Spatial Distribution and Feeding Habits of Larval and Juvenile Pleuragramma Antarcticum in the Western Ross Sea (Antarctica)

Abstract

During four spring-summer Italian Antarctic expeditions to Terra Nova Bay and the Western Ross Sea in 1988, 1989–90, 1994–95 and 1996, zooplankton and ichtyoplankton were collected by a multinet BIONESS (250 μm and 500 μm) and PHN (500 μm), respectively. Nototheniids were predominant in the whole study area, representing 97.3% of all larvae. Pleuragramma antarcticum was the dominant species sampled in the spring-summer cruises of 1987–88 (98.6%), 1989–90 (95.9%) and 1995–96 (99.8%), while it predominated in association with Trematomus lepidorhinus in 1994–95. The early life history of Pleuragramma antarcticum was described from 891 samples. Maximum abundances of P. antarcticum postlarvae were confined to Terra Nova Bay. The mean relative abundance in the whole area in summer 1988, for positive 0- to 150-m hauls, was 434 (±720 SD) specimens 100 m−3 and 158 (±212 SD) specimens per 100 m3 in January 1990 for positive 0-140-m hauls. Off Terra Nova Bay, postlarval abundance did not exceed 5-86 individuals 100 m−3. Juvenile Pleuragramma antarcticum showed a more oceanic occurrence than postlarvae of the same species. Low concentrations between 0.1 and 12.5 ind 100 m−3 (mean 1.66 ± 2.88 SD) were recorded in offshore waters between 170° and 175°E and in the southern part of Terra Nova Bay. Postlarvae (n = 4385) ranged between 7 and 23 mm SL (mean 13.9 ± 1.62 SD) and juveniles (n = 388) were between 28 and 53 mm SL (mean 40.0 ± 4.78 SD). A growth rate of 0.08 mm day−1 was confirmed for the first year. The daily increments of Pleuragramma antarcticum postlarvae estimated from sequential sampling in 1987–88 from Terra Nova Bay and in 1995–96 from the southern western Ross Sea were about 0.21–0.25 mm. The co-occurrence of different postlarval size classes during the investigations confirm that two hatching periods occurred in Terra Nova Bay between early and late December. The presence of early stages in January 1988, 1990 and 1996 (7–9 mm SL) confirms that Pleuragramma antarcticum spawn in Terra Nova Bay. Highest concentrations of postlarvae occurred at the edge near Cape Washington and in the inshore waters of Drygalski Ice Tongue. These areas seem to represent a nursery ground of early stages, where the presence of permanent polynyas could provide favourable food conditions for the development of the first stages of life. More than 62% of the postlarvae were collected in well-stratified Antarctic Surface Waters (AASW) with temperature values that exceed 0°C and vertical distribution appears to be related to the summertime thermocline. From these sites, early stages were drifted by a westward-flowing current of the limb of the Antarctic coastal current and the southern limb of the Ross Sea Gyre. These large gyres favoured postlarval retention mostly in northern and southern areas of Terra Nova Bay, preventing the drift north of Cape Washington, south of Drygalski Ice Tongue and in offshore waters beyond 170°E. Thus, the youngest planktonic stages of Pleuragramma antarcticum concentrate in the surface mixed layer of Terra Nova Bay where feeding conditions are favourable. In the size classes 8-17 mm the diet was dominated by calanoid eggs (35.5%), Limacina (32.1%) and tintinnids (17.6%). The cyclopoid copepod Oncaea curvata contributed to 7.1% of the diet.