, month-to-month, considered artifacts on sampling. However, adult growth appeared negligible during January to March. After midsummer (January) there were distinct changes in relative abundances of males to females of >42 mm: within 43–47 mm the modal frequencies favored females, and within 48–55 mm males were dominant, suggesting differential survivorship. The Gerlache-Bransfield Straits and similar coastal habitats must be considered nursery areas, not only for immature krill as has been previously proposed, but also for early summer larvae. In particularly productive years, as in 1986-87, these coastal waters may become spawning localities for small adult krill before production by large adults develops farther away from the Peninsula. Immatures of <12 mm in December and ca 16–30 mm by March, comprised a subgroup of the smallest immature krill though not consistently abundant enough to be distinguished as a year class. Their distributions suggest origins and early growth outside of the study area, possibly in the Weddell Sea and coastal waters of the Bellingshausen Sea.