Comparison of geostatistical and random sample survey analyses of Antarctic krill acoustic data

Data from the acoustic surveys of MV SA “Agulhas” and FRV “Walther Herwig”, and the 1981 RRS “John Biscoe” South Georgia acoustic survey were analysed by geostatistical methods. Estimates of mean density (g m−2) of krill and their variances are compared with published results from statistical analyses based on random sampling theory. A further high-resolution geostatistical analysis of the MV SA “Agulhas” (ping-by-ping) data set of the density of each individual aggregation is also presented. These analyses illustrate the problems of applying geostatistical methods to data from highly aggregated species which can show marked skewness in their histogram of density

Abundance and diurnal vertical distribution of fish larvae in early spring and summer in a fjord at South Georgia

The diurnal vertical distribution and abundance of fish larvae was studied by net samples in the 265 m deep fjord of Cumberland East Bay, South Georgia. In early spring the six most abundant species of fish larvae in the upper 200 m of the water column were Champsocephalus gunnari, Lepidonotothen nudifrons, Pseudochaenichthys georgianus, Chaenocephalus aceratus, Parachaenichthys' georgianus and Electrona antarctica. In summer, the larvae of C. gunnari, Gobionotothen gibberifrons, Lepidonotothen larseni and Trematomus hansoni were most abundant, and the majority were found in the upper 140 m of the water column, with greatest densities in the top 2m. During both seasons most species showed some evidence of diurnal vertical migration. A distinct pattern was found for C. gunnari in summer; they were at 0–20 m during the day and 60–100 m at midnight. Net avoidance by the larvae of most species was greater before sunset in early spring, and during all periods of daylight in summer, than at other times of day

Response of two Antarctic bryophytes to stratospheric ozone depletion

We report a study which measured changes to the radiative environment arising from stratospheric O3 depletion at Rothera Point on the western Antarctic Peninsula (67°S, 68°W) and subsequent associations between these changes and the pigmentation and maximum quantum yield of photochemistry (Fv/Fm) of two Antarctic bryophytes, the liverwort Cephaloziella varians and the moss Sanionia uncinata. We found a strong relationship between O3 column depth and the ratio of UV-B to PAR irradiance (FUV-B/FPAR) recorded at ground level. Weaker, but significant, associations were also found between O3 column depth and noon irradiances and daily doses of unweighted and biologically effective UV-B radiation received at ground level. Regression analyses indicated that FUV-B/FPAR and daily dose of unweighted UV-B were best predictors for concentrations of total carotenoids and UV-B screening pigments extracted from bryophyte tissues. Concentrations of these pigments were loosely but significantly positively associated with O3-dependent irradiance parameters. HPLC analyses of carotenoids also suggested that both species increased the synthesis of neoxanthin during periods of O3 depletion. Violaxanthin, lutein, zeaxanthin and b,bββ-carotene concentrations were also apparently influenced by O3 reduction, but not consistently across both bryophyte species. Concentrations of chlorophylls a and b were apparently unaffected by O3 depletion. No direct associations between Fv/Fm and O3-dependent irradiance parameters were found. However stepwise multiple regression analyses suggested that the production of UV-B screening pigments conferred protection from elevated FUV-B/FPAR on Fv/Fm in both species and that carotenoids conferred protection on Fv/Fm in Sanionia. Our data suggest that changes to the radiative environment associated with stratospheric O3 depletion influence the pigmentation of two Antarctic bryophytes, but that Fv/Fm is unaffected, at least in part because of rapid synthesis of protective pigments

Seasonal variation in the effects of hard substratum biofilming on settlement of marine invertebrate larvae

The effect of season on “biofilming”;, as a cue for the settlement of marine invertebrate larvae, was investigated in a long‐term field study during the years 1992–1994. The series of settlement experiments was conducted in a tidal rapid on the west coast of Scotland, and involved manipulations of artificial panels. Biofilming of substrata, whilst excluding larval settlement, was achieved by the enclosure of panels within tight‐fitting (but removable) mesh screens so that the number of settlers on filmed and unfilmed substrata were counted in the initial absence of other incumbent post‐larvae. Depending on larval species, the effects of biofilming were found to be either facilitatory or inhibitory. Significant within‐ and between‐species seasonal differences in the settlement responses were detected, and a reversal of the effect of biofilming on larval settlement response, from inhibitory to facilitatory and vice versa, was noted with season in the case of some taxonomic groups and species (e.g. Tubulipora sp., Plagioecia sp., Electra pilosa (L.)). The present study emphasizes the need for extended field studies of larval responses to environmental cues, when the focus of interest is in drawing general inferences about naturally occurring behavioural patterns at settlement

Growth and tissue mass cycles in the infaunal bivalve Yoldia eightsi at Signy Island, Antarctica

Shell growth in Yoldia eightsi was measured over an austral summer and winter in 1992. In specimens 27 mm length did not increase in length. Tissue dry and ash-free dry mass (AFDM) cycles were assessed at monthly intervals between December 1988 and January 1991. ANCOVA indicated significant interannual and seasonal effects on this cycle. Tissue mass increased in the summer, coinciding with the phytoplankton bloom and the period of maximum sedimentation of organic material from the water column. A standard 20-mm-length animal reached a maximum AFDM of 114 mg in February 1990. The minimum value (68 mg AFDM) throughout the 2 years of measurements was in early December 1988, at the end of the austral winter. Periods of tissue mass increase were, therefore, decoupled from shell growth, at least in juveniles. Tissue mass was significantly higher in 1990 than 1989, which was mainly due to high organic contents in the summer (January to May). This was not consistent with the pattern of organic content in the sediments at the study site, but was in phase with the cycle in sediment chlorophyll a content. Tissue mass increase depended on major resource input during the summer, but Y. eightsi was capable of maintaining winter condition from stocks of benthic microalgae in years of poor ice cover. Tissue mass declined between April and July each year. This was accompanied by large falls in tissue ash content, and coincided with the spawning period in early June. These are the first monthly tissue mass data collected over a 2-year period for an Antarctic mollusc. They are the first such data indicating seasonal variation in tissue mass and showing a decoupling of shell and tissue growth in a polar bivalve. The P/B ratio calculated from these data was 0.106, which is slightly lower than previous values found for this species, but is in line with general values for Antarctic marine benthos

Life cycle, demography and reproductive investment in the myopsid squid Alloteuthis subulata

Eleven monthly samples of the myopsid squid Alloteuthis subulata were collected from the English Channel with a small otter trawl in 1985-86. Squid were sexed and assigned a maturity stage. Measurements were made of mantle length and dry weight of gonad and somatic tissue. Maximum length of life for squid from this population is ca 1 yr. Immature squid are recruited into the population twice during the year, in spring and summer. Males and females start to mature at 30 to 40 mm mantle length. The demography of the population is complex, and more difficult to interpret for males. There were apparently 3 spawning groups of females in the year sampled. The length exponent for body weight is very low in both males and females. Males mature earlier than females and male gonad weight is poorly correlated with body weight in squid > 90 mm mantle length. Gonad weight increases logarithmically in relation to total body weight in females but gonad production is not appreciable until mantle length exceeds 100 mm. The gonad accounts for ca 25 O/O of the body mass of females at the maximum mantle length of 140 mm. This suggests that A. subulata, In common with some other cephalopods, has a low reproductive effort by comparison with the lifetime reproductive effort of species of benthic iteroparous mollusc

Cephalopod prey of the wandering albatross Diomedea exulans

Cephalopod beaks from the stomach contents of “wandering albatross” (Diomedea exulans L.) chicks from Bird Island, South Georgia, were sampled between May and September in 1983 and 1984. Lower beaks were identified and measured, and allometric data were used to calculated mantle length and biomass of the species consumed. A total of 3421 lower beaks were examined, representing 35 species in the 1983 sample and 45 species in the 1984 sample. Eight of the twenty families contributed over 95% of the biomass. In 1984 there were less Onychoteuthidae and more Ommastrephidae than in 1983 and a decrease in the number of species known to occur south of the Antarctic Polar Front. There was a difference in the size-frequency distribution of the cephalopod diet in the two years; in 1984 there was a higher frequency of intermediate-sized specimens, reflecting the greater importance of ommastrephids, especially Illex sp. The energy content of cephalopods in 1984 may have been greater than in 1983. Serial sampling of cephalopod beaks during the austral winter did not reveal evidence of growth. By the age of 200 d, wandering albatross chicks have consumed a total of approximately 100 kg wet weight of cephalopods each

Cephalopod prey of the grey-headed albatross Diomedea chrysostoma

Cephalopod remains were collected, at regular intervals throughout the fledging period, from the stomach contents of chicks of the grey-headed albatrossDiomedea chrysostoma at Bird Island, South Georgia, in 1984 and 1986 and from regurgitations of adults at the nest in 1986. The 1984 sample was taken during a season characterised by abnormal local oceanographic conditions in which the breeding success was very low; in 1986 conditions were normal and breeding success was high. Cephalopod beaks (289 from adults; 5 651 from chicks) were identified, and allometric equations were used to estimate the biomass represented. Five cephalopod species belonging to five families (Gonatidae, Onychoteuthidae, Psychroteuthidae, Ommastrephidae and Cranchiidae) contributed 98% by number and 97% of the biomass fed to chicks. The most important species was the ommastrephidMartialia hyadesi, contributing 68.9 to 77.4% by number and 72.5 to 79.3% of the total biomass fed to chicks. The relative proportions of cephalopod species in the chicks' diet were similar between 1984 and 1986, but the total number and biomass was significantly less in 1984. There is evidence of growth ofM. hyadesi between January and June

Cephalopod predation by the king penguin Aptenodytes patagonicus from South Georgia

The king penguin Aptenodytes patagonicus is a sub-Antarctic species that feeds primarily on mesopelagic fish and cephalopods in the vicinity of the Antarctic Polar Front (APF). We examined the cephalopod prey of adult penguins at a breeding colony on South Georgia during 4 austral summers, 1990 to 1994. The most important prey (~97% by mass) of the king penguins was found, in a related study, to be mesopelagic fish, mainly myctophids. The penguins¹ cephalopod prey, which constituted the remaining 3%, was shown in this study to be dominated by the ommastrephid squid Martialia hyadesi, both in terms of numbers and biomass. Other squid species were typical of the South Georgia/Antarctic Polar Front (APF) area but only relatively small specimens were present and, apart from Gonatus antarcticus, the presence of flesh indicated that they were probably mostly caught close to the island. M. hyadesi generally fell in the same size range as M. hyadesi exploited by commercial fisheries in the South Atlantic. In the 1992/93 summer, when krill was abundant at South Georgia, the amount of squid consumed, especially M. hyadesi, was substantially less than in the other years. This coincided with a reduction in the amount of the myctophid Krefftichthys anderssoni, which is the major prey of M. hyadesi, in the penguin diet. The total consumption of cephalopods by the king penguin at South Georgia is estimated to be about 75000 t yr-1. Of this, some 3600 to 6000 t yr-1 is estimated to be M. hyadesi. This might underestimate consumption if the penguins consume a greater proportion of squid in the winter, as they do in other sectors of the Southern Ocean