Taro Ichii

Distribution of salps near the South Shetland Islands during austral summer, 1990–1991 with special reference to krill distribution

Distribution and biomass of salps and Antarctic krill (Euphausia superba) were investigated near the South Shetland Islands during austral summer 1990–1991. Salp biomass ranged between 0 and 556 mgC·m−3 and was greatest at a station in the Bransfield Strait in late December 1990. Salp biomass was lower than that of E. superba. Two species of salps; Salpa thompsoni and Ihlea racovitzai were found, and the former was dominant numerically. Spatial distribution and generation composition of these two species was different. Spatial distributions of salps and E. superba did not overlap particularly so the January–February period. While E. superba was found mainly in the coastal area which showed high-chlorophyll a values, salps exhibited high biomass in the oceanic area with low chlorophyll a concentrations. Predation by salps on small krill and the competitive removal of food by them, are discussed as potential reasons for the relatively low abundance of E. superba at the stations where salps were present in great numbers.

Relationship between the distribution of euphausiids and baleen whales in the Antarctic (35°E – 145°W)

Abstract Simultaneous whale sighting and hydroacoustic surveys were conducted from a research vessel in the Antarctic to examine the relationship between the distribution of euphausiids and baleen whales. High densities of minke whales and large aggregations of euphausiids were observed along the ice edge over the continental slope in the southeast region of area IV and in the southwest region of area V. The results suggest that the continental slope zone that coincides with the ice edge would be an important minke whale feeding area. Minke whales were rarely sighted in the offshore region even if euphausiids were abundant. Distributions of humpback whales were correlated with high euphausiid density zones, regardless of the bottom topographic features. Several groups of blue whales were sighted in the small area along the ice edge where euphausiids were abundant, but sightings were too few to draw any general conclusion about the relationship between blue whales and euphausiids. Both baleen whales and euphausiids were scarce in the area east of 170°W where sea ice covered the continental shelf and slope zone.

Identification of ommastrephid squid paralarvae collected in northern Hawaiian waters and phylogenetic implications for the family Ommastrephidae using mtDNA analysis

A total of 110 adult individuals from four ommastrephid (family Ommastrephidae) squid species (Ommastrephes bartramii, Sthenoteuthis oualaniensis, Eucleoteuthis luminosa, and Hyaloteuthis pelagica) were used to obtain diagnostic DNA markers for species identification. Restriction fragment length polymorphism (RFLP) analysis of a partial segment (855 bp) of the mitochondrial DNA cytochrome oxidase I (COI) gene amplified by polymerase chain reaction (PCR) revealed that the restriction profiles of two endonucleases (Alu I and Tsp509 I) were diagnostic for species identification. The restriction assay partially supplemented with nucleotide sequence analysis successfully assigned 69 damaged and morphologically equivocal ommastrephid paralarvae collected in northern Hawaiian waters, identifying 60 O. bartramii, eight S. oualaniensis, and one E. luminosa. The family Ommastrephidae appears to be monophyletic. Although the phylogenetic relationships among genera were not resolved well due to apparent homoplasy and large genetic divergence between species, COI sequence data without transitions provided support for subfamily level relationships.

Green krill, the indicator of micro- and nano-size phytoplankton availability to krill

Abstract Size-fractionated chlorophyll-a concentrations of surface seawater were measured for pico-, nano-, and micro-size fractions (<2 μm, 2–10 μm, and >10 μm respectively) during commercial krill fishery operations in the waters north of the South Shetland Islands. The proportion of green krill (individuals discoloured due to active feeding on phytoplankton) had significant regressions with chlorophyll-a concentrations in micro- and nano-size fractions. Between these two fractions, chlorophyll-a concentration in the micro-size fraction showed the higher partial regression coefficient. This result shows the importance of phytoplankton larger than nano-phytoplankton, especially micro-phytoplankton, in terms of a phytoplanktonic food source for Antarctic krill in the natural environment.

Competition between the krill fishery and penguins in the South Shetland Islands

Among the South Shetland Islands, the potential competition for krill (Euphausia superba) between the Japanese fishery and krill-eating breeding penguins was assessed. A low level of competition was apparent due to several factors. Spatial overlap between the main fishing and foraging areas was insignificant Firstly, the large colonies of the dominant penguin (chinstrap penguins,Pygoscelis antarctica) occurred where sea ice disappears early in spring, and not necessarily where krill are abundant in summer, i.e. the area of krill fishery. Secondly, overlap between trawling depth and foraging dive depth of penguins was marginal, with the latter depth being shallower. Moreover, overlap in the size-frequency of krill caught by trawlers and those captured by penguins was not complete; the penguins took larger krill on average. Finally, the present small fishery is unlikely to impact upon local krill biomass in the region. Krill biomass was estimated to be 250–1500×103 tonnes within the preferred fishing areas during summer. The present catch rate by the fishery (≤13×103 tonnes/half-month period) is smaller by an order of magnitude, and the fishing area does not include the main foraging areas of breeding penguins.

Diet and sexual maturation of the neon flying squid Ommastrephes bartramii during autumn and spring in the Kuroshio -Oyashio transition region

Diet and sexual maturation were examined in the winter-spring cohort of the neon flying squid, Ommastrephes bartramii, during its southward migration through the Kuroshio - Oyashio transition region of the western North Pacific. The main prey items are micronektonic animals and small pelagic fish, which were abundantly distributed throughout the study area. Among the prey species, 0. bartramii was dependent on the Japanese anchovy Engraulis japonicus, which also migrate from the northern to southern transition region during the winter, and the micronektonic squid Watasenia scintillans throughout the study period. Other common prey items were Gonatopsis borealis, Diaphus theta, and Ceratoscopelus warmingii from October to November or December, and Tarletonbeania taylori after December. These dietary changes can be explained by the difference in the seasonal north-south migration patterns of the predator and prey species. Male sexual maturation progressed throughout the season, and most individuals were fully mature in January. In contrast, most of the females were immature throughout the study period. The feeding strategy of the squid in relation to their seasonal north to south migration and sexual maturation was discussed.

Molecular evidence for synonymy of the genera Moroteuthis and Onykia and identification of their paralarvae from northern Hawaiian waters

It has been claimed that most squid species in the genus Onykia may be immature stages of species in the genus Moroteuthis. To evaluate the generic status of Moroteuthis and Onykia and to identify paralarvae collected in northern Hawaiian waters, we performed morphological investigation and nucleotide sequence analysis of the mitochondrial cytochrome oxidase I (COI) gene. Of 42 Onykia paralarvae (1.8–8.5 mm dorsal mantle length, DML) examined, 41 had a nucleotide sequence identical to that of M. robusta and one (designated Onykia sp. A) could not be assigned to any known Moroteuthis species. Nucleotide sequence diversity estimates based on Kimura’s two-parameter distances between Onykia sp. A and Moroteuthis spp. (0.109–0.150) fell well within the range of congeneric species, suggesting that Onykia sp. A is a member of the genus Moroteuthis. Molecular data supported the conclusion that the genus Moroteuthis is a junior synonym of the genus Onykia. Morphological investigation revealed that paralarvae of O. robusta (= M. robusta) <4.0 mm DML were distinct from other Onykia paralarvae in the chromatophore pattern on the mantle. The key characters for distinguishing O. robusta paralarvae from Onykia sp. A were the number and arrangement of chromatophores on the funnel.

Validation of band counts in eyestalks for the determination of age of Antarctic krill, Euphausia superba

Using known-age Antarctic krill (Euphausia superba) grown from eggs hatched at two different laboratories, we validate the annual pattern of bands deposited in the eyestalks of krill and determine the absolute age of these animals. Ages two through five years were validated, and these animals ranged from 37.1 to 62.6 mm in total length. The band counts in these individuals were either identical to their absolute ages, or only failed to agree by a few months, which demonstrates the accuracy of this method. Precision and bias were estimated graphically using Chang’s index (Coefficient of Variation = 5.03%). High accuracy and precision between readers and low ageing bias indicate that longitudinal sections of eyestalks can be used to age krill in wild samples and to develop age-based stock assessment models for krill. Archival samples preserved in formalin (5%) and stored in ambient conditions were also readable. Ageing preserved krill will provide the opportunity to examine changes in growth among krill populations within the Southern Ocean and to retrospectively examine changes in krill production over the last century to better understand the historical and future impacts of climate change on this critical Southern Ocean species.