Influence of environmental conditions on spatial distribution and abundance of early life stages of Antarctic silverfish,Pleuragramma antarcticum(Nototheniidae), in the Ross Sea

Abstract

The Antarctic silverfishPleuragramma antarcticumBoulenger is the dominant fish species in the high Antarctic zone, playing a key role in the Ross Sea midwater shelf ecosystem. Unlike other notothenioids, it is holoplanktonic species, spending its entire life cycle in the water column. Early life stages ofP. antarcticumare generally found in the upper 200 m and their spatial distribution is largely affected by water masses and general circulation. To understand better the mechanisms involved in the geographical distribution of the Antarctic silverfish within the western Ross Sea, an analysis of abundance and distribution was carried out in relation to oceanographic conditions. Samples were collected in summer during the 1998, 2000 and 2004 Italian cruises, covering the majority of the western sector of the Ross Sea. Overall 127 stations were sampled using standard plankton nets for biological samples and CTD and XBT to record abiotic parameters. Although all surveys were in December–January, the yearly results differed in terms of relative abundance of larval developmental stages and of oceanographic characteristics. The 1997–98 samples were characterized by very low abundance overall and by the virtual absence of early larvae. In summers 1999–2000 and 2003–04 the abundance ofP. antarcticumwas one order of magnitude higher than in the earlier season. In 1999–2000 catches were mainly composed of pre-flexion larvae and late postlarvae, while in 2003–04 catches were made up of pre-flexion larvae and juveniles. In January 2000 the Ross Sea summer polynya was fully open as the pack ice was almost completely melted, whereas in January 1998 and 2004 the opening of the polynya was considerably delayed. As a consequence, a delay in phytoplankton blooms and a decrease in primary production were observed in the summer seasons 1998 and 2004 with respect to 2000. The spatial distribution of early life stages, that were confined to the continental shelf and shelf break of the Ross Sea, generally appeared to be positively influenced by transition zones (oceanographic fronts). In addition, most of catches were recorded on or in close proximity to the banks (Pennell, Mawson, Ross and Crary) that characterize the continental shelf of the Ross Sea. On the basis of present findings and literature data, a link between the general circulation in the western Ross Sea and the distribution pattern of the early life stages ofP. antarcticumhas been developed.

Some biological characteristics of early larvae Dacodraco hunteri (Notothenioidei: Channichthyidae) in the western Ross Sea

“…In both areas, persistent katabatic winds cause the formation of large coastal polynyas during spring (Bromwich et al 1998), triggering rapid increase in phytoplankton blooms until December (Arrigo and Van Dijken 2004). Locally, this provides favorable food conditions for the newly hatched larvae of P. antarcticum (La Mesa et al 2010), which represent the main prey of D. hunteri larvae, confirming once again the importance of coastal polynyas in the early life cycle of notothenioids as reported elsewhere (Kellermann 1986;Hubold 1990). Larvae of D. hunteri were mainly caught at the stations near the ice shelf also in the Weddell Sea, where they dominated the neritic fish community off Halley Bay (White and Piatkowski 1993).…”

Distributions of krill and Antarctic silverfish and correlations with environmental variables in the western Ross Sea, Antarctica

“…The slow surface speeds that emerged as a characteristic for the Antarctic silverfish habitat will facilitate retention of the early life stages in the productive polynya region. Unlike the crystal krill, the distribution of Antarctic silverfish observations (La Mesa et al 2010) was sufficient to capture speed as a habitat characteristic, which highlights the importance of designing sampling strategies to include scales of environmental controls.…”

Impact of Climate Change on Fishes in Complex Antarctic Ecosystems

“…The sea ice region is apparently also the spawning ground (La Mesa and Eastman, 2012) as the pelagic eggs were found floating under the sea ice . The reproductive cycle of P. antarcticum seems to be closely coupled to seasonal sea ice dynamics, and early stages depend on the temporal and spatial match with the seasonal zooplankton production (La Mesa and Eastman, 2012;La Mesa et al, 2010). The hatching period of P. borchgrevinki seems to be less strongly coupled to production peaks (Pankhurst, 1990), but both species are expected to be significantly affected by alterations in seasonal sea ice dynamics by loss of habitat/ refuge and spawning ground and alterations at the base of the food web.…”

“…Shifts in any of these three parameters might significantly compromise larval condition and survival. Most notothenioid larvae depend on seasonal timing as well (Efremenko, 1983;La Mesa and Eastman, 2012;La Mesa et al, 2010), and the capacity to avoid a 399 Impact of Climate Change on Fishes in Complex Antarctic Ecosystems mismatch depends on species' plasticity in their reproductive cycle. In some species, a certain plasticity to adapt the reproductive cycle might exist, as indicated by differences in spawning time among populations in different locations (see Kock and Kellermann, 1991), but it is unknown whether all species are able to adapt their reproductive cycle and by which factors the timing is triggered.…”